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<h1><span class=chap_num>Chapter 6</span>The Secret Life of Objects</h1>

<p><a class="p_ident" id="p_gDFRhOpA5B" href="#p_gDFRhOpA5B" tabindex="-1" role="presentation"></a>An abstract data type is realized by writing a special kind of program […] which defines the type in terms of the operations which can be performed on it.</p>

<footer>Barbara Liskov, <cite>Programming with Abstract Data Types</cite></footer>

</blockquote><figure class="chapter framed"><img src="img/chapter_picture_6.jpg" alt="Picture of a rabbit with its proto-rabbit"></figure>

<p><a class="p_ident" id="p_KkunxYMaeO" href="#p_KkunxYMaeO" tabindex="-1" role="presentation"></a><a href="04_data.html">Chapter 4</a> introduced JavaScript’s objects. In programming culture, we have a thing called <em>object-oriented programming</em>, a set of techniques that use objects (and related concepts) as the central principle of program organization.</p>

<p><a class="p_ident" id="p_iS8mjYhuYY" href="#p_iS8mjYhuYY" tabindex="-1" role="presentation"></a>Though no one really agrees on its precise definition, object-oriented programming has shaped the design of many programming languages, including JavaScript. This chapter will describe the way these ideas can be applied in JavaScript.</p>

<h2><a class="h_ident" id="h_hn18MBjoh2" href="#h_hn18MBjoh2" tabindex="-1" role="presentation"></a>Encapsulation</h2>

<p><a class="p_ident" id="p_76wqEvwMj/" href="#p_76wqEvwMj/" tabindex="-1" role="presentation"></a>The core idea in object-oriented programming is to divide programs into smaller pieces and make each piece responsible for managing its own state.</p>

<p><a class="p_ident" id="p_Ww3ymqEfW3" href="#p_Ww3ymqEfW3" tabindex="-1" role="presentation"></a>This way, some knowledge about the way a piece of the program works can be kept <em>local</em> to that piece. Someone working on the rest of the program does not have to remember or even be aware of that knowledge. Whenever these local details change, only the code directly around it needs to be updated.</p>

<p id="interface"><a class="p_ident" id="p_m1XIFUcUIg" href="#p_m1XIFUcUIg" tabindex="-1" role="presentation"></a>Different pieces of such a program interact with each other through <em>interfaces</em>, limited sets of functions or bindings that provide useful functionality at a more abstract level, hiding their precise implementation.</p>

<p><a class="p_ident" id="p_n3o8Ctd3cn" href="#p_n3o8Ctd3cn" tabindex="-1" role="presentation"></a>Such program pieces are modeled using objects. Their interface consists of a specific set of methods and properties. Properties that are part of the interface are called <em>public</em>. The others, which outside code should not be touching, are called <em>private</em>.</p>

<p><a class="p_ident" id="p_RExXHtzJn1" href="#p_RExXHtzJn1" tabindex="-1" role="presentation"></a>Many languages provide a way to distinguish public and private properties and prevent outside code from accessing the private ones altogether. JavaScript, once again taking the minimalist approach, does not—not yet at least. There is work underway to add this to the language.</p>

<p><a class="p_ident" id="p_h9lEzfGlTT" href="#p_h9lEzfGlTT" tabindex="-1" role="presentation"></a>Even though the language doesn’t have this distinction built in, JavaScript programmers <em>are</em> successfully using this idea. Typically, the available interface is described in documentation or comments. It is also common to put an underscore (<code>_</code>) character at the start of property names to indicate that those properties are private.</p>

<p><a class="p_ident" id="p_ysFjuxKxY0" href="#p_ysFjuxKxY0" tabindex="-1" role="presentation"></a>Separating interface from implementation is a great idea. It is usually called <em>encapsulation</em>.</p>

<h2 id="obj_methods"><a class="h_ident" id="h_fkrGgDyRWc" href="#h_fkrGgDyRWc" tabindex="-1" role="presentation"></a>Methods</h2>

<p><a class="p_ident" id="p_DKj01h8Gzf" href="#p_DKj01h8Gzf" tabindex="-1" role="presentation"></a>Methods are nothing more than properties that hold function values. This is a simple method:</p>

<pre class="snippet cm-s-default" data-language="javascript" ><a class="c_ident" id="c_BYfVVcFY2P" href="#c_BYfVVcFY2P" tabindex="-1" role="presentation"></a><span class="cm-keyword">let</span> <span class="cm-def">rabbit</span> <span class="cm-operator">=</span> {};

<span class="cm-variable">rabbit</span>.<span class="cm-property">speak</span> <span class="cm-operator">=</span> <span class="cm-keyword">function</span>(<span class="cm-def">line</span>) {

<span class="cm-variable">console</span>.<span class="cm-property">log</span>(<span class="cm-string-2">`The rabbit says '${</span><span class="cm-variable-2">line</span><span class="cm-string-2">}</span><span class="cm-string-2">'`</span>);

};

<span class="cm-variable">rabbit</span>.<span class="cm-property">speak</span>(<span class="cm-string">&quot;I'm alive.&quot;</span>);

<span class="cm-comment">// → The rabbit says 'I'm alive.'</span></pre>

<p><a class="p_ident" id="p_N+6e0UGvFo" href="#p_N+6e0UGvFo" tabindex="-1" role="presentation"></a>Usually a method needs to do something with the object it was called on. When a function is called as a method—looked up as a property and immediately called, as in <code>object.method()</code>—the binding called <code>this</code> in its body automatically points at the object that it was called on.</p>

