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implementation.js

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'use strict';

/* eslint no-invalid-this: 1 */

var ERROR_MESSAGE = 'Function.prototype.bind called on incompatible ';

var toStr = Object.prototype.toString;

var max = Math.max;

var funcType = '[object Function]';

var concatty = function concatty(a, b) {

var arr = [];

for (var i = 0; i < a.length; i += 1) {

arr[i] = a[i];

}

for (var j = 0; j < b.length; j += 1) {

arr[j + a.length] = b[j];

}

return arr;

};

var slicy = function slicy(arrLike, offset) {

var arr = [];

for (var i = offset || 0, j = 0; i < arrLike.length; i += 1, j += 1) {

arr[j] = arrLike[i];

}

return arr;

};

var joiny = function (arr, joiner) {

var str = '';

for (var i = 0; i < arr.length; i += 1) {

str += arr[i];

if (i + 1 < arr.length) {

str += joiner;

}

}

return str;

};

module.exports = function bind(that) {

var target = this;

if (typeof target !== 'function' || toStr.apply(target) !== funcType) {

throw new TypeError(ERROR_MESSAGE + target);

}

var args = slicy(arguments, 1);

var bound;

var binder = function () {

if (this instanceof bound) {

var result = target.apply(

this,

concatty(args, arguments)

);

if (Object(result) === result) {

return result;

}

return this;

}

return target.apply(

that,

concatty(args, arguments)

);

};

var boundLength = max(0, target.length - args.length);

var boundArgs = [];

for (var i = 0; i < boundLength; i++) {

boundArgs[i] = '$' + i;

}

bound = Function('binder', 'return function (' + joiny(boundArgs, ',') + '){ return binder.apply(this,arguments); }')(binder);

if (target.prototype) {

var Empty = function Empty() {};

Empty.prototype = target.prototype;

bound.prototype = new Empty();

Empty.prototype = null;

}

return bound;

};