JavaScript'使用严格'; 内部功能

Question

Tested some js code in Chrome Dev Console and I'm a bit confused.

I know that in strict mode functions that are not methods of an object when referred to this keyword should receive undefined instead of global object.

function test(){
    "use strict";
    return this===undefined;}
test(); 

Outputs false .

"use strict";
function test(){
    return this===undefined;}
test(); 

Still false .

(function test(){
    "use strict";
    return this===undefined;}());

Outputs true .

Just wanted to clarify. ʕ •ᴥ•ʔ I'm new to js.

Answer 1

What you have noticed is simply a side-effect of the way the developer console works. When you enter code there, this is effectively what happens (see this answer for more details):

eval.call(null, "with (window) { \
                     function test() { \
                         'use strict'; \
                         console.log(this); \
                     } test(); \
                 }");

This is an indirect call to eval , which means it will always execute in the global execution context (in the browser, that's window ).

Effectively, the function is bound to the global object and therefore this holds a reference to the global object, as if you did this in a web page (rather than in the console):

function test(){
    "use strict";
    return this === undefined;
}

test(); // true
test.call(window); // false

Answer 2

Everything's fine. If you run your code via some HTML page (not a dev console), results meet expectations (always this===undefined ).

Additionally in latest Firefox (Firebug):

function test(){
    "use strict";
    return this===undefined;}
test(); 
>> true

So this seems to be just another Chrome's bug (feature?). It feels like it has a slightly different approach to the code that is passed via dev console.

Also note that order matters:

<script>
    console.log( 'Me First!' );

    "use strict";

    function test(){
        console.log( this );
    }
    test();

</script>

>>> "Me First!"
>>> Window {top: Window, window: Window, location: Location, external: Object, chrome: Object…}

But:

<script>
    "use strict";

    console.log( 'Me later!' );

    function test(){
        console.log( this );
    }
    test();

</script>

>>> undefined
>>> "Me later!"

Answer 3

It is a bug in the Chromium Developer Console that causes this to still refer to the global object. The same code works as specified with javascript: in the location bar, and in documents.

You can test that like so (2 console inputs):

var global = (function () { return this; }());

"use strict";
function test () { return this === global; }
test();

and (one or more console inputs)

var script = document.createElement("script");
script.type = "text/javascript";
script.appendChild(document.createTextNode(
  'function test () { "use strict"; return this === undefined; }; console.log(test());'
));
document.body.appendChild(script);

Tested in Chromium Version 25.0.1364.97 Debian 7.0 (183676).