typeof window ==='object'`始终为真吗？

Question

Assuming window isn't shadowed, will typeof window === 'object' always be true? Is this safe?

Context: I'm trying to export a module to the global scope using browserify and I want to include a check to see if this module is run in the browser and if it is, I want to export it to the global object.

Answer 1

If you have the real window , defined by the runtime, it will always be an object. The typeof operator (§12.5.6) has a limited number of return values (table 15) , and there's no special one for window . If they expand that table, it could change, but the only applicable result in ES1 through ES7 is 'object' .

Bear in mind that Node doesn't define window at all (so typeof would be 'undefined' ) and you can shadow it, as you mention. In a browser with a standard DOM implementation, window will be the object defined in §7.3 of the DOM specification . For JavaScript, this should be an object and I believe all browsers expose it as such.

In webpack, you would use the output.library property and set output.libraryTarget to 'var' , then the bundle sets up the global for you. I imagine browserify has an equivalent.

Answer 2

The answer is: It depends.

Short answer:

If you're in a browser, then yes, window will always be an object. If you're on a JavaScript standalone engine such as Node.js, then probably no.

More details:

A window is one of the top-hierarchy objects on a browser. It is the parent of the whole DOM tree present inside a browser window (or tab), and the context reference where most of functions and event listeners will often run.

So in any browser that expects to work properly, there will be a window object. (some side/experimental projects may have modified JavaScript engines and not implement window , but this is highly unlikely and definitely not the kind of thing that is going to be used by "mainstream users")

However, when we talk about JavaScript engines running outside browsers, things get a little different. On a server-side JavaScript runtime such as Node.js, the purpose is not to work with windows/tabs, URLs and DOM tree parsing such as a browser do. So you will not have this object available and running typeof window will return 'undefined' on those environments.

Answer 3

To add a variable to global scope, browserify exposes global which in the browser represents window .

Per the browserify docs (emphasis mine)

Additionally, if you use any of these variables, they will be defined in the bundled output in a browser-appropriate way:

• process
• Buffer
• global - top-level scope object (window)
• __filename - file path of the currently executing file
• __dirname - directory path of the currently executing file

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As far as the question being asked:

Assuming window isn't shadowed, will typeof window === 'object' always be true?

Yes given that it's running in a browser environment.