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Home Interviews What is a closure in JavaScript? Provide an example. – MindStick
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1 Answers

Ravi Vishwakarma
Ravi Vishwakarma 17 Jun 2024
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A closure in JavaScript is a feature where an inner function has access to its outer (enclosing) function's variables even after the outer function has finished executing. Closures are created whenever a function is created, allowing the function to remember the scope in which it was created.

Key Characteristics of Closures:

  1. Access to Outer Function’s Variables: The inner function can access variables and parameters of its outer function even after the outer function has returned.
  2. Private Data: Closures can be used to create private variables or methods.

Example of a Closure:

function outerFunction() {
    let outerVariable = 'I am an outer variable';

    function innerFunction() {
        console.log(outerVariable); // Can access the outer variable
    }

    return innerFunction;
}

const closure = outerFunction(); // outerFunction returns innerFunction, which has a reference to outerVariable
closure(); // Logs: 'I am an outer variable'

In this example:

  • outerFunction declares a variable outerVariable and defines an innerFunction.
  • innerFunction has access to outerVariable because it is within the scope of outerFunction.
  • When outerFunction is called, it returns innerFunction. The innerFunction maintains access to outerVariable even after outerFunction has completed execution.
  • Calling closure() (which is the returned innerFunction) logs the value of outerVariable.

Practical Example :

<script type="text/javascript">
	function createCounter() {
	    let count = 0; // Private variable

	    return {
	        increment: function() {
	            count++;
	            return count;
	        },
	        decrement: function() {
	            count--;
	            return count;
	        },
	        getCount: function() {
	            return count;
	        }
	    };
	}

	const counter = createCounter();

	console.log(counter.increment()); 
	console.log(counter.increment()); 
	console.log(counter.decrement()); 
	console.log(counter.getCount()); 
	console.log(counter.count); 

</script>

Output:

Ravi Vishwakarma

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