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Use strict

The "use strict" directive was introduced in ECMAScript 5 (ES5). It's a way to opt in to a restricted variant of JavaScript which does not allow certain actions and throws more exceptions.

Enabling "use strict" mode:

  • Placed at the top of a JavaScript file, it applies to the entire script.
  • Placed at the beginning of a function, it applies only to that function.
  • "use strict" mode is automatically enabled within ECMAScript 6 (ES6) classes and modules.

Here's a comparison table illustrating the differences:

Scenario Non-strict Mode Strict Mode
Assigning to undeclared variable, read-only global property, or read-only declared variable Allowed, no error SyntaxError
this value outside any function Global object undefined
Function declaration within a block Visible outside the block Visible only inside the block
Deleting variables, function, or function parameters Allowed, no error SyntaxError
Changing a non-allowed property descriptor Allowed, no error TypeError