What is a “closure” in JavaScript? Provide an example.
A closure is an inner function that has access to the outer (enclosing) function’s variables—scope chain. The closure has three scope chains: it has access to its own scope (variables defined between its curly brackets), it has access to the outer function’s variables, and it has access to the global variables.
var add = (function () {
// this is the "self invoking" function that is only run once.
// it sets the "counter" variable to 0 and returns a function
// this return function gets assigned to the "add" variable, which makes "add" a function that can be called with add();
var counter = 0;
function private_function(){ alert('foo'); }
return function () {
// this is the "closure". it has access to the private variables defined in the "self invoking" function above.
return counter += 1;
}
})();
add();
add();
add();
// the counter is now 3
private_function(); // doesn't work because we don't have access to the private scope within the "add" variable, only the closure can access this private function.
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