This chapter briefly recaps the features of JavaScript that we’ve learned by now, paying special attention to subtle moments.

Statements are delimited with a semicolon:

alert('Hello'); alert('World');

Usually, a line-break is also treated as a delimiter, so that would also work:

alert('Hello')
alert('World')

That’s called “automatic semicolon insertion”. Sometimes it doesn’t work, for instance:

alert("There will be an error after this message")

[1, 2].forEach(alert)

Most codestyle guides agree that we should put a semicolon after each statement.

Semicolons are not required after code blocks {...} and syntax constructs with them like loops:

function f() {
  // no semicolon needed after function declaration
}

for(;;) {
  // no semicolon needed after the loop
}

... But even if we can put an “extra” semicolon somewhere, that’s not an error. It will be ignored.

More in: Code structure.

To fully enable all features of modern JavaScript, we should start scripts with "use strict".

'use strict';

...

The directive must be at the top of a script or at the beginning of a function.

Without "use strict", everything still works, but some features behave in the old-fashion, “compatible” way. We’d generally prefer the modern behavior.

Some modern features of the language (like classes that we’ll study in the future) enable strict mode implicitly.

More in: The modern mode, "use strict".

Can be declared using:

• let

• const (constant, can’t be changed)

• var (old-style, will see later)

A variable name can include:

• Letters and digits, but the first character may not be a digit.

• Characters $ and _ are normal, on par with letters.

• Non-Latin alphabets and hieroglyphs are also allowed, but commonly not used.

Variables are dynamically typed. They can store any value:

let x = 5;
x = "John";

There are 7 data types:

• number for both floating-point and integer numbers,

• string for strings,

• boolean for logical values: true/false,

• null – a type with a single value null, meaning “empty” or “does not exist”,

• undefined – a type with a single value undefined, meaning “not assigned”,

• object and symbol – for complex data structures and unique identifiers, we haven’t learnt them yet.

The typeof operator returns the type for a value, with two exceptions:

typeof null == "object" // error in the language
typeof function(){} == "function" // functions are treated specially

More in: Variables and Data types.

We’re using a browser as a working environment, so basic UI functions will be:

prompt(question[, default])

Ask a question, and return either what the visitor entered or null if he pressed “cancel”.

confirm(question)

Ask a question and suggest to choose between Ok and Cancel. The choice is returned as true/false.

alert(message)

Output a message.

All these functions are modal, they pause the code execution and prevent the visitor from interacting with the page until he answers.

For instance:

let userName = prompt("Your name?", "Alice");
let isTeaWanted = confirm("Do you want some tea?");

alert( "Visitor: " + userName ); // Alice
alert( "Tea wanted: " + isTeaWanted ); // true

More in: Interaction: alert, prompt, confirm.

JavaScript supports the following operators:

Arithmetical

Regular: * + - /, also % for the remainder and ** for power of a number.

Binary plus + concatenates strings. And if any of the operands is a string, the other one is converted to string too:

alert( '1' + 2 ); // '12', string
alert( 1 + '2' ); // '12', string

Assignments

There is a simple assignment: a = b and combined ones like a *= 2.

Bitwise

Bitwise operators work with integers on bit-level: see the docs when they are needed.

Ternary

The only operator with three parameters: cond ? resultA : resultB. If cond is truthy, returns resultA, otherwise resultB.

Logical operators

Logical AND && and OR || perform short-circuit evaluation and then return the value where it stopped.

Comparisons

Equality check == for values of different types converts them to a number (except null and undefined that equal each other and nothing else), so these are equal:

alert( 0 == false ); // true
alert( 0 == '' ); // true

Other comparisons convert to a number as well.

The strict equality operator === doesn’t do the conversion: different types always mean different values for it, so:

Values null and undefined are special: they equal == each other and don’t equal anything else.

Greater/less comparisons compare strings character-by-character, other types are converted to a number.

Logical operators

There are few others, like a comma operator.

More in: Operators, Comparisons, Logical operators.

• We covered 3 types of loops:

  // 1
  while (condition) {
    ...
  }
  
  // 2
  do {
    ...
  } while (condition);
  
  // 3
  for(let i = 0; i < 10; i++) {
    ...
  }
  

• The variable declared in for(let...) loop is visible only inside the loop. But we can also omit let and reuse an existing variable.

• Directives break/continue allow to exit the whole loop/current iteration. Use labels to break nested loops.

Details in: Loops: while and for.

Later we’ll study more types of loops to deal with objects.

The “switch” construct can replace multiple if checks. It uses === for comparisons.

For instance:

let age = prompt('Your age?', 18);

switch (age) {
  case 18:
    alert("Won't work"); // the result of prompt is a string, not a number

  case "18":
    alert("This works!");
    break;

  default:
    alert("Any value not equal to one above");
}

Details in: The "switch" statement.

We covered three ways to create a function in JavaScript:

1. Function Declaration: the function in the main code flow

   function sum(a, b) {
     let result = a + b;
   
     return result;
   }
   

2. Function Expression: the function in the context of an expression

   let sum = function(a, b) {
     let result = a + b;
   
     return result;
   }
   

   Function expression can have a name, like sum = function name(a, b), but that name is only visible inside that function.

3. Arrow functions:

   // expression at the right side
   let sum = (a, b) => a + b;
   
   // or multi-line syntax with { ... }, need return here:
   let sum = (a, b) => {
     // ...
     return a + b;
   }
   
   // without arguments
   let sayHi = () => alert("Hello");
   
   // with a single argument
   let double = n => n * 2;
   

• Functions may have local variables: those declared inside its body. Such variables are only visible inside the function.

• Parameters can have default values: function sum(a = 1, b = 2) {...}.

• Functions always return something. If there’s no return statement, then the result is undefined.

More: see Functions, Function expressions and arrows.

That was a brief list of JavaScript features. As of now we’ve studied only basics. Further in the tutorial you’ll find more specials and advanced features of JavaScript.