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Strict Mode

Easy

In plain terms

Strict mode makes JavaScript throw errors for common mistakes like using undeclared variables. Add "use strict" at the top of a file or function. Modern modules are strict by default.

What you need to know

  • •Prevents undeclared variables
  • •Disallows duplicate parameter names
  • •Modules and classes are strict by default

Try it yourself

Copy the code below and run it in your browser console or a code editor:

'use strict';

x = 10;  // Error! x is not defined

function foo() {
  'use strict';
  y = 5;  // Error in strict mode
}

// Deleting variables not allowed
let z = 1;
// delete z;  // Error in strict mode

Learn more

Dive deeper with these trusted resources:

MDNjavascript.infoTutorialsPointW3Schools