What is a strict mode in javascript and Why do you need strict mode?

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Strict Mode is a feature introduced in ECMAScript 5 that allows you to enforce a stricter parsing and error handling in your JavaScript code. It helps in catching common coding errors and improving the security and performance of JavaScript code.

Why Use Strict Mode?

  • Error Detection: It makes it easier to write secure JavaScript code by throwing errors for bad syntax and unsafe operations that are otherwise silently ignored in non-strict mode.
  • Prevents Global Variables: It prevents the accidental creation of global variables, which helps avoid potential bugs and conflicts.
  • Enforces Safe Coding Practices: It enforces stricter parsing and error handling, such as throwing errors for assignment to non-writable properties, using undefined variables, or other unsafe operations.
  • Improves Performance: Some JavaScript engines can optimize code more aggressively when it is running in strict mode.

How to Enable Strict Mode

Strict mode is declared by adding "use strict"; at the beginning of a script or a function.

Global Strict Mode:

"use strict";
// This code is in strict mode
x = 3.14; // Error: x is not declared

Key Differences with Non-Strict Mode

  • Variable Declarations: In strict mode, all variables must be declared with var, let, or const. Undeclared variables will cause an error.
  • Assignment Errors: Strict mode throws an error for assignments to read-only properties, non-existent properties, or the this keyword used in an inappropriate context.
  • Reserved Keywords: Certain keywords are reserved for future use and cannot be used as identifiers in strict mode.

Additional Information

  • Strict Mode in ES6: In ES6 and later, strict mode can also be applied to modules and classes automatically, without the need for "use strict";.
  • Limitations: Strict mode does not affect existing code and is only enforced in the blocks where it is explicitly declared.