Script-wide validation
Enable stricter checks in a traditional script.
JS Puzzles · Const
Strict Mode
Strict mode is enabled with the "use strict" directive at the top of a script or function. It changes JavaScript to a stricter execution mode; classes and modules enable it automatically.
How it works
Strict mode is enabled by placing the exact directive "use strict" at the beginning of a script or function. A script-level directive affects the whole script, while a function-level directive affects only that function. Classes and modules enable strict mode automatically.
Syntax
"use strict";Key points
- Use the exact string directive to opt a script into strict mode.
- At script level, the directive applies to the entire script.
- Placing the directive at the beginning of a function limits strict mode to that function.
- Classes and modules activate strict mode automatically, so an explicit directive is unnecessary there.
- Strict mode is one-way for the current execution context.
Common mistakes
- The directive must appear at the top of the script; code before it can prevent strict mode from being enabled.
- Strict mode cannot be turned off once execution has entered it.
Example: Strict mode rejects accidental globals
'use strict';
function assignUndeclared() {
undeclared = 1;
}
try {
assignUndeclared();
} catch (error) {
console.log(error instanceof ReferenceError);
}The strict function throws instead of silently creating a global variable.
When to use it
Function-scoped strict mode
Constrain legacy code to a stricter function body.
Automatic strict mode
Use classes or modules without adding a directive.
Related
classesmodulesscriptsfunctions