Syntax

• ‘use strict’;
• “use strict”;
• use strict;

Remarks

Strict mode is an option added in ECMAScript 5 to enable a few backwards-incompatible enhancements. Behaviour changes in “strict mode” code include:

• Assigning to undefined variables raises an error instead of defining new global variables;
• Assigning to or deleting non-writable properties (such as window.undefined) raises an error instead of executing silently;
• Legacy octal syntax (ex. 0777) is unsupported;
• The with statement is unsupported;
• eval cannot create variables in the surrounding scope;
• Functions’ .caller and .arguments properties are unsupported;
• A function’s parameter list cannot have duplicates;
• window is no longer automatically used as the value of this.

NOTE:- ‘strict’ mode is NOT enabled by default as if a page uses JavaScript which depends on features of non - strict mode, then that code will break. Thus, it has to be turned on by the programmer himself / herself.

For entire scripts

For functions

Changes to properties

Changes to global properties

Duplicate Parameters

Function scoping in strict mode

Behaviour of a functions arguments list

Non-Simple parameter lists

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