<pre class="snippet cm-s-default" data-language="javascript" ><a class="c_ident" id="c_cXuIwPt+3C" href="#c_cXuIwPt+3C" tabindex="-1" role="presentation"></a><span class="cm-keyword">function</span> <span class="cm-def">speak</span>(<span class="cm-def">line</span>) {

<span class="cm-variable">console</span>.<span class="cm-property">log</span>(<span class="cm-string-2">`The ${</span><span class="cm-keyword">this</span>.<span class="cm-property">type</span><span class="cm-string-2">}</span> <span class="cm-string-2">rabbit says '${</span><span class="cm-variable-2">line</span><span class="cm-string-2">}</span><span class="cm-string-2">'`</span>);

}

<span class="cm-keyword">let</span> <span class="cm-def">whiteRabbit</span> <span class="cm-operator">=</span> {<span class="cm-property">type</span>: <span class="cm-string">&quot;white&quot;</span>, <span class="cm-property">speak</span>};

<span class="cm-keyword">let</span> <span class="cm-def">hungryRabbit</span> <span class="cm-operator">=</span> {<span class="cm-property">type</span>: <span class="cm-string">&quot;hungry&quot;</span>, <span class="cm-property">speak</span>};

<span class="cm-variable">whiteRabbit</span>.<span class="cm-property">speak</span>(<span class="cm-string">&quot;Oh my ears and whiskers, &quot;</span> <span class="cm-operator">+</span>

<span class="cm-string">&quot;how late it's getting!&quot;</span>);

<span class="cm-comment">// → The white rabbit says 'Oh my ears and whiskers, how</span>

<span class="cm-comment">// late it's getting!'</span>

<span class="cm-variable">hungryRabbit</span>.<span class="cm-property">speak</span>(<span class="cm-string">&quot;I could use a carrot right now.&quot;</span>);

<span class="cm-comment">// → The hungry rabbit says 'I could use a carrot right now.'</span></pre>

<p id="call_method"><a class="p_ident" id="p_llBwR5t6LB" href="#p_llBwR5t6LB" tabindex="-1" role="presentation"></a>You can think of <code>this</code> as an extra parameter that is passed in a different way. If you want to pass it explicitly, you can use a function’s <code>call</code> method, which takes the <code>this</code> value as its first argument and treats further arguments as normal parameters.</p>

<pre class="snippet cm-s-default" data-language="javascript" ><a class="c_ident" id="c_9lDr0S2nEw" href="#c_9lDr0S2nEw" tabindex="-1" role="presentation"></a><span class="cm-variable">speak</span>.<span class="cm-property">call</span>(<span class="cm-variable">hungryRabbit</span>, <span class="cm-string">&quot;Burp!&quot;</span>);

<span class="cm-comment">// → The hungry rabbit says 'Burp!'</span></pre>

<p><a class="p_ident" id="p_8ZeOAlGKXe" href="#p_8ZeOAlGKXe" tabindex="-1" role="presentation"></a>Since each function has its own <code>this</code> binding, whose value depends on the way it is called, you cannot refer to the <code>this</code> of the wrapping scope in a regular function defined with the <code>function</code> keyword.</p>

<p><a class="p_ident" id="p_C4AqzW0fAV" href="#p_C4AqzW0fAV" tabindex="-1" role="presentation"></a>Arrow functions are different—they do not bind their own <code>this</code> but can see the <code>this</code> binding of the scope around them. Thus, you can do something like the following code, which references <code>this</code> from inside a local function:</p>

<pre class="snippet cm-s-default" data-language="javascript" ><a class="c_ident" id="c_wg++tcauWw" href="#c_wg++tcauWw" tabindex="-1" role="presentation"></a><span class="cm-keyword">function</span> <span class="cm-def">normalize</span>() {

<span class="cm-variable">console</span>.<span class="cm-property">log</span>(<span class="cm-keyword">this</span>.<span class="cm-property">coords</span>.<span class="cm-property">map</span>(<span class="cm-def">n</span> <span class="cm-operator">=&gt;</span> <span class="cm-variable-2">n</span> <span class="cm-operator">/</span> <span class="cm-keyword">this</span>.<span class="cm-property">length</span>));

}

<span class="cm-variable">normalize</span>.<span class="cm-property">call</span>({<span class="cm-property">coords</span>: [<span class="cm-number">0</span>, <span class="cm-number">2</span>, <span class="cm-number">3</span>], <span class="cm-property">length</span>: <span class="cm-number">5</span>});

<span class="cm-comment">// → [0, 0.4, 0.6]</span></pre>

<p><a class="p_ident" id="p_YkWiHcMuVP" href="#p_YkWiHcMuVP" tabindex="-1" role="presentation"></a>If I had written the argument to <code>map</code> using the <code>function</code> keyword, the code wouldn’t work.</p>

<h2 id="prototypes"><a class="h_ident" id="h_SumMlRB7yn" href="#h_SumMlRB7yn" tabindex="-1" role="presentation"></a>Prototypes</h2>

<p><a class="p_ident" id="p_hi1TWnD/2p" href="#p_hi1TWnD/2p" tabindex="-1" role="presentation"></a>Watch closely.</p>

<pre class="snippet cm-s-default" data-language="javascript" ><a class="c_ident" id="c_P0GVdA5c8J" href="#c_P0GVdA5c8J" tabindex="-1" role="presentation"></a><span class="cm-keyword">let</span> <span class="cm-def">empty</span> <span class="cm-operator">=</span> {};

