Can 'this' ever be null in Javascript

问题:

I have a function along the lines of the following:

    doSomething: function () {
        var parent = null;

        if (this === null) {
            parent = 'some default value';
        } else {
            parent = this.SomeValue();
        }
    }

Could parent ever be set to 'some default value' or is the check for null superfluous?

Alternatively, what if I used the less restrictive:

    doSomething: function () {
        var parent = this ? this.SomeValue() : 'some default value';
    }

Could parent ever be set to 'some default value' in this case?

回答1:

In non-strict mode, this has undergone an Object(this) transformation, so it's always truthy. The exceptions are null and undefined which map to the global object. So this is never null and always truthy, making both checks superfluous.

In strict mode, however, this can be anything so in that case you'd have to watch out. But then again you have to opt in for strict mode yourself, so if you don't do that there are no worries.

(function() {               return this; }).call(null); // global object
(function() { "use strict"; return this; }).call(null); // null

The specification of ES5 says:

The thisArg value is passed without modification as the this value. This is a change from Edition 3, where a undefined or null thisArg is replaced with the global object and ToObject is applied to all other values and that result is passed as the this value.

回答2:

Although not a direct answer to your question.. in a browser 'this' will, by default, refer to the 'window' object. On nodejs it will refer to the global object.

I'm not sure if there's ever a case where it could be null, but it would at the very least be unusual.

回答3:

The this keyword shouldn't ever be null, but it might not be what you expect. You can end up referencing the window object if you're not careful.

In your case, I believe this should refer to the doSomething function.

var myObject = {
    myMethod: function() {
        console.log(this);
    }
};

myObject.myMethod(); // prints out a reference to the myMethod function

Here's a jsFiddle.

回答4:

You can see what occurs if you try to force Javascript to use a null scope in any browser. I made an example here:

http://jsfiddle.net/rXsWj/

In short, this empirically proves that the browser changes 'null' to 'window' on the fly. If instead you change null to 'a', you'll see an alert of 'a' like you would expect. Code from link as reference:

function a() {
    alert(this)
}

a.apply(null)

​

回答5:

No. The value of this will never be the text 'some default value'.

回答6:

this can not be null. if it was null, then you couldn't be in method scope.