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JavaScript Closures.(Count Local Variable in function)

A closure is a function having access to the parent scope, even after the parent function has closed.
The variable add is assigned the return value of a self-invoking function.
The self-invoking function only runs once. It sets the counter to zero (0), and returns a function expression.
This way add becomes a function. The "wonderful" part is that it can access the counter in the parent scope.
This is called a JavaScript closure. It makes it possible for a function to have "private" variables.
The counter is protected by the scope of the anonymous function, and can only be changed using the add function.
CODING:
<!DOCTYPE html>
<html>
<body>

<p>Counting with a local variable.</p>

<button type="button" onclick="myFunction()">Count!</button>

<p id="demo">0</p>

<script>
var add = (function () {
    var counter = 0;
    return function () {return counter += 1;}
})();

function myFunction(){
    document.getElementById("demo").innerHTML = add();
}
</script>

</body>
</html>

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