Implement bind analysis step by step using native javascript

bind

Official description

bind() function creates a new function (called a bound function), which has the same function body as the called function (the target function of the bound function) (in the ECMAScript 5 specification built-in call attribute). When the target function is called the this value is bound to the first parameter of bind() and cannot be overridden. When the bound function is called, bind() also accepts preset parameters to provide to the original function. A bound function can also create objects using the new operator: this behaves like the original function as a constructor. The supplied this value is ignored, and the arguments to the call are supplied to the mock function.

Introduction to use

Since the scope in JavaScript is determined by the environment in which it is run, often the function definition is different from the environment in which it is actually run, and the scope will also change accordingly.

For example, the following situation:

var id = 'window'; 
//定义一个函数，但是不立即执行 
var test = function(){ 
    console.log(this.id) 
} 
test() // window 
//把test作为参数传递 
var obj = { 
    id:'obj', 
    hehe:test 
} 
//此时test函数运行环境发生了改变 
obj.hehe() // 'obj' 
//为了避免这种情况，javascript里面有一个bind方法可以在函数运行之前就绑定其作用域，修改如下 
 
var id = 'window'; 
var test = function(){ 
    console.log(this.id) 
}.bind(window) 
var obj = { 
    id:'obj', 
    hehe:test 
} 
test() // window 
obj.hehe() // window

As introduced above, an important role of the bind method is to bind the scope to a function, but the bind method is not compatible with lower version browsers. Here we can implement it manually.

Split the key ideas

Because the bind method will not execute the function immediately, it needs to return a function to be executed (closure is used here, which can return a function) return function(){}

Scope binding, Here you can use the apply or call method to implement xx.call(yy)/xx.apply(yy)

parameter transfer. Due to the uncertainty of the parameters, you need to use apply to transfer the array (the example is more clear) xx.apply(yy, [...Array...]), it is inconvenient to use call, because the parameters behind the call need to be listed one by one

implementation

With the above ideas, the rough prototype is already clear, the code should It is also easy to implement

binding scope and binding parameters

Function.prototype.testBind = function(that){ 
    var _this = this, 
        /* 
        *由于参数的不确定性，统一用arguments来处理，这里的arguments只是一个类数组对象，有length属性 
        *可以用数组的slice方法转化成标准格式数组，除了作用域对象that以外， 
        *后面的所有参数都需要作为数组参数传递 
        *Array.prototype.slice.apply(arguments,[1])/Array.prototype.slice.call(arguments,1) 
        */ 
        slice = Array.prototype.slice, 
        args = slice.apply(arguments,[1]); 
    //返回函数     
    return function(){ 
        //apply绑定作用域，进行参数传递 
        return _this.apply(that,args) 
    }     
}

test

var test = function(a,b){ 
    console.log('作用域绑定 '+ this.value) 
    console.log('testBind参数传递 '+ a.value2) 
    console.log('调用参数传递 ' + b) 
} 
var obj = { 
    value:'ok' 
} 
var fun_new = test.testBind(obj,{value2:'also ok'}) 
 
fun_new ('hello bind') 
// 作用域绑定 ok 
// testBind参数传递 also ok 
// 调用参数传递  undefined

dynamic parameters

The scope binding of the bind method has been implemented above, but the fly in the ointment is that since we return a function , it should support passing parameters when calling. Obviously, the above fun_new does not support passing parameters when calling. It can only pass parameters when testBind is bound, because what we end up calling is this return function

function(){ 
        return _this.apply(that,args) 
    }     
 
这里面的args在绑定的时候就已经确定了，调用的时候值已经固定， 
我们并没有处理这个function传递的参数。

We will Transformation

return function(){ 
        return _this.apply(that, 
            args.concat(Array.prototype.slice.apply(arguments,[0])) 
        ) 
    }

Array.prototype.slice.apply(arguments,[0]) here refers to a series of parameters passed when the return function is executed, so it starts from the first parameter [0], the previous one args = slice.apply(arguments,[1]) refers to the parameters passed when the testBind method is executed, so starting from the second one [1], the two are essentially different and cannot be confused. Only after merging the two It is the complete parameter of the return function

So there is the following implementation

Function.prototype.testBind = function(that){ 
    var _this = this, 
        slice = Array.prototype.slice, 
        args = slice.apply(arguments,[1]); 
    return function(){ 
        return _this.apply(that, 
                    args.concat(Array.prototype.slice.apply(arguments,[0])) 
                ) 
    }     
}

