This is the shortest article about JS closures.

Function A () {var I = 0;} ();

Each time a executes, a new execution environment is created and the variable I is created in this environment. That is to say, each time I is different. This is called the closure of function a on variable I.

Such CodeIs a typical code without a closure:

For (VAR I = 0; I <9; I ++) {A. childnodes [I]. onclick = function () {alert (I );};}

It has a problem, that is, there is only one variable I, and the final output is the current I (= 9 ). To save the I during execution, use the closure. Make each I different

For (VAR I = 0; I <9; I ++) {var fn = function (m) {var I = m; // use FN to save I. Return function () {alert (I); // we have used the closure to save the variable I};. childnodes [I]. onclick = FN (I );}