Question

Is there a definite source on variable capture in Javascript besides the standard (it’s a pain to read the standard)?

In the following code i is copied by value:

for (var i = 0; i < 10; i++)
{
    (function (i)
    {
        process.nextTick(function ()
        {
            console.log(i)
        })
    }) (i)
}

So it prints 1..10. process.nextTick is an analog of setTimeout(f,0) in node.

But in the next code i doesn’t seem to be copied:

for (var i = 0; i < 10; i++)
{
        var j = i
        process.nextTick(function ()
        {
            console.log(j)
        })
}

It prints 9 10 times. Why? I’m more interested in a reference/general article than in explaining this concrete case of capture.

Answer 1

I don’t have a handy reference. But the bottom line is: In the first, you’re explicitly passing in i to an anonymous function, which creates a new scope. You are not creating a new scope for either i or j in the second. Also, JavaScript always captures variables, not values. So you would be able to modify i too.

The JavaScript var keyword has function scope, not block scope. So a for loop does not create a scope.

As a note, the non-standard let keyword has local scope.