摘要：關于在絕大多數情況下，函數的調用方式決定了的值。不能在執行期間被賦值，并且在每次函數被調用時的值也可能會不同。它們除了參數略有不同，其功能完全一樣。它們的第一個參數都為將要指向的對象。

在絕大多數情況下，函數的調用方式決定了this的值。this不能在執行期間被賦值，并且在每次函數被調用時this的值也可能會不同。

window.something = "I love JavaScript"
console.log(this.something) // "I love JavaScript"
console.log(window === this) // true

var a = 1
function test() { console.log(this.a) }
test() // 1 - still remains the window reference

this.a = "I am in the global scope"
function Test() {
  this.a = "I am in the test scope"
  this.show = function() { console.log(this.a) }
}
Test.prototype.display = function () { console.log(this.a) }
var test = new Test() // updated the scope of this
test.show() // I am in the test scope
test.display() // I am in the test scope 

JavaScript 內部提供了一種機制，讓我們可以自行手動設置 this 的指向。它們就是 call 與 apply。所有的函數都具有著兩個方法。它們除了參數略有不同，其功能完全一樣。它們的第一個參數都為 this 將要指向的對象。

function Laptop(name, storage) {
  this.name = name
  this.storage = storage
}

function Dell(name, storage, company) {
  Laptop.call(this, "Dell", 1024)
  this.company = company
}

console.log(new Dell("Dell", 1024, "Dell Inc").storage)

var obj = {
  entry: "mammals-banana-tower",
  duration: 0
}

function breed(name) {
  console.log("Show this breeding info", name, this.entry, this.duration)
  console.log(this === obj)
}

breed() // this => window
breed.call(obj, "Frank") // this => obj

注：當沒有傳遞任何參數作為 call() 的第一個參數時，在非嚴格模式下，this 會指向 window。

var _call = function (that) {
  that = that ? Object(that) : window
  that.func = this

  function formatArgs(oArgs, sign) {
    var _args
    for (var i = 1, len = oArgs.length; i < len; i++) {
      _args.push(sign ? ("_param_" + i) : oArgs[i])
    }
    return _args
  }

  var args = formatArgs(arguments)
  var newFunc = (new Function("args", "return that.func(" + formatArgs(args, true).toString() + ")"))(args)

  that.func = null
  return newFunc
}

用過 React 的同學都知道，當使用 class component 時，需要在 constructor 綁定當前的成員函數，或者針對事件委托的情況下，也需要進行綁定；ES6 箭頭函數可以讓我們更專注于具體的實現邏輯，簡化了 this 操作

// ES5
// 
// constructor() { this.handleClick = this.handleClick.bind(this) }

// ES6
//  handleClick()}>
// handleClick = () => {}

var f = function() { console.log(this.text) }
f = f.bind({ text: "I was bound" }).bind({ text: "I won"t be bound" })
f() // I was bound

很容易發現，f.bind() 返回的綁定函數對象僅在創建是保留當前的上下文（或者傳入的參數），因此無法在第二次進行重綁定。

var _bind = function (that) {

  var fBound,
    target = this,
    slice = Array.prototype.slice,
    toStr = Object.prototype.toString,
    args = slice.call(arguments, 1); // except that

  if (typeof target !== "function" || toStr.call(target) !== "[object Function]") {
    throw new TypeError("Function.prototype.bind - what is trying to be bound is not callable");
  }

  var binder = function () {
    var oArgs = args.concat(slice.call(arguments))
    if (this instanceof fBound) {
      var result = target.apply(this, oArgs);
      return Object(result) === result ? result : this;
    } else {
      return target.apply(that, oArgs);
    }
  };

  var i = 0,
    params = [],
    paramLength = Math.max(0, target.length - args.length);

  for (; i < paramLength; i++) {
    params.push("_param_" + i);
  }

  fBound = (new Function(
    "binder",
    "return function(" + params.join(",") + ") { return binder.apply(this,arguments); }"
  ))(binder);

  // maintain the reference of prototype
  if (target.prototype) {
    var fNOP = function () { };
    fNOP.prototype = target.prototype;
    fBound.prototype = new fNOP();
    fNOP.prototype = null;
  }

  return fBound;
};

https://developer.mozilla.org...
https://developer.mozilla.org...
https://developer.mozilla.org...
https://developer.mozilla.org...
https://developer.mozilla.org...
https://developer.mozilla.org...
https://www.ecma-internationa...
https://javascript.info/bind
https://juejin.im/post/5c0605...
https://github.com/mqyqingfen...