Closures

Closures

Use case

var nodes = document.getElementsByTagName('button');
for (var i = 0; i < nodes.length; i++) {
   nodes[i].addEventListener('click', function() {
      console.log('You clicked element #' + i);
   });
}

The code prints two times You clicked element #NODES_LENGTHwhere NODES_LENGTH is the number of the nodes retrieved.

The value of the variables in closures isn’t static, hence the value of i isn’t the value at the time the handler was added (0 for the first button in the list, 1 for the second, and so on). At the time the handler will be executed, on the console will be printed the current value of the variable i, that is equal to the length of the nodes list.

Solutions

First solution: use an IIFE to create another closure so that the value of i will be the one expected:

var nodes = document.getElementsByTagName('button');
for (var i = 0; i < nodes.length; i++) {
   nodes[i].addEventListener('click', (function(i) {
      return function() {
         console.log('You clicked element #' + i);
      }
   })(i));
}

Second solution: move the function outside the loop. :

function handlerWrapper(i) {
   return function() {
      console.log('You clicked element #' + i);
   }
}

var nodes = document.getElementsByTagName('button');
for (var i = 0; i < nodes.length; i++) {
   nodes[i].addEventListener('click', handlerWrapper(i));
}