<span class="cm-variable">console</span>.<span class="cm-property">log</span>(<span class="cm-variable">empty</span>.<span class="cm-property">toString</span>);

<span class="cm-comment">// → function toString(){…}</span>

<span class="cm-variable">console</span>.<span class="cm-property">log</span>(<span class="cm-variable">empty</span>.<span class="cm-property">toString</span>());

<span class="cm-comment">// → [object Object]</span></pre>

<p><a class="p_ident" id="p_NUxqIXvg/G" href="#p_NUxqIXvg/G" tabindex="-1" role="presentation"></a>I pulled a property out of an empty object. Magic!</p>

<p><a class="p_ident" id="p_bJubL+gx0p" href="#p_bJubL+gx0p" tabindex="-1" role="presentation"></a>Well, not really. I have simply been withholding information about the way JavaScript objects work. In addition to their set of properties, most objects also have a <em>prototype</em>. A prototype is another object that is used as a fallback source of properties. When an object gets a request for a property that it does not have, its prototype will be searched for the property, then the prototype’s prototype, and so on.</p>

<p><a class="p_ident" id="p_XPuG8mFwbT" href="#p_XPuG8mFwbT" tabindex="-1" role="presentation"></a>So who is the prototype of that empty object? It is the great ancestral prototype, the entity behind almost all objects, <code>Object.prototype</code>.</p>

<pre class="snippet cm-s-default" data-language="javascript" ><a class="c_ident" id="c_7Q/HaNra3M" href="#c_7Q/HaNra3M" tabindex="-1" role="presentation"></a><span class="cm-variable">console</span>.<span class="cm-property">log</span>(<span class="cm-variable">Object</span>.<span class="cm-property">getPrototypeOf</span>({}) <span class="cm-operator">==</span>

<span class="cm-variable">Object</span>.<span class="cm-property">prototype</span>);

<span class="cm-comment">// → true</span>

<span class="cm-variable">console</span>.<span class="cm-property">log</span>(<span class="cm-variable">Object</span>.<span class="cm-property">getPrototypeOf</span>(<span class="cm-variable">Object</span>.<span class="cm-property">prototype</span>));

<span class="cm-comment">// → null</span></pre>

<p><a class="p_ident" id="p_GfVMu4b4D+" href="#p_GfVMu4b4D+" tabindex="-1" role="presentation"></a>As you guess, <code>Object.<wbr>getPrototypeOf</code> returns the prototype of an object.</p>

<p><a class="p_ident" id="p_4S0XbM8aBm" href="#p_4S0XbM8aBm" tabindex="-1" role="presentation"></a>The prototype relations of JavaScript objects form a tree-shaped structure, and at the root of this structure sits <code>Object.prototype</code>. It provides a few methods that show up in all objects, such as <code>toString</code>, which converts an object to a string representation.</p>

<p><a class="p_ident" id="p_j+MLWf3JXr" href="#p_j+MLWf3JXr" tabindex="-1" role="presentation"></a>Many objects don’t directly have <code>Object.prototype</code> as their prototype but instead have another object that provides a different set of default properties. Functions derive from <code>Function.<wbr>prototype</code>, and arrays derive from <code>Array.prototype</code>.</p>

<pre class="snippet cm-s-default" data-language="javascript" ><a class="c_ident" id="c_NgntUaXZ1S" href="#c_NgntUaXZ1S" tabindex="-1" role="presentation"></a><span class="cm-variable">console</span>.<span class="cm-property">log</span>(<span class="cm-variable">Object</span>.<span class="cm-property">getPrototypeOf</span>(<span class="cm-variable">Math</span>.<span class="cm-property">max</span>) <span class="cm-operator">==</span>

<span class="cm-variable">Function</span>.<span class="cm-property">prototype</span>);

<span class="cm-comment">// → true</span>

<span class="cm-variable">console</span>.<span class="cm-property">log</span>(<span class="cm-variable">Object</span>.<span class="cm-property">getPrototypeOf</span>([]) <span class="cm-operator">==</span>

<span class="cm-variable">Array</span>.<span class="cm-property">prototype</span>);

<span class="cm-comment">// → true</span></pre>

<p><a class="p_ident" id="p_1fIXMD62Nu" href="#p_1fIXMD62Nu" tabindex="-1" role="presentation"></a>Such a prototype object will itself have a prototype, often <code>Object.prototype</code>, so that it still indirectly provides methods like <code>toString</code>.</p>

<p><a class="p_ident" id="p_H2XhzSHHJP" href="#p_H2XhzSHHJP" tabindex="-1" role="presentation"></a>You can use <code>Object.create</code> to create an object with a specific prototype.</p>

<pre class="snippet cm-s-default" data-language="javascript" ><a class="c_ident" id="c_gSGrvGTpkW" href="#c_gSGrvGTpkW" tabindex="-1" role="presentation"></a><span class="cm-keyword">let</span> <span class="cm-def">protoRabbit</span> <span class="cm-operator">=</span> {

<span class="cm-property">speak</span>(<span class="cm-def">line</span>) {

<span class="cm-variable">console</span>.<span class="cm-property">log</span>(<span class="cm-string-2">`The ${</span><span class="cm-keyword">this</span>.<span class="cm-property">type</span><span class="cm-string-2">}</span> <span class="cm-string-2">rabbit says '${</span><span class="cm-variable-2">line</span><span class="cm-string-2">}</span><span class="cm-string-2">'`</span>);

}

};

