JavaScript: Can ECMAScript 5’s Strict Mode (“use strict”) be enabled using single quotes (‘use strict’)

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JavaScript doesn't care if your Strings are double-quoted "double" or single-quoted 'single'.

Every example of ECMAScript 5's strict mode has it enabled by "use strict" in double-quotes. Can I do the following (single-quotes):

alert(function(){
  'use strict';
  return !this;
}());

This will return true if Strict mode is enabled, and false if it is not.

Best Answer

For you, without using a browser that supports strict mode:

A Use Strict Directive is an ExpressionStatement in a Directive Prologue whose StringLiteral is either the exact character sequences "use strict" or 'use strict'. A Use Strict Directive may not contain an EscapeSequence or LineContinuation.

Related Solutions

Javascript – When should I use double or single quotes in JavaScript

The most likely reason for use of single vs. double in different libraries is programmer preference and/or API consistency. Other than being consistent, use whichever best suits the string.

Using the other type of quote as a literal:

alert('Say "Hello"');
alert("Say 'Hello'");

This can get complicated:

alert("It's \"game\" time.");
alert('It\'s "game" time.');

Another option, new in ECMAScript 6, is template literals which use the backtick character:

alert(`Use "double" and 'single' quotes in the same string`);
alert(`Escape the \` back-tick character and the \${ dollar-brace sequence in a string`);

Template literals offer a clean syntax for: variable interpolation, multi-line strings, and more.

Note that JSON is formally specified to use double quotes, which may be worth considering depending on system requirements.

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

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...

Which 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 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.

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

Best Answer

Related Solutions

Javascript – When should I use double or single quotes in JavaScript

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

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