JavaScript Strict mode
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
withstatement is unsupported; evalcannot create variables in the surrounding scope;- Functions'
.callerand.argumentsproperties are unsupported; - A function's parameter list cannot have duplicates;
windowis no longer automatically used as the value ofthis.
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.