<span class="cm-keyword">let</span> <span class="cm-def">killerRabbit</span> <span class="cm-operator">=</span> <span class="cm-variable">Object</span>.<span class="cm-property">create</span>(<span class="cm-variable">protoRabbit</span>);

<span class="cm-variable">killerRabbit</span>.<span class="cm-property">type</span> <span class="cm-operator">=</span> <span class="cm-string">&quot;killer&quot;</span>;

<span class="cm-variable">killerRabbit</span>.<span class="cm-property">speak</span>(<span class="cm-string">&quot;SKREEEE!&quot;</span>);

<span class="cm-comment">// → The killer rabbit says 'SKREEEE!'</span></pre>

<p><a class="p_ident" id="p_eBm7AuQUBb" href="#p_eBm7AuQUBb" tabindex="-1" role="presentation"></a>A property like <code>speak(line)</code> in an object expression is a shorthand way of defining a method. It creates a property called <code>speak</code> and gives it a function as its value.</p>

<p><a class="p_ident" id="p_y8kzFoQw1W" href="#p_y8kzFoQw1W" tabindex="-1" role="presentation"></a>The “proto” rabbit acts as a container for the properties that are shared by all rabbits. An individual rabbit object, like the killer rabbit, contains properties that apply only to itself—in this case its type—and derives shared properties from its prototype.</p>

<h2 id="classes"><a class="h_ident" id="h_7RhGr+474h" href="#h_7RhGr+474h" tabindex="-1" role="presentation"></a>Classes</h2>

<p><a class="p_ident" id="p_iYbDLfqSFW" href="#p_iYbDLfqSFW" tabindex="-1" role="presentation"></a>JavaScript’s prototype system can be interpreted as a somewhat informal take on an object-oriented concept called <em>classes</em>. A class defines the shape of a type of object—what methods and properties it has. Such an object is called an <em>instance</em> of the class.</p>

<p><a class="p_ident" id="p_GhnffIpINq" href="#p_GhnffIpINq" tabindex="-1" role="presentation"></a>Prototypes are useful for defining properties for which all instances of a class share the same value, such as methods. Properties that differ per instance, such as our rabbits’ <code>type</code> property, need to be stored directly in the objects themselves.</p>

<p id="constructors"><a class="p_ident" id="p_hflVCtYTKN" href="#p_hflVCtYTKN" tabindex="-1" role="presentation"></a>So to create an instance of a given class, you have to make an object that derives from the proper prototype, but you <em>also</em> have to make sure it, itself, has the properties that instances of this class are supposed to have. This is what a <em>constructor</em> function does.</p>

<pre class="snippet cm-s-default" data-language="javascript" ><a class="c_ident" id="c_oOKUeIzSVa" href="#c_oOKUeIzSVa" tabindex="-1" role="presentation"></a><span class="cm-keyword">function</span> <span class="cm-def">makeRabbit</span>(<span class="cm-def">type</span>) {

<span class="cm-keyword">let</span> <span class="cm-def">rabbit</span> <span class="cm-operator">=</span> <span class="cm-variable">Object</span>.<span class="cm-property">create</span>(<span class="cm-variable">protoRabbit</span>);

<span class="cm-variable-2">rabbit</span>.<span class="cm-property">type</span> <span class="cm-operator">=</span> <span class="cm-variable-2">type</span>;

<span class="cm-keyword">return</span> <span class="cm-variable-2">rabbit</span>;

}</pre>

<p><a class="p_ident" id="p_j8irpNziDb" href="#p_j8irpNziDb" tabindex="-1" role="presentation"></a>JavaScript provides a way to make defining this type of function easier. If you put the keyword <code>new</code> in front of a function call, the function is treated as a constructor. This means that an object with the right prototype is automatically created, bound to <code>this</code> in the function, and returned at the end of the function.</p>

<p><a class="p_ident" id="p_svyxVljkeg" href="#p_svyxVljkeg" tabindex="-1" role="presentation"></a>The prototype object used when constructing objects is found by taking the <code>prototype</code> property of the constructor function.</p>

<pre class="snippet cm-s-default" data-language="javascript" ><a class="c_ident" id="c_p+u3OtMv8K" href="#c_p+u3OtMv8K" tabindex="-1" role="presentation"></a><span class="cm-keyword">function</span> <span class="cm-def">Rabbit</span>(<span class="cm-def">type</span>) {

<span class="cm-keyword">this</span>.<span class="cm-property">type</span> <span class="cm-operator">=</span> <span class="cm-variable-2">type</span>;

}

<span class="cm-variable">Rabbit</span>.<span class="cm-property">prototype</span>.<span class="cm-property">speak</span> <span class="cm-operator">=</span> <span class="cm-keyword">function</span>(<span class="cm-def">line</span>) {

<span class="cm-variable">console</span>.<span class="cm-property">log</span>(<span class="cm-string-2">`The ${</span><span class="cm-keyword">this</span>.<span class="cm-property">type</span><span class="cm-string-2">}</span> <span class="cm-string-2">rabbit says '${</span><span class="cm-variable-2">line</span><span class="cm-string-2">}</span><span class="cm-string-2">'`</span>);

};

<span class="cm-keyword">let</span> <span class="cm-def">weirdRabbit</span> <span class="cm-operator">=</span> <span class="cm-keyword">new</span> <span class="cm-variable">Rabbit</span>(<span class="cm-string">&quot;weird&quot;</span>);</pre>

