As of late, I run a portion of my JavaScript code through Crockford's JSLint, and it gave the following error:

The problem at line 1 character 1: Missing "use strict" statement.

After doing some searching and RND, I realized that many people added "use strict"; into their JavaScript code. Once I added the statement, the error stopped appearing. But Google or other SE did not reveal much of the history behind this strong statement. Certainly, it must have something to do with how the JavaScript is interpreted by the browser, but I have no idea what the effect would be.

So what is "use strict"; all about, what does it imply, and is it still relevant?

Do any of the current browsers respond to the "use strict"; string or is it only for future use?

javascript syntax use-strict 
1 Answers

Original answer

This article about Javascript Strict Mode might interest you: John Resig - ECMAScript 5 Strict Mode, JSON, and More

To quote some interesting parts:

Strict Mode is a new feature in ECMAScript 5 that allows you to place a program, or a function, in a "strict" operating context. This strict context prevents certain actions from being taken and throws more exceptions.

And:

Strict mode helps out in a couple ways:

  • It catches some common coding bloopers, throwing exceptions.
  • It prevents, or throws errors, when relatively "unsafe" actions are taken (such as gaining access to the global object).
  • It disables features that are confusing or poorly thought out.

Also, note you can apply "strict mode" to the whole file... Or you can use it only for a specific function (still quoting from John Resig's article):

// Non-strict code...

(function(){
  "use strict";

  // Define your library strictly...
})();

// Non-strict code...

This might be helpful if you have to mix old and new code ;-)

So, I suppose it's a bit like the "use strict" you can use it in Perl (hence the name?): it helps you make fewer errors, by detecting more things that could lead to breakages.

Strict mode is now supported by all major browsers.

Update for ES6 modules

Inside native ECMAScript modules (with import and export statements) and ES6 classes, strict mode is always enabled and cannot be disabled.

answered Dec 21


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