Are Anonymous Functions Always Closures in JavaScript?

Closures vs. Anonymous Functions in JavaScript

JavaScript closures have become a subject of confusion for some developers. While all JavaScript functions are closures, a specific subset is of particular theoretical interest. This article aims to clarify the concept and determine which of two code blocks accurately utilizes closures.

Understanding Closures:

A closure is a function that captures and maintains access to variables from its enclosing scope, even after that scope is no longer active. In essence, a closure is a function that has a memory and can access data from its parent scope, known as "parent scope variables" or "upvalues."

Identifying Closures:

To determine if a function is a closure, we must examine its variables. Functions that have no non-local variables (i.e., freely variable variables) cannot be closures. Functions with non-local variables must be referenced from outside their parent scope to become closures.

Case 1: Friend's Program

<code class="javascript">for(var i = 0; i < 10; i++) {
    (function(){
        var i2 = i;
        setTimeout(function(){
            console.log(i2);
        }, 1000)
    })();
}</code>

In this case, the function lacks any closed-over parent scope variables and is therefore not a closure. However, the inner function passed to setTimeout is a closure because it captures and maintains access to the free variable i2.

Case 2: Author's Program

<code class="javascript">for(var i = 0; i < 10; i++) {
    setTimeout((function(i2){
        return function() {
            console.log(i2);
        }
    })(i), 1000);
}</code>

Here, the inner function maintains access to the free variable i2 and remains a closure even when referenced outside its parent scope.

Conclusion:

In both cases, closures are utilized. The author's program employs a closure to properly capture the value of i. By returning another function and passing it to setTimeout, the inner function can access the closed-over variable i2. This ensures that the correct value is printed to the console after the delay.

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