<p><a class="p_ident" id="p_IZA8w6crnz" href="#p_IZA8w6crnz" tabindex="-1" role="presentation"></a>Constructors (all functions, in fact) automatically get a property named <code>prototype</code>, which by default holds a plain, empty object that derives from <code>Object.prototype</code>. You can overwrite it with a new object if you want. Or you can add properties to the existing object, as the example does.</p>

<p><a class="p_ident" id="p_DJG7BO1oTU" href="#p_DJG7BO1oTU" tabindex="-1" role="presentation"></a>By convention, the names of constructors are capitalized so that they can easily be distinguished from other functions.</p>

<p><a class="p_ident" id="p_9bg1buEAma" href="#p_9bg1buEAma" tabindex="-1" role="presentation"></a>It is important to understand the distinction between the way a prototype is associated with a constructor (through its <code>prototype</code> property) and the way objects <em>have</em> a prototype (which can be found with <code>Object.<wbr>getPrototypeOf</code>). The actual prototype of a constructor is <code>Function.<wbr>prototype</code> since constructors are functions. Its <code>prototype</code> <em>property</em> holds the prototype used for instances created through it.</p>

<pre class="snippet cm-s-default" data-language="javascript" ><a class="c_ident" id="c_KDYP9dCWfS" href="#c_KDYP9dCWfS" tabindex="-1" role="presentation"></a><span class="cm-variable">console</span>.<span class="cm-property">log</span>(<span class="cm-variable">Object</span>.<span class="cm-property">getPrototypeOf</span>(<span class="cm-variable">Rabbit</span>) <span class="cm-operator">==</span>

<span class="cm-variable">Function</span>.<span class="cm-property">prototype</span>);

<span class="cm-comment">// → true</span>

<span class="cm-variable">console</span>.<span class="cm-property">log</span>(<span class="cm-variable">Object</span>.<span class="cm-property">getPrototypeOf</span>(<span class="cm-variable">weirdRabbit</span>) <span class="cm-operator">==</span>

<span class="cm-variable">Rabbit</span>.<span class="cm-property">prototype</span>);

<span class="cm-comment">// → true</span></pre>

<h2><a class="h_ident" id="h_hPv1gHC33s" href="#h_hPv1gHC33s" tabindex="-1" role="presentation"></a>Class notation</h2>

<p><a class="p_ident" id="p_T5Ghz5me/w" href="#p_T5Ghz5me/w" tabindex="-1" role="presentation"></a>So JavaScript classes are constructor functions with a prototype property. That is how they work, and until 2015, that was how you had to write them. These days, we have a less awkward notation.</p>

<pre class="snippet cm-s-default" data-language="javascript" ><a class="c_ident" id="c_kqKA+SZ6vS" href="#c_kqKA+SZ6vS" tabindex="-1" role="presentation"></a><span class="cm-keyword">class</span> <span class="cm-def">Rabbit</span> {

<span class="cm-property">constructor</span>(<span class="cm-def">type</span>) {

<span class="cm-keyword">this</span>.<span class="cm-property">type</span> <span class="cm-operator">=</span> <span class="cm-variable-2">type</span>;

}

<span class="cm-property">speak</span>(<span class="cm-def">line</span>) {

<span class="cm-variable">console</span>.<span class="cm-property">log</span>(<span class="cm-string-2">`The ${</span><span class="cm-keyword">this</span>.<span class="cm-property">type</span><span class="cm-string-2">}</span> <span class="cm-string-2">rabbit says '${</span><span class="cm-variable-2">line</span><span class="cm-string-2">}</span><span class="cm-string-2">'`</span>);

}

}

<span class="cm-keyword">let</span> <span class="cm-def">killerRabbit</span> <span class="cm-operator">=</span> <span class="cm-keyword">new</span> <span class="cm-variable">Rabbit</span>(<span class="cm-string">&quot;killer&quot;</span>);

<span class="cm-keyword">let</span> <span class="cm-def">blackRabbit</span> <span class="cm-operator">=</span> <span class="cm-keyword">new</span> <span class="cm-variable">Rabbit</span>(<span class="cm-string">&quot;black&quot;</span>);</pre>

<p><a class="p_ident" id="p_wpM2aH78Jp" href="#p_wpM2aH78Jp" tabindex="-1" role="presentation"></a>The <code>class</code> keyword starts a class declaration, which allows us to define a constructor and a set of methods all in a single place. Any number of methods may be written inside the declaration’s braces. The one named <code>constructor</code> is treated specially. It provides the actual constructor function, which will be bound to the name <code>Rabbit</code>. The others are packaged into that constructor’s prototype. Thus, the earlier class declaration is equivalent to the constructor definition from the previous section. It just looks nicer.</p>

<p><a class="p_ident" id="p_rJ0WbDXCuo" href="#p_rJ0WbDXCuo" tabindex="-1" role="presentation"></a>Class declarations currently allow only <em>methods</em>—properties that hold functions—to be added to the prototype. This can be somewhat inconvenient when you want to save a non-function value in there. The next version of the language will probably improve this. For now, you can create such properties by directly manipulating the prototype after you’ve defined the class.</p>

<p><a class="p_ident" id="p_pp17mMu8If" href="#p_pp17mMu8If" tabindex="-1" role="presentation"></a>Like <code>function</code>, <code>class</code> can be used both in statements and in expressions. When used as an expression, it doesn’t define a binding but just produces the constructor as a value. You are allowed to omit the class name in a class expression.</p>

