Strict Mode in JavaScript

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.

Advantages:

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.

1. No duplicate keys in objects

   const myObject = {
        name: "Parwinder",
        age: 34,
        car: "MiniCoop",
        name: "Bhagat"
    }
   console.log(myObject.name); // Bhagat

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

1. Throw error when trying to delete undeletable properties of an object

   "use strict"
   delete Object.prototype; // throws a TypeError

1. 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.

1. 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.