【转】理解JavaScript的 “this”关键字

It's safe to say that the this keyword is probably one of the misunderstood parts of JavaScript. Admittedly, I used to throw the this keyword around until my script worked and it confused the hell out of me (and still does many other JS developers) around the world. Only when I learned about lexical scope, how functions are invoked, scope context, a few context changing methods - I really understood it.

Before you dive into this article, here's a few very important points to takeaway and remember about the thiskeyword:

• The this keyword's value has nothing to do with the function itself, how the function is called determines the this value
• It can be dynamic, based on how the function is called
• You can change the this context through .call(), .apply() and .bind()

Default this context

There are a few different ways the this value changes, and as we know it's usually the call-site that creates the context.

Window Object, global scope

Let's take a quick example at how simply calling regular functions binds the this value differently:

// define a function
var myFunction = function () {
  console.log(this);
};

// call it
myFunction();

What can we expect the this value to be? By default, this should always be the window Object, which refers to the root - the global scope. So when we console.log(this); from our function, as it's invoked by the window (simply just called), we should expect the this value to be our window Object:

// define a function
var myFunction = function () {
  console.log(this); // [object Window]
};

// call it
myFunction();

Object literals

Inside Object literals, the this value will always refer to it's own Object. Nice and simple to remember. That is good news when invoking our functions, and one of the reasons I adopt patterns such as the module pattern for organising my objects.

Here's how that might look:

// create an object
var myObject = {};

// create a method on our object
myObject.someMethod = function () {
  console.log(this);
};

// call our method
myObject.someMethod();

Here, our window Object didn't invoke the function - our Object did, so this will refer to the Object that called it:

// create an object
var myObject = {};

// create a method on our object
myObject.someMethod = function () {
  console.log(this); // myObject
};

// call our method
myObject.someMethod();

Prototypes and Constructors

The same applies with Constructors:

var myConstructor = function () {
    this.someMethod = function () {
        console.log(this);
    };
};

var a = new myConstructor();
a.someMethod();

And we can add a Prototype Object as well:

var myConstructor = function () {
    this.someMethod = function () {
        console.log(this);
    };
};

myConstructor.prototype = {
    somePrototypeMethod: function () {
        console.log(this);
    }
};

var a = new myConstructor();
a.someMethod();
a.somePrototypeMethod();

Interestingly, in both cases the this value will refer to the Constructor object, which will bemyConstructor.

Events

When we bind events, the same rule applies, the this value points to the owner. The owner in the following example would be the element.

// let's assume .elem is <div class="elem"></div>
var element = document.querySelector('.elem');
var someMethod = function () {
  console.log(this);
};
element.addEventListener('click', someMethod, false);

Here, this would refer to <div class="elem"></div>.

Dynamic this

The second point I made in the intro paragraph was that this is dynamic, which means the value could change. Here's a real simple example to show that:

// let's assume .elem is <div class="elem"></div>
var element = document.querySelector('.elem');

// our function
var someMethod = function () {
  console.log(this);
};

// when clicked, `this` will become the element
element.addEventListener('click', someMethod, false); // <div>

// if we just invoke the function, `this` becomes the window object
someMethod(); // [object Window]

Changing this context

There are often many reasons why we need to change the context of a function, and thankfully we have a few methods at our disposal, these being .call(), .apply() and .bind().

Using any of the above will allow you to change the context of a function, which in effect will change the thisvalue. You'll use this when you want this to refer to something different than the scope it's in.

Using .call(), .apply() and .bind()

"Functions are first class Objects" you'll often hear, this means they can also have their own methods!

The .call() method allows you to change the scope with a specific syntax ref:

.call(thisArg[, arg1[, arg2[, ...]]]);

Usage would look something like this:

someMethod.call(anotherScope, arg1, arg1);

You'll notice further arguments are all comma separated - this is the only difference between .call() and.apply():

someMethod.call(anotherScope, arg1, arg1); // commas
someMethod.apply(anotherScope, [arg1, arg1]); // array

With any of the above, they immediately invoke the function. Here's an example:

var myFunction = function () {
  console.log(this);
};
myFunction.call();

Without any arguments, the function is just invoked and this will remain as the window Object.

Here's a more practical usage, this script will always refer to the window Object:

var numbers = [{
  name: 'Mark'
},{
  name: 'Tom'
},{
  name: 'Travis'
}];
for (var i = 0; i < numbers.length; i++) {
  console.log(this); // window
}

The forEach method also has the same effect, it's a function so it creates new scope:

var numbers = [{
  name: 'Mark'
},{
  name: 'Tom'
},{
  name: 'Travis'
}];
numbers.forEach(function () {
  console.log(this); // window
});

We could change each iteration's scope to the current element's value inside a regular for loop as well, and use this to access object properties:

var numbers = [{
  name: 'Mark'
},{
  name: 'Tom'
},{
  name: 'Travis'
}];
for (var i = 0; i < numbers.length; i++) {
  (function () {
    console.log(this.name); // Mark, Tom, Travis
  }).call(numbers[i]);
}

This is especially extensible when passing around other Objects that you might want to run through the exact same functions.

forEach scoping

Not many developers using forEach know that you can change the initial scope context via the second argument:

numbers.forEach(function () {
  console.log(this); // this = Array [{ name: 'Mark' },{ name: 'Tom' },{ name: 'Travis' }]
}, numbers); // BOOM, scope change!