Test

var test = function(a,b){ 
    console.log('作用域绑定 '+ this.value) 
    console.log('testBind参数传递 '+ a.value2) 
    console.log('调用参数传递 ' + b) 
} 
var obj = { 
    value:'ok' 
} 
var fun_new = test.testBind(obj,{value2:'also ok'}) 
 
fun_new ('hello bind') 
// 作用域绑定 ok 
// testBind参数传递 also ok 
// 调用参数传递  hello bind

Among the above two parameter passing methods, bind has the highest priority, starting from args.concat(Array.prototype.slice.apply(arguments,[ 0])) It can also be seen that the parameters of bind are in front of the array.

Prototype chain

There is a sentence in the official document:

A bound function may also be constructed using the new operator: doing

so acts as though the target function had instead been constructed.

The provided this value is ignored , while prepended arguments are

provided to the emulated function.

Indicates that after the bound function is instantiated by new, it needs to inherit the prototype chain method of the original function, and this provided during the binding process is ignored (inherits the original function this object), but the parameters will still be used.

Here we need a transfer function to pass the prototype chain

fNOP = function () {} //创建一个中转函数 
fNOP.prototype = this.prototype; 
xx.prototype = new fNOP()  
修改如下 
Function.prototype.testBind = function(that){ 
    var _this = this, 
        slice = Array.prototype.slice, 
        args = slice.apply(arguments,[1]), 
        fNOP = function () {}, 
        //所以调用官方bind方法之后 有一个name属性值为 'bound ' 
        bound = function(){ 
            return _this.apply(that, 
                args.concat(Array.prototype.slice.apply(arguments,[0])) 
            ) 
        }     
 
    fNOP.prototype = _this.prototype; 
 
    bound.prototype = new fNOP(); 
 
    return bound; 
}

And the first parameter this of the bind method does not need to be passed. It needs to be divided into two situations

Directly call the method after bind

var f = function () { console.log('不传默认为'+this)  };f.bind()() 
// 不传默认为 Window

Javascript native step Step-by-step implementation of bind analysis

bind() function will create a new function (called a binding function). The new function has the same function body (built-in in the ECMAScript 5 specification) as the called function (the target function of the binding function) call attribute). When the target function is called the this value is bound to the first parameter of bind() and cannot be overridden. When the bound function is called, bind() also accepts preset parameters to provide to the original function.源 Author: Who is the source: Segmentfault | 2016-11-02 18: 54

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Official description

Bind () function will create a new function (referred to as called bound function), the new function has the same function body (the built-in call attribute in the ECMAScript 5 specification) as the callee function (the target function of the bound function). When the target function is called the this value is bound to the first parameter of bind() and cannot be overridden. When the bound function is called, bind() also accepts preset parameters to provide to the original function. A bound function can also create objects using the new operator: this behaves like the original function as a constructor. The supplied this value is ignored, and the arguments to the call are supplied to the mock function.

Introduction to use

Since the scope in JavaScript is determined by the environment in which it is run, often the function definition is different from the environment in which it is actually run, and the scope will also change accordingly.

🎜For example, the following situation: 🎜

var id = 'window'; 
//定义一个函数，但是不立即执行 
var test = function(){     
console.log(this.id) } 
test() 
// window 
//把test作为参数传递 
var obj = {     
id:'obj',     
hehe:test } 
//此时test函数运行环境发生了改变 obj.hehe() 
// 'obj' 
//为了避免这种情况，javascript里面有一个bind方法可以在函数运行之前就绑定其作用域，修改如下  
var id = 'window'; 
var test = function(){     
console.log(this.id) }.bind(window) 
var obj = {     
id:'obj',     
hehe:test } 
test() 
// window obj.hehe() 
// window

上面介绍了bind方法的一个重要作用就是为一个函数绑定作用域，但是bind方法在低版本浏览器不兼容，这里我们可以手动实现一下。

拆分一下关键思路

因为bind方法不会立即执行函数，需要返回一个待执行的函数(这里用到闭包，可以返回一个函数)return function(){}

作用域绑定，这里可以使用apply或者call方法来实现 xx.call(yy)/xx.apply(yy)

