JavaScript Use of Strict Statement

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The JavaScript language developed without losing compatibility for a long time. New features were added to the language, but the old never changed, so as not to break existing HTML / JS-page usage.

However, this has led to the fact that any error in the design of the language became “frozen” forever.

It was so until the appearance of the standard ECMAScript 5 (ES5), which also added new features and made a number of corrections in the language that could lead to the fact that the old code became non-working.

To avoid this, it was decided that by default these dangerous changes will be turned off, and the code will work as before. And in order to put the code in a correspondence with the modern standards, it’s needed to specify the special directive “use strict”.

IE9 does not support this directive.

“use strict” directive

The directive looks like the line “use strict”; or ‘use strict’; and is placed at the beginning of the script.

For example:

"Use strict";

// This code will work in a modern ES5 style 

Do you need “use strict”?

Speaking abstractly, yes, you do. In strict mode, some bugs in the design of the language are fixed, and the modern standard is good in general. However, there are two problems.

Support for IE9 browsers that ignore the “use strict”

If you have a code that works perfectly in Google Chrome, there’s no guarantee that it will work in IE9 browsers. IE9 always works in old standard, and all the problems should be solved in IE, which is much more complicated than in Google Chrome.

Libraries written without regard to “use strict”

Some libraries are written without “use strict”. This does not always work correctly if the calling code has “use strict”.

There are only a few such libraries, but you will spend a lot of time fixing all of the issues.

Therefore, the conclusion is that writing code with “use strict” follows only if you are sure that the problems described above will not occur.


The IE8 browser only supports a very old version of the JavaScript standard, namely ES3.

Fortunately, many of the features of the modern standard can be added to the browser by connecting the library ES5 shim, namely the scripts es5-shim.js and es5-sham.js from it.


In this guide, we looked through the “use strict” directive and understood that it should be used carefully.

Thanks for your attention!

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