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Closure In Javascript


A closure is a mechanism in JavaScript where an inner function has access to variables in its outer (enclosing) function's scope, even after the outer function has finished executing. This allows the inner function to remember and access variables from the environment in which it was created, preserving the state of those variables.

Consider the following example:

function outerFunc(outerParam) {
  const outerVar = 'I am outside!';

  function innerFunc(innerParam) {
    const innerVar = 'I am inside!';
    console.log(outerParam, outerVar, innerParam, innerVar);

  return innerFunc;

const myInnerFunc = outerFunc('Hello');

In this code:

  • outerFunc is defined, taking a parameter outerParam.
  • Inside outerFunc, a variable outerVar is defined.
  • An inner function innerFunc is defined, which has access to both outerParam and outerVar from its outer scope.
  • outerFunc returns innerFunc, creating a closure that captures the variables outerParam and outerVar.
  • myInnerFunc is assigned the returned closure.
  • When myInnerFunc is called with the argument 'World', it can still access the values of outerParam and outerVar from its outer scope, even though outerFunc has finished executing.


Hello I am outside! World I am inside!

In this example, innerFunc forms a closure, allowing it to access outerParam and outerVar from its outer scope, even after outerFunc has completed execution. This behavior is possible because the closure "closes over" the variables from its outer scope, preserving their values for future use.

Closures are widely used in JavaScript for various purposes, such as data privacy, event handling, and creating private methods and variables in object-oriented programming.