参数传递，由于参数的不确定性，需要用apply传递数组(实例更明了)xx.apply(yy,[...Array...])，如果用call就不太方便了，因为call后面的参数需要一个个列出来

实现

有了上述的思路，大致的雏形已经明了了，代码应该也很容易实现

绑定作用域，绑定传参

Function.
prototype.
testBind = function(that){     
var _this = this,         
/*         
*由于参数的不确定性，统一用arguments来处理，这里的arguments只是一个类数组对象，有length属性         
*可以用数组的slice方法转化成标准格式数组，除了作用域对象that以外，         
*后面的所有参数都需要作为数组参数传递         
*Array.prototype.slice.apply(arguments,[1])/Array.prototype.slice.call(arguments,1)         
*/         
slice = Array.prototype.slice,         
args = slice.apply(arguments,[1]);     
//返回函数         
return function(){         
//apply绑定作用域，进行参数传递         
return _this.apply(that,args)     
}     
}

测试

var test = function(a,b){     
console.log('作用域绑定 '+ this.value)     
console.log('testBind参数传递 '+ a.value2)     
console.log('调用参数传递 ' + b) 
} 
var obj = {     
value:'ok' } 
var fun_new = test.testBind(obj,{value2:'also ok'})  
fun_new ('hello bind') 
// 作用域绑定 ok 
// testBind参数传递 also ok 
// 调用参数传递  undefined

动态参数

上面已经实现了bind方法的作用域绑定，但是美中不足的是，既然我们返回的是一个函数，调用的时候应该支持传递参数，很显然，上面的 fun_new 调用的时候并不支持传参，只能在 testBind 绑定的时候传递参数，因为我们最终调用的是这个返回函数

function(){         
return _this.apply(that,args)     
}      
这里面的args在绑定的时候就已经确定了，调用的时候值已经固定， 我们并没有处理这个function传递的参数。

我们对其进行改造

return function(){         
return _this.apply(that,             
args.concat(Array.prototype.slice.apply(arguments,[0]))         
)    
}

这里的 Array.prototype.slice.apply(arguments,[0]) 指的是这个返回函数执行的时候传递的一系列参数，所以是从第一个参数开始 [0] ,之前的args = slice.apply(arguments,[1])指的是 testBind方法执行时候传递的参数，所以从第二个开始 [1]，两则有本质区别，不能搞混，只有两者合并了之后才是返回函数的完整参数

所以有如下实现

Function.
prototype.testBind = function(that){     
var _this = this,         
slice = Array.prototype.slice,         
args = slice.apply(arguments,[1]);     
return function(){         
return _this.apply(that,                     
args.concat(Array.prototype.slice.apply(arguments,[0]))                 
)     
}     
}

测试

var test = function(a,b){     
console.log('作用域绑定 '+ this.value)     
console.log('testBind参数传递 '+ a.value2)     
console.log('调用参数传递 ' + b) } 
var obj = {     value:'ok' } 
var fun_new = test.testBind(obj,{value2:'also ok'})  
fun_new ('hello bind') 
// 作用域绑定 ok 
// testBind参数传递 also ok 
// 调用参数传递  hello bind

在以上2种传参方式中，bind的优先级高，从 args.concat(Array.prototype.slice.apply(arguments,[0])) 也可以看出来，bind的参数在数组前面。

原型链

官方文档上有一句话:

A bound function may also be constructed using the new operator: doing

so acts as though the target function had instead been constructed.

The provided this value is ignored, while prepended arguments are

provided to the emulated function.

说明绑定过后的函数被new实例化之后，需要继承原函数的原型链方法，且绑定过程中提供的this被忽略(继承原函数的this对象)，但是参数还是会使用。

这里就需要一个中转函数把原型链传递下去

fNOP = function () {} 
//创建一个中转函数 
fNOP.prototype = this.prototype; 
xx.prototype = new fNOP()  
修改如下 
Function.prototype.testBind = function(that){     
var _this = this,         
slice = Array.prototype.slice,         
args = slice.apply(arguments,[1]),         
fNOP = function () {},         
//所以调用官方bind方法之后 有一个name属性值为 'bound '         
bound = function(){             
return _this.apply(that,                 
args.concat(Array.prototype.slice.apply(arguments,[0]))             
)         
}          
fNOP.prototype = _this.prototype;      
bound.prototype = new fNOP();      
return bound; }

