My guidelines for understanding JavaScript closures are as follows:
In order for a JavaScript closure to be established, there must be inner functions nested within outer functions where the inner function has access to a variable in the outer function. If either of these conditions is not met, then a closure will not be created.
It's worth noting that memory leaks can occur when a variable from the outer function happens to be a DOM element.
Let's examine the example provided in the Mozilla article:
function addHandler() {
var el = document.getElementById('el');
el.onclick = function() {
this.style.backgroundColor = 'red';
};
}
This code clearly demonstrates a closure being formed with an inner function accessing a variable (el) from the outer scope which is a DOM element, leading to a memory leak as explained in the article.
Now, looking at the second example you mentioned:
function addHandler() {
var clickHandler = function() {
this.style.backgroundColor = 'red';
};
(function() {
var el = document.getElementById('el');
el.onclick = clickHandler;
})();
}
In this case, while there are two nested functions, they are at the same level within the outer function. The second nested function does create a closure by referencing the local variable clickHandler, but since this variable is not a DOM element, there is no memory leak concern. Additionally, the local variable el within the second nested function does not contribute to memory leaks because it is isolated within its own scope and not accessible to other functions, preventing any potential issues.