<pre class="snippet cm-s-default" data-language="javascript" ><a class="c_ident" id="c_79re+GWcTJ" href="#c_79re+GWcTJ" tabindex="-1" role="presentation"></a><span class="cm-keyword">let</span> <span class="cm-def">object</span> <span class="cm-operator">=</span> <span class="cm-keyword">new</span> <span class="cm-keyword">class</span> { <span class="cm-property">getWord</span>() { <span class="cm-keyword">return</span> <span class="cm-string">&quot;hello&quot;</span>; } };

<span class="cm-variable">console</span>.<span class="cm-property">log</span>(<span class="cm-variable">object</span>.<span class="cm-property">getWord</span>());

<span class="cm-comment">// → hello</span></pre>

<h2><a class="h_ident" id="h_oUlUep3Os8" href="#h_oUlUep3Os8" tabindex="-1" role="presentation"></a>Overriding derived properties</h2>

<p><a class="p_ident" id="p_Xbxf2Ooo0z" href="#p_Xbxf2Ooo0z" tabindex="-1" role="presentation"></a>When you add a property to an object, whether it is present in the prototype or not, the property is added to the object <em>itself</em>. If there was already a property with the same name in the prototype, this property will no longer affect the object, as it is now hidden behind the object’s own property.</p>

<pre class="snippet cm-s-default" data-language="javascript" ><a class="c_ident" id="c_vja6JbO0si" href="#c_vja6JbO0si" tabindex="-1" role="presentation"></a><span class="cm-variable">Rabbit</span>.<span class="cm-property">prototype</span>.<span class="cm-property">teeth</span> <span class="cm-operator">=</span> <span class="cm-string">&quot;small&quot;</span>;

<span class="cm-variable">console</span>.<span class="cm-property">log</span>(<span class="cm-variable">killerRabbit</span>.<span class="cm-property">teeth</span>);

<span class="cm-comment">// → small</span>

<span class="cm-variable">killerRabbit</span>.<span class="cm-property">teeth</span> <span class="cm-operator">=</span> <span class="cm-string">&quot;long, sharp, and bloody&quot;</span>;

<span class="cm-variable">console</span>.<span class="cm-property">log</span>(<span class="cm-variable">killerRabbit</span>.<span class="cm-property">teeth</span>);

<span class="cm-comment">// → long, sharp, and bloody</span>

<span class="cm-variable">console</span>.<span class="cm-property">log</span>(<span class="cm-variable">blackRabbit</span>.<span class="cm-property">teeth</span>);

<span class="cm-comment">// → small</span>

<span class="cm-variable">console</span>.<span class="cm-property">log</span>(<span class="cm-variable">Rabbit</span>.<span class="cm-property">prototype</span>.<span class="cm-property">teeth</span>);

<span class="cm-comment">// → small</span></pre>

<p><a class="p_ident" id="p_HM5YtS3KgJ" href="#p_HM5YtS3KgJ" tabindex="-1" role="presentation"></a>The following diagram sketches the situation after this code has run. The <code>Rabbit</code> and <code>Object</code> prototypes lie behind <code>killerRabbit</code> as a kind of backdrop, where properties that are not found in the object itself can be looked up.</p><figure><img src="img/rabbits.svg" alt="Rabbit object prototype schema"></figure>

<p><a class="p_ident" id="p_or3/lz1DV8" href="#p_or3/lz1DV8" tabindex="-1" role="presentation"></a>Overriding properties that exist in a prototype can be a useful thing to do. As the rabbit teeth example shows, overriding can be used to express exceptional properties in instances of a more generic class of objects, while letting the nonexceptional objects take a standard value from their prototype.</p>

<p><a class="p_ident" id="p_GIhnbh3MdO" href="#p_GIhnbh3MdO" tabindex="-1" role="presentation"></a>Overriding is also used to give the standard function and array prototypes a different <code>toString</code> method than the basic object prototype.</p>

<pre class="snippet cm-s-default" data-language="javascript" ><a class="c_ident" id="c_DrIRvUgeOD" href="#c_DrIRvUgeOD" tabindex="-1" role="presentation"></a><span class="cm-variable">console</span>.<span class="cm-property">log</span>(<span class="cm-variable">Array</span>.<span class="cm-property">prototype</span>.<span class="cm-property">toString</span> <span class="cm-operator">==</span>

<span class="cm-variable">Object</span>.<span class="cm-property">prototype</span>.<span class="cm-property">toString</span>);

<span class="cm-comment">// → false</span>

<span class="cm-variable">console</span>.<span class="cm-property">log</span>([<span class="cm-number">1</span>, <span class="cm-number">2</span>].<span class="cm-property">toString</span>());

<span class="cm-comment">// → 1,2</span></pre>

<p><a class="p_ident" id="p_FYf2A2jS65" href="#p_FYf2A2jS65" tabindex="-1" role="presentation"></a>Calling <code>toString</code> on an array gives a result similar to calling <code>.<wbr>join(&quot;,&quot;)</code> on it—it puts commas between the values in the array. Directly calling <code>Object.<wbr>prototype.<wbr>toString</code> with an array produces a different string. That function doesn’t know about arrays, so it simply puts the word <em>object</em> and the name of the type between square brackets.</p>

<pre class="snippet cm-s-default" data-language="javascript" ><a class="c_ident" id="c_XpqFUrDFJE" href="#c_XpqFUrDFJE" tabindex="-1" role="presentation"></a><span class="cm-variable">console</span>.<span class="cm-property">log</span>(<span class="cm-variable">Object</span>.<span class="cm-property">prototype</span>.<span class="cm-property">toString</span>.<span class="cm-property">call</span>([<span class="cm-number">1</span>, <span class="cm-number">2</span>]));

