JavaScript: Can ECMAScript 5's Strict Mode ("use strict") be enabled using single quotes ('use strict')?

Question

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.

@Myles Gray - I don't have a browser that supports Strict mode at work :( — Rudiger
– Rudiger, Commented Mar 7, 2011 at 0:00
@Felix Kling - None of the major browsers support Strict mode :( See kangax.github.com/es5-compat-table — Rudiger
– Rudiger, Commented Mar 7, 2011 at 0:06

Michael Thompson · Accepted Answer · 2019-05-23 09:38:09Z

61

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.

edited May 23, 2019 at 9:38

Michael Thompson

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answered Mar 7, 2011 at 0:06

Felix Kling

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5 Comments

Eric Over a year ago

If you are using gzip to compress the content then I would recommend using double quotes or single quotes depending on which one shows up most in the document.

user123444555621 Over a year ago

Okay, but how about JS engine support? I don't think there are any known quirks at the moment, but I'm not sure.

Felix Kling Over a year ago

@Pumbaa80: You mean if there is a problem if strict mode is not supported? I highly doubt it. The syntax was chosen to make it backwards compatible. An engine that does not support strict mode simply sees an expression statement with a lone string literal.

user123444555621 Over a year ago

@FelixKling That's not what I meant. I was wondering whether there might be an engine that triggers strict mode when seeing "use strict", but not when seeing 'use strict', even though the two are supposed to be equivalent.

Felix Kling Over a year ago

@Pumbaa80: Ah. I don't know, I don't think so.

user113716 · Accepted Answer · 2011-03-07 00:06:04Z

http://ecma262-5.com/ELS5_HTML.htm#Section_14.1

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.

Elian Ebbing · Accepted Answer · 2011-03-07 00:07:49Z

5

According to the mozilla documentation you can use both "use strict"; and 'use strict';.

answered Mar 7, 2011 at 0:07

Elian Ebbing

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