closure is a function bound to one or more external variables
An example of this concept is that the function bar is bound to the external variables x, y, and z:
function foo(x, y) {
var z = 3;
return function bar(a, b, c) {
return (a + b + c) * (x + y + z);
};
}
var closure = foo(1, 2);
closure(5, 6, 7); // (5 + 6 + 7) * (1 + 2 + 3) = 24
The variable closure
refers to the inner function bar
returned from the call to foo
. Invoking closure
is like reentering the scope within foo
, which gives visibility into all of foo
's local variables and parameters.
even after the parent function has terminated
This means that after foo
is executed, the returned function stored in the closure
variable persists the state of foo
. You can even create multiple independent closures by invoking foo
again:
var closure = foo(1, 2);
closure(5, 6, 7); // (5 + 6 + 7) * (1 + 2 + 3) = 24
var closure2 = foo(0, 0);
closure2(5, 6, 7); // (5 + 6 + 7) * (0 + 0 + 3) = 21
/* closure2 does not affect the other closure */
closure(5, 6, 7); // (5 + 6 + 7) * (1 + 2 + 3) = 24
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