The strict mode was introduced in ES5. It is a way of enforcing strict rules while coding in JS. Not all browsers support strict mode, so always test your code.


  1. Disable some silent JS errors and throw them instead.
  2. Performant code in specific instances where a JS engine supports performance optimizations
  3. Code improvements
    • No duplicate keys in objects
    • Variable declaration without var keyword
    • Error on duplicate function arguments

Enabling strict mode

  • File-level: add "use strict" on top of the file before any other statements.
  • Function level: add the same "use strict" on top of the function body before any other statements.
  • Module level: Modules introduced in ES6/ES2015 are in strict mode by default.

Changes applied in strict mode

  1. Variable declaration without var keyword
    someVariable = 17;
    console.log(someVariable); // 17

    This is a completely valid code. Even though we did not use let, var or const to declare the variable, JavaScript works without an issue. This would force JS to create a global property, and that could cause side effects in a large application (variable name conflict and modifying global variables).

    Strict mode solves this by throwing an error.

  2. No duplicate keys in objects
    const myObject = {
         name: "Parwinder",
         age: 34,
         car: "MiniCoop",
         name: "Bhagat"
    console.log(; // Bhagat

    Again, completely valid under non-strict mode (sloppy mode) but throws an error in strict mode.

  3. Throw error when trying to delete undeletable properties of an object
    "use strict"
    delete Object.prototype; // throws a TypeError
  4. No duplicate arguments in functions
    function sumOfNumbers(a, a, b) {
        return a + a + b;
    console.log(sumOfNumbers(1, 2, 3)); // 7

    The above code is valid too. It ends up with unexpected results. The value of a is set to 2, and the sum returns 7 instead of our expectation, 6. Strict mode throws a syntax error.

  5. Assigning NaN to a variable has no failure feedback in sloppy mode. Strict mode throws an exception

🚨 There’s no way to cancel “use strict”. Once we enter strict mode, there’s no way to turn it off.