Mark Needham

Thoughts on Software Development

Javascript: Function scoping

with 6 comments

My colleague John Hume wrote an interesting post about his experience with the ‘const’ keyword in ActionScript where he describes the problems with trying to capture a loop variable in a closure and then evaluating it later on in the code.

Since ActionScript and JavaScript are both dialects of ECMAscript, this is a problem in JavaScript as well, and is due to the fact that variables in JavaScript have function scope rather than block scope which is the case in many other languages.

This problem would tend to reveal itself in code where we try to capture a loop variable in an anonymous function and use it later on, like so:

function getValues() {
    var x = new Array();
    for(var i=0; i < 10; i++) {
       x[i] = function() { return i; }
    }
    return x;
};
 
var values = getValues();
for(var j=0; j < values.length; j++) {
    console.log(values[j]());
}

We might expect that to print the sequence of numbers 0-9 on the screen but what we actually get is ’10’ printed 10 times.

There are a couple of things that I initially found strange about this:

  1. Why doesn’t it print out the numbers 0-9?
  2. Given that it doesn’t do that why does it print out ’10’ 10 times instead of ‘9’ 10 times?

The answer to the first question is that ‘i’ gets assigned a new value on each iteration of the loop and we don’t evaluate ‘i’ until we evaluate the anonymous function on line 11.

The value when we do evaluate it would be the last value that it was set to by the loop which in this case that would be ’10’ because that’s the value that ‘i’ has to be in order for the loop to terminate.

This is actually a problem in C# as well – the following code will output ’10’ 10 times as well:

[Test]
public void ClosureOnTheSameValue()
{
    var values = new List<Func<int>>();
    for(int i=0; i < 10; i++)
    {
        values.Add(() => i);
    }
 
    foreach (var value in values)
    {
        Console.WriteLine(value());
    }
}

Again we capture ‘i’ inside a closure and since we only evaluate that value when it’s actually used it will always refer to the last value that ‘i’ was set to which in this case means that it will always output a value of 10.

To fix this in C# we could just create a temporary variable – something which Resharper will actually suggest to us:

[Test]
public void ClosureOnDifferentValue()
{
    var values = new List<Func<int>>();
    for(int i=0; i < 10; i++)
    {
        var idash = i;
        values.Add(() => idash);
    }
 
    foreach (var value in values)
    {
        Console.WriteLine(value());
    }
}

This works in C# because variables have block scope which means that we have a new version of ‘idash’ for each of the functions that we add to the ‘values’ collection.

Sadly the same trick doesn’t work in JavaScript because variables have function scope in Javascript:

function getValues() {
    var x = new Array();
    for(var i=0; i < 10; i++) {
       var idash = i;
       x[i] = function() { return idash; }
    }
    return x;
};
 
var values = getValues();
for(var j=0; j < values.length; j++) {
    console.log(values[j]());
}

The ‘idash’ temporary variable that we created to try and solve the problem gets assigned a new value in each iteration of the loop because that variable is only declared once for the whole function.

The code above could be written like this to make that clearer:

function getValues() {
    var x = new Array();
    var idash;
 
    for(var i=0; i < 10; i++) {
       idash = i;
       x[i] = function() { return idash; }
    }
    return x;
};
 
var values = getValues();
for(var j=0; j < values.length; j++) {
    console.log(values[j]());
}

As John points out:

Here’s something I either never knew or at some point forgot about JavaScript: variables are lexically scoped, but only function bodies introduce new lexical scopes.

In this case we actually end up printing ‘9’ 10 times because that’s the maximum value that gets assigned to ‘idash’.

One solution is to create a temporary variable inside an anonymous function that we execute immediately, like this:

function getValues() {
    var x = new Array();
    for(var i=0; i < 10; i++) {
        (function() {
            var idash = i;
            x[i] = function() { return idash; } })();
    }
    return x;
};
 
var values = getValues();
for(var j=0; j < values.length; j++) {
    console.log(values[j]());
}

Now ‘idash’ is scoped inside the anonymous function and we therefore end up with a new value each time like we want.

Raph pointed out that we could achieve the same thing in a simpler way with the following code:

function getValues() {
    var x = new Array();
    for(var i=0; i < 10; i++) (function(i) {
        x[i] = function() { return i; };
    })(i);
    return x;
};
 
var values = getValues();
for(var j=0; j < values.length; j++) {
    console.log(values[j]());
}

Here we define a for loop with just a single statement so we can lose the ‘{}’ and just call an anonymous function passing in ‘i’.

Of course this example is truly contrived but I wanted to pick something simple enough that I could try and follow exactly how it worked.

I’m not entirely sure of the terminology around closures and scoping so if I’ve described anything incorrectly then please correct me!

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Written by Mark Needham

March 10th, 2010 at 11:06 pm

Posted in Javascript

Tagged with

  • steve

    Sadly, yes, that is the correct answer.

  • http://blog.felipel.com Felipe Lima

    Very cool post. Thanks a lot for clarifying this. I’ve caught myself striving with closures in js several times and never had the chance to realize that variables are function scoped, instead of block scoped.

  • Patrick

    Not only is it function scope but variables are hoisted. I personally find it helpful to adopt the one var per function style in JavaScript. Your own mileage may vary.

  • Thomas

    Unfortunately I must say: I read about this so many times, and it is such a well known topic (at least for experienced JS developers), I really wonder what this artice here is good for. But at least the article is written quite well.

  • simon

    yes, this stumbling block in JS is often mentioned.

    everyone uses the for-loop example, which i find highly contrived (though easy to grasp).

    A JS developer will always use Array.forEach(callback) anyways – solving the problem without ever encountering it.

  • http://twitter.com/JohnBubriski John Bubriski

    I think that you could further simplify the last code snippet to remove the inner-most function that returns a function for i, and instead just set it to the value (unless that was really the purpose):

    function getValues() {
    var x = new Array();
    for(var i=0; i < 10; i++) (function(i) {
    x[i] = i;
    })(i);
    return x;
    };
     
    var values = getValues();
    for(var j=0; j < values.length; j++) {
    console.log(values[j]);
    }

    Although I think I would prefer your original style, with my modifications:

    function getValues() {
    var x = new Array();
    for(var i=0; i < 10; i++) { (function(i) {
    x[i] = i;
    })(i); } return x;
    };
     
    var values = getValues();
    for(var j=0; j < values.length; j++) {
    console.log(values[j]);
    }