Comprehensive analysis of this in JavaScript
JavaScript Of course it is indispensable to have objects in it, the this keyword! With the this keyword JavaScript, the code will be reduced. Today we will analyze this in JavaScript!
Implicit binding
Regarding this, generally speaking, whoever calls a method will point to whom this method points to, such as:
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If there are multiple calls, the objectPropertiesOnly the upper or last layer of the reference chain works in the calling location, such as:
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Implicit loss
One of the most common this binding problems is that implicitly bound functions will lose the binding object. That is to say, it should apply the default binding, thereby binding this to the global object or undefined, depending on whether it is in strict mode.
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Although bar is a reference to obj.foo, in fact, it refers to the foo function itself, so bar() at this time is actually a function call without any modification, so it is applied Default binding
A subtler, more common and more unexpected situation occurs when passing in the callback function:
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Parameter passing is actually an implicit formula assignment, so when we pass in the function, it will also be implicitly assigned, so the result is the same as the previous example. If the function is passed into the language's built-in function instead of passing in the self-declared function (such as setTimeout, etc.), the result will be the same.
Explicit binding
Simply put, it means specifying this, such as: call, apply, bind, new binding, etc.
Hard binding
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Here is a brief explanation: In the bar function, foo uses the apply function to bind obj, which means that this in foo will point to obj, at the same time, uses arguments (no limit on the number of parameters passed in) as parameters and passes them into the foo function; so when running bar(3), first output obj.a, which is 2 and the passed in 3, and then foo returns the sum of the two, so the value of b is 5
Similarly, this example can also use bind:
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new binding
In traditional class-oriented languages, when using new to initialize a class, the constructor in the class will be called, but new in JS The mechanism is actually completely different from class-oriented and language-oriented.
Use new to call a function, or when a constructor call occurs, the following operations will be automatically performed:
Create (or construct) a brand new object
This The new object will be executed [[Prototype]] connection
This new object will be bound to this
of the function call If the function does not return other objects, then newExpression## The function in # will automatically return this new object such as:
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##This application in arrow functions The arrow function does not use the four standard rules of this, but determines this based on the outer (function or global) scope.
我们来看一下箭头函数的词法作用域:
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foo()内部创建的箭头函数会捕获调用时foo()的this。由于foo()的this绑定到obj1,bar(引用箭头函数)的this也会绑定到obj1,箭头函数的绑定无法被修改。(new也不行!)
总结
如果要判断一个运行中的函数的this绑定,就需要找到这个函数的直接调用位置。找到之后就可以顺序应用下面这四条规则来判断this的绑定对象。
由new调用?绑定到新创建的对象。
由call或者apply(或者bind)调用?绑定到指定的对象。
由上下文对象调用?绑定到那个上下文对象。
默认:在严格模式下绑定到undefined,否则绑定到全局对象。
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