<span class="cm-comment">// → [object Array]</span></pre>

<h2><a class="h_ident" id="h_gAcc11EHzV" href="#h_gAcc11EHzV" tabindex="-1" role="presentation"></a>Maps</h2>

<p><a class="p_ident" id="p_5v6QzULS7C" href="#p_5v6QzULS7C" tabindex="-1" role="presentation"></a>We saw the word <em>map</em> used in the <a href="05_higher_order.html#map">previous chapter</a> for an operation that transforms a data structure by applying a function to its elements. Confusing as it is, in programming the same word is also used for a related but rather different thing.</p>

<p><a class="p_ident" id="p_++bw9zDmtH" href="#p_++bw9zDmtH" tabindex="-1" role="presentation"></a>A <em>map</em> (noun) is a data structure that associates values (the keys) with other values. For example, you might want to map names to ages. It is possible to use objects for this.</p>

<pre class="snippet cm-s-default" data-language="javascript" ><a class="c_ident" id="c_Wu6a8ObZI0" href="#c_Wu6a8ObZI0" tabindex="-1" role="presentation"></a><span class="cm-keyword">let</span> <span class="cm-def">ages</span> <span class="cm-operator">=</span> {

<span class="cm-property">Boris</span>: <span class="cm-number">39</span>,

<span class="cm-property">Liang</span>: <span class="cm-number">22</span>,

<span class="cm-property">Júlia</span>: <span class="cm-number">62</span>

};

<span class="cm-variable">console</span>.<span class="cm-property">log</span>(<span class="cm-string-2">`Júlia is ${</span><span class="cm-variable">ages</span>[<span class="cm-string">&quot;Júlia&quot;</span>]<span class="cm-string-2">}</span><span class="cm-string-2">`</span>);

<span class="cm-comment">// → Júlia is 62</span>

<span class="cm-variable">console</span>.<span class="cm-property">log</span>(<span class="cm-string">&quot;Is Jack's age known?&quot;</span>, <span class="cm-string">&quot;Jack&quot;</span> <span class="cm-keyword">in</span> <span class="cm-variable">ages</span>);

<span class="cm-comment">// → Is Jack's age known? false</span>

<span class="cm-variable">console</span>.<span class="cm-property">log</span>(<span class="cm-string">&quot;Is toString's age known?&quot;</span>, <span class="cm-string">&quot;toString&quot;</span> <span class="cm-keyword">in</span> <span class="cm-variable">ages</span>);

<span class="cm-comment">// → Is toString's age known? true</span></pre>

<p><a class="p_ident" id="p_EQqc7pcOKT" href="#p_EQqc7pcOKT" tabindex="-1" role="presentation"></a>Here, the object’s property names are the people’s names, and the property values are their ages. But we certainly didn’t list anybody named toString in our map. Yet, because plain objects derive from <code>Object.prototype</code>, it looks like the property is there.</p>

<p><a class="p_ident" id="p_enf1/9ItBM" href="#p_enf1/9ItBM" tabindex="-1" role="presentation"></a>As such, using plain objects as maps is dangerous. There are several possible ways to avoid this problem. First, it is possible to create objects with <em>no</em> prototype. If you pass <code>null</code> to <code>Object.create</code>, the resulting object will not derive from <code>Object.prototype</code> and can safely be used as a map.</p>

<pre class="snippet cm-s-default" data-language="javascript" ><a class="c_ident" id="c_AkRQLQc4AG" href="#c_AkRQLQc4AG" tabindex="-1" role="presentation"></a><span class="cm-variable">console</span>.<span class="cm-property">log</span>(<span class="cm-string">&quot;toString&quot;</span> <span class="cm-keyword">in</span> <span class="cm-variable">Object</span>.<span class="cm-property">create</span>(<span class="cm-atom">null</span>));

<span class="cm-comment">// → false</span></pre>

<p><a class="p_ident" id="p_jGC2iO9E1x" href="#p_jGC2iO9E1x" tabindex="-1" role="presentation"></a>Object property names must be strings. If you need a map whose keys can’t easily be converted to strings—such as objects—you cannot use an object as your map.</p>

<p><a class="p_ident" id="p_nIsq9E5wmZ" href="#p_nIsq9E5wmZ" tabindex="-1" role="presentation"></a>Fortunately, JavaScript comes with a class called <code>Map</code> that is written for this exact purpose. It stores a mapping and allows any type of keys.</p>

<pre class="snippet cm-s-default" data-language="javascript" ><a class="c_ident" id="c_dd6KsGgAGP" href="#c_dd6KsGgAGP" tabindex="-1" role="presentation"></a><span class="cm-keyword">let</span> <span class="cm-def">ages</span> <span class="cm-operator">=</span> <span class="cm-keyword">new</span> <span class="cm-variable">Map</span>();

<span class="cm-variable">ages</span>.<span class="cm-property">set</span>(<span class="cm-string">&quot;Boris&quot;</span>, <span class="cm-number">39</span>);

<span class="cm-variable">ages</span>.<span class="cm-property">set</span>(<span class="cm-string">&quot;Liang&quot;</span>, <span class="cm-number">22</span>);

<span class="cm-variable">ages</span>.<span class="cm-property">set</span>(<span class="cm-string">&quot;Júlia&quot;</span>, <span class="cm-number">62</span>);

