Why using `strict mode` in JavaScript libraries?

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What does “use strict” do in JavaScript, and what is the reasoning behind it?

Actually I know what the use strict does in JavaScript as the question asked here:
What does "use strict" do in JavaScript, and what is the reasoning behind it?

But I can't understand why we should use strict mode in JavaScript libraries? I mean what's the benefits of using that?

Your answer is already [here on SO][1]. [1]: http://stackoverflow.com/questions/1335851/what-does-use-strict-do-in-javascript-and-what-is-the-reasoning-behind-it — rg88, Nov 21 '12 at 05:51
The reasons for using it in libraries are the same as using it generally, as linked in your question. Why do you think there's a difference just because it's library code? — T.J. Crowder, Nov 21 '12 at 05:53
I voted to close as duplicate because I don't see an answer here being any different or any clearer than the one already given. — moribvndvs, Nov 21 '12 at 05:55
@T.J.Crowder But I can't understand the using it generally. I said in libraries because I saw it in `jQuery`, not more. — Afshin Mehrabani, Nov 21 '12 at 05:55
@AfshinMehrabani: You can't understand the benefit of using strict mode, having read the question you linked and its answers and references? Perhaps you could ask a specific question about one of the many listed benefits and why you don't see the point? — T.J. Crowder, Nov 21 '12 at 05:59

T.J. Crowder · Accepted Answer · 2019-05-18T08:03:35.687

The question you linked, its answers, and the references it gives list a bunch of reasons for using strict mode.

Let me call out just one of them: The Horror of Implicit Globals¹

Non-strict code:

function foo(a) {
    var myvar;

    myar = a * 4;

    // ...

    return myvar;
}

Now, this code:

console.log(foo(2));

...should log "8", right? But it doesn't, it always logs "undefined":

function foo(a) {
    var myvar;

    myar = a * 4;

    // ...

    return myvar;
}
console.log(foo(2));

And what's more, it silently creates a global variable called myar. Why? Because I had a typo in my code (I missed out the v in myvar when setting it to a * 4).

Compare with:

function foo(a) {
    "use strict";
    var myvar;

    myar = a * 4;

    // ...

    return myvar;
}
console.log(foo(2));

Now, instead of a weird return value and a global variable getting created, I get a nice error message: ReferenceError: "myar" is not defined

Now, that particular aspect of strict mode could be accomplished using a lint tool instead. But you don't always involve a lint tool in your coding-in-anger toolchain, when you're just trying to fix something and bouncing between your editor and the browser. So it's nice when the browser helps you out.

Separately, strict mode does things that can't be done by lint tools, such as disallowing with, changing the default value of this in function calls that don't set it, etc.

¹ (that's a post on my anemic little blog)

Why using `strict mode` in JavaScript libraries?

1 Answers1