Tutorial :Why aren't these two bits of JavaScript equivalent?



Question:

in jquery 1.4.2, ff 3.6.6:

The following code produces three divs, which write messages to the firebug console as you would expect. However, if you uncomment out the loop and comment out the 3 lines doing it manually, it doesn't work - mousing over any of the divs results in "three" being written to the console.

Why are these two methods any different than each other? In each one you use a selector to find the element and add an event to it.

<head>  <script type="text/javascript" src="/media/js/jquery.js"></script>  <script>    $( document ).ready( function() {      $("#one").mouseenter(function(){console.log("one")})    $("#two").mouseenter(function(){console.log("two")})    $("#three").mouseenter(function(){console.log("three")})      //  names=['one','two','three'];    //  for (k in names){    //    id=names[k]    //    $("#"+id).mouseenter(function(){console.log(id)})    //  }  })  </script>  </head>    <body>    <span id="one">ONE</span>    <p><span id="two">TWO</span></p>    <p><span id="three">THREE</span></p>  </body>  


Solution:1

You would be having a very common closure problem in the for in loop.

Variables enclosed in a closure share the same single environment, so by the time the mouseenter callback is called, the loop will have run its course and the id variable will be left pointing to the value of the last element of the names array.

This can be quite a tricky topic, if you are not familiar with how closures work. You may want to check out the following article for a brief introduction:

You could solve this with even more closures, using a function factory:

function makeMouseEnterCallback (id) {      return function() {        console.log(id);    };    }    // ...    var id, k,      names = ['one','two','three'];    for (k = 0; k < names.length; k++) {    id = names[k];    $("#" + id).mouseenter(makeMouseEnterCallback(id));  }  

You could also inline the above function factory as follows:

var id, k,       names = ['one','two','three'];    for (k = 0; k < names.length; k++) {    id = names[k];    $("#" + id).mouseenter((function (p_id) {        return function() {          console.log(p_id);      };      })(id));  }  

Any yet another solution could be as @d.m suggested in another answer, enclosing each iteration in its own scope:

var k,       names = ['one','two','three'];    for (k = 0; k < names.length; k++) {    (function() {      var id = names[k];      $("#" + id).mouseenter(function () { console.log(id) });    })();  }  

Although not related to this problem, it is generally recommended to avoid using a for in loop to iterate over the items of an array, as @CMS pointed out in a comment below (Further reading). In addition, explicitly terminating your statements with a semicolon is also considered a good practice in JavaScript.


Solution:2

This happens because when the code of the anonymous function function(){console.log(id)} is executed, the value of id is three indeed.

To can use the following trick to enclose the value on each loop iteration into the anonymous callback scope:

names=['one', 'two', 'three'];  for (var k in names) {      (function() {          var id = names[k];          $("#"+id).mouseenter(function(){ console.log(id) });      })();  }  

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