<span class="cm-variable">console</span>.<span class="cm-property">log</span>(<span class="cm-string-2">`Júlia is ${</span><span class="cm-variable">ages</span>.<span class="cm-property">get</span>(<span class="cm-string">&quot;Júlia&quot;</span>)<span class="cm-string-2">}</span><span class="cm-string-2">`</span>);

<span class="cm-comment">// → Júlia is 62</span>

<span class="cm-variable">console</span>.<span class="cm-property">log</span>(<span class="cm-string">&quot;Is Jack's age known?&quot;</span>, <span class="cm-variable">ages</span>.<span class="cm-property">has</span>(<span class="cm-string">&quot;Jack&quot;</span>));

<span class="cm-comment">// → Is Jack's age known? false</span>

<span class="cm-variable">console</span>.<span class="cm-property">log</span>(<span class="cm-variable">ages</span>.<span class="cm-property">has</span>(<span class="cm-string">&quot;toString&quot;</span>));

<span class="cm-comment">// → false</span></pre>

<p><a class="p_ident" id="p_iIdp+mGl3Y" href="#p_iIdp+mGl3Y" tabindex="-1" role="presentation"></a>The methods <code>set</code>, <code>get</code>, and <code>has</code> are part of the interface of the <code>Map</code> object. Writing a data structure that can quickly update and search a large set of values isn’t easy, but we don’t have to worry about that. Someone else did it for us, and we can go through this simple interface to use their work.</p>

<p><a class="p_ident" id="p_tx3xnowcEp" href="#p_tx3xnowcEp" tabindex="-1" role="presentation"></a>If you do have a plain object that you need to treat as a map for some reason, it is useful to know that <code>Object.keys</code> returns only an object’s <em>own</em> keys, not those in the prototype. As an alternative to the <code>in</code> operator, you can use the <code>hasOwnProperty</code> method, which ignores the object’s prototype.</p>

<pre class="snippet cm-s-default" data-language="javascript" ><a class="c_ident" id="c_qBrd35Qiln" href="#c_qBrd35Qiln" tabindex="-1" role="presentation"></a><span class="cm-variable">console</span>.<span class="cm-property">log</span>({<span class="cm-property">x</span>: <span class="cm-number">1</span>}.<span class="cm-property">hasOwnProperty</span>(<span class="cm-string">&quot;x&quot;</span>));

<span class="cm-comment">// → true</span>

<span class="cm-variable">console</span>.<span class="cm-property">log</span>({<span class="cm-property">x</span>: <span class="cm-number">1</span>}.<span class="cm-property">hasOwnProperty</span>(<span class="cm-string">&quot;toString&quot;</span>));

<span class="cm-comment">// → false</span></pre>

<h2><a class="h_ident" id="h_mJ/JHQRHg9" href="#h_mJ/JHQRHg9" tabindex="-1" role="presentation"></a>Polymorphism</h2>

<p><a class="p_ident" id="p_ozkqUookhO" href="#p_ozkqUookhO" tabindex="-1" role="presentation"></a>When you call the <code>String</code> function (which converts a value to a string) on an object, it will call the <code>toString</code> method on that object to try to create a meaningful string from it. I mentioned that some of the standard prototypes define their own version of <code>toString</code> so they can create a string that contains more useful information than <code>&quot;[object Object]&quot;</code>. You can also do that yourself.</p>

<pre class="snippet cm-s-default" data-language="javascript" ><a class="c_ident" id="c_V+CBn0VJnW" href="#c_V+CBn0VJnW" tabindex="-1" role="presentation"></a><span class="cm-variable">Rabbit</span>.<span class="cm-property">prototype</span>.<span class="cm-property">toString</span> <span class="cm-operator">=</span> <span class="cm-keyword">function</span>() {

<span class="cm-keyword">return</span> <span class="cm-string-2">`a ${</span><span class="cm-keyword">this</span>.<span class="cm-property">type</span><span class="cm-string-2">}</span> <span class="cm-string-2">rabbit`</span>;

};

<span class="cm-variable">console</span>.<span class="cm-property">log</span>(<span class="cm-variable">String</span>(<span class="cm-variable">blackRabbit</span>));

<span class="cm-comment">// → a black rabbit</span></pre>

<p><a class="p_ident" id="p_HgUFdFB6vM" href="#p_HgUFdFB6vM" tabindex="-1" role="presentation"></a>This is a simple instance of a powerful idea. When a piece of code is written to work with objects that have a certain interface—in this case, a <code>toString</code> method—any kind of object that happens to support this interface can be plugged into the code, and it will just work.</p>

<p><a class="p_ident" id="p_qNZih5k+vu" href="#p_qNZih5k+vu" tabindex="-1" role="presentation"></a>This technique is called <em>polymorphism</em>. Polymorphic code can work with values of different shapes, as long as they support the interface it expects.</p>

<p><a class="p_ident" id="p_g9W26O8p4+" href="#p_g9W26O8p4+" tabindex="-1" role="presentation"></a>I mentioned in <a href="04_data.html#for_of_loop">Chapter 4</a> that a <code>for</code>/<code>of</code> loop can loop over several kinds of data structures. This is another case of polymorphism—such loops expect the data structure to expose a specific interface, which arrays and strings do. And we can also add this interface to your own objects! But before we can do that, we need to know what symbols are.</p>

<h2><a class="h_ident" id="h_Iq1mTp65i3" href="#h_Iq1mTp65i3" tabindex="-1" role="presentation"></a>Symbols</h2>