而且bind方法的第一个参数this是可以不传的，需要分2种情况

直接调用bind之后的方法

var f = function () { console.log('不传默认为'+this)  };f.bind()() // 不传默认为 Window

所以直接调用绑定方法时候 apply(that, 建议改为 apply(that||window,，其实不改也可以，因为不传默认指向window

使用new实例化被绑定的方法

容易糊涂，重点在于弄清楚标准的bind方法在new的时候做的事情，然后就可以清晰的实现

这里我们需要看看 new 这个方法做了哪些操作 比如说 var a = new b()

创建一个空对象 a = {}，并且this变量引用指向到这个空对象a

继承被实例化函数的原型 ：a.__proto__ = b.prototype

被实例化方法b的this对象的属性和方法将被加入到这个新的 this 引用的对象中： b的属性和方法被加入的 a里面

新创建的对象由 this 所引用 ：b.call(a)

通过以上可以得知，如果是var after_new = new bindFun(); 由于这种行为是把原函数当成构造器，那么那么最终实例化之后的对象this需要继承自原函数, 而这里的 bindFun 目前是

function(){ 
            return _this.apply(that || window, 
                args.concat(Array.prototype.slice.apply(arguments,[0])) 
            ) 
        }

这里apply的作用域是绑定的that || window，在执行 testBind()的时候就已经固定，并没有把原函数的this对象继承过来，不符合我们的要求，我们需要根据apply的特性解决这个问题：

在一个子构造函数中，你可以通过调用父构造函数的 `apply/call` 方法来实现继承

例如

function Product(name, price) { 
  this.name = name; 
  this.price = price; 
 
  if (price < 0) { 
    throw RangeError(&#39;Cannot create product &#39; + 
                      this.name + &#39; with a negative price&#39;); 
  } 
} 
 
function Food(name, price) { 
  Product.call(this, name, price);  
  this.category = &#39;food&#39;; 
} 
 
//等同于（其实就是把Product放在Food内部执行了一次） 
function Food(name, price) {  
    this.name = name; 
    this.price = price; 
    if (price < 0) { 
        throw RangeError(&#39;Cannot create product &#39; + 
                this.name + &#39; with a negative price&#39;); 
    } 
 
    this.category = &#39;food&#39;;  
}

所以在new新的实例的时候实时将这个新的this对象 进行 apply 继承原函数的 this 对象，就可以达到 new 方法里面的第 3 步的结果

apply(that||window, 
//修改为 如果是new的情况，需要绑定new之后的作用域，this指向新的实例对象 
apply(isNew ?　this : that||window,  ==> 
 
Function.prototype.testBind = function(that){ 
    var _this = this, 
        slice = Array.prototype.slice, 
        args = slice.apply(arguments,[1]), 
        fNOP = function () {}, 
        //所以调用官方bind方法之后 有一个name属性值为 'bound ' 
        bound = function(){ 
            return _this.apply(isNew ?　this : that||window, 
                args.concat(Array.prototype.slice.apply(arguments,[0])) 
            ) 
        }     
 
    fNOP.prototype = _this.prototype; 
 
    bound.prototype = new fNOP(); 
 
    return bound; 
}

这里的 isNew 是区分 bindFun 是直接调用还是被 new 之后再调用，通过原型链的继承关系可以知道，

bindFun 属于 after_new的父类，所以 after_new instanceof bindFun 为 true,同时

bindFun.prototype = new fNOP() 原型继承; 所以 fNOP 也是 after_new的父类， after_new instanceof fNOP 为 true

最终结果

Function.prototype.testBind = function(that){ 
        var _this = this, 
            slice = Array.prototype.slice, 
            args = slice.apply(arguments,[1]), 
            fNOP = function () {}, 
            bound = function(){ 
                //这里的this指的是调用时候的环境 
                return _this.apply(this instanceof  fNOP ?　this : that||window, 
                    args.concat(Array.prototype.slice.apply(arguments,[0])) 
                ) 
            }     
        fNOP.prototype = _this.prototype; 
     
        bound.prototype = new fNOP(); 
     
        return bound; 
    }

我看到有些地方写的是

this instanceof fNOP && that ? this : that || window,

我个人觉得这里有点不正确，如果绑定时候不传参数，那么that就为空，那无论怎样就只能绑定 window作用域了。

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