Of course the above example doesn't change the scope to how we want it, as it changes the functions scope for every iteration, not each individual one - though it has use cases for sure!

To get the ideal setup, we need:

var numbers = [{
  name: 'Mark'
},{
  name: 'Tom'
},{
  name: 'Travis'
}];
numbers.forEach(function (item) {
  (function () {
    console.log(this.name); // Mark, Tom, Travis
  }).call(item);
});

.bind()

Using .bind() is an ECMAScript 5 addition to JavaScript, which means it's not supported in all browsers (but can be polyfilled so you're all good if you need it). Bind has the same effect as .call(), but instead binds the function's context prior to being invoked, this is essential to understand the difference. Using .bind() will notinvoke the function, it just "sets it up".

Here's a really quick example of how you'd setup the context for a function, I've used .bind() to change the context of the function, which by default the this value would be the window Object.

var obj = {};
var someMethod = function () {
  console.log(this); // this = obj
}.bind(obj);
someMethod();

This is a really simple use case, they can also be used in event handlers as well to pass in some extra information without a needless anonymous function:

var obj = {};
var element = document.querySelector('.elem');
var someMethod = function () {
  console.log(this);
};
element.addEventListener('click', someMethod.bind(obj), false); // bind

"Jumping scope"

I call this jumping scope, but essentially it's just some slang for accessing a lexical scope reference (also a bit easier to remember...).

There are many times when we need to access lexical scope. Lexical scope is where variables and functions are still accessible to us in parent scopes.

var obj = {};

obj.myMethod = function () {
  console.log(this); // this = `obj`
};

obj.myMethod();

In the above scenario, this binds perfectly, but what happens when we introduce another function. How many times have you encountered a scope challenge when using a function such as setTimeout inside another function? It totally screws up any this reference:

var obj = {};
obj.myMethod = function () {
  console.log(this); // this = obj
    setTimeout(function () {
        console.log(this); // window object :O!!!
    }, 100);
};
obj.myMethod();

So what happened there? As we know, functions create scope, and setTimeout will be invoked by itself, defaulting to the window Object, and thus making the this value a bit strange inside that function.

Important note: this and the arguments Object are the only objects that don't follow the rules of lexical scope.

How can we fix it? There are a few options! If we're using .bind(), it's an easy fix, note the usage on the end of the function:

var obj = {};
obj.myMethod = function () {
  console.log(this); // this = obj
    setTimeout(function () {
        console.log(this); // this = obj
    }.bind(this), 100); // .bind() #ftw
};
obj.myMethod();

We can also use the jumping scope trick, var that = this;:

var obj = {};
obj.myMethod = function () {
  var that = this;
  console.log(this); // this = obj
    setTimeout(function () {
        console.log(that); // that (this) = obj
    }, 100);
};
obj.myMethod();

We've cut the this short and just simply pushed a reference of the scope into the new scope. It's kind of cheating, but works wonders for "jumping scope". With newcomers such as .bind(), this technique is sometimes frowned upon if used and abused.

One thing I dislike about .bind() is that you could end up with something like this:

var obj = {};
obj.myMethod = function () {
  console.log(this);
    setTimeout(function () {
        console.log(this);
        setTimeout(function () {
            console.log(this);
            setTimeout(function () {
                console.log(this);
                setTimeout(function () {
                    console.log(this);
                }.bind(this), 100); // bind
            }.bind(this), 100); // bind
        }.bind(this), 100); // bind
    }.bind(this), 100); // bind
};
obj.myMethod();

A tonne of .bind() calls, which look totally stupid. Of course this is an exaggerated issue, but it can happen very easily when switching scopes. In my opinion this would be easier - it will also be tonnes quicker as we're saving lots of function calls:

var obj = {};
obj.myMethod = function () {
  var that = this; // one declaration of that = this, no fn calls
  console.log(this);
    setTimeout(function () {
        console.log(that);
        setTimeout(function () {
            console.log(that);
            setTimeout(function () {
                console.log(that);
                setTimeout(function () {
                    console.log(that);
                }, 100);
            }, 100);
        }, 100);
    }, 100);
};
obj.myMethod();

Do what makes sense!

jQuery $(this)

Yes, the same applies, don't use $(this) unless you actually know what it's doing. What it is doing is passing the normal this value into a new jQuery Object, which will then inherit all of jQuery's prototypal methods (such as addClass), so you can instantly do this:

$('.elem').on('click', function () {
  $(this).addClass('active');
});

Happy scoping ;)

Understanding the “this” keyword in JavaScript