admin管理员组

文章数量:1178539

I have seen anonymous functions inside for loops to induce new scope on the web in one or two places and would like to know if it makes sense.

for example:

var attr, colors = ['green','blue','red'];

for ( attr = 0; attr < colors.length; attr++) {
    (function() {
        var colorAttr = colors[attr];

        // do something with colorAttr
    })();
}

I understand it has something to do with keeping the scope inside the for loop clean, but in what situations would this be necessary? Would it be good practice to do this everywhere you need to declare a new var inside the for loop?

I have seen anonymous functions inside for loops to induce new scope on the web in one or two places and would like to know if it makes sense.

for example:

var attr, colors = ['green','blue','red'];

for ( attr = 0; attr < colors.length; attr++) {
    (function() {
        var colorAttr = colors[attr];

        // do something with colorAttr
    })();
}

I understand it has something to do with keeping the scope inside the for loop clean, but in what situations would this be necessary? Would it be good practice to do this everywhere you need to declare a new var inside the for loop?

• javascript
• for-loop
• closures

Share Improve this question Follow asked Dec 20, 2012 at 17:02 EvanEvan 6,11599 gold badges3535 silver badges6464 bronze badges 7

• Here is an example where closure in loops make sense: stackoverflow.com/questions/1552941/… – Salman Arshad Commented Dec 20, 2012 at 17:05
• 1 attr changes as the loop progresses, so if you use colors[attr] in a callback function, it won't refer to the colors[attr] that you actually want. – Blender Commented Dec 20, 2012 at 17:05
• I use async calls only when waiting for something to complete. In this case, it may be problematic to track the colors[] changes. – mkey Commented Dec 20, 2012 at 17:07
• so does a pattern like this make more sense if you are attaching an event handler inside? – Evan Commented Dec 20, 2012 at 17:08
• This isn't a callback - it is an immediately executing function. colorAttr is scoped to that immediately executing function, but it can walk up to the parent scope and happily access attr. – Fenton Commented Dec 20, 2012 at 17:09

 |  Show 2 more comments

2 Answers 2

Sorted by: Reset to default 39

2021 Update

var used to be the only way to declare a variable. But we now have const and let which solve this problem in a better way. These variable declarations do respect the loop as a scope to bind to, which means the following snippet works fine and there is no need for an anonymous function to capture those values.

const colors = ['green', 'blue', 'red'];

for (let i = 0; i < colors.length; i++) {
    const color = colors[i];
    setTimeout(function() {
        alert(color);
    }, i * 1000);
}

What follows below is my original answer to this question from 2012.

---

When you have inner functions that are not executed immediately, as part of the loop.

var i, colors = ['green', 'blue', 'red'];

for (i = 0; i < colors.length; i++) {
    var color = colors[i];
    setTimeout(function() {
        alert(color);
    }, i * 1000);
}

// red
// red
// red

Even though var color is inside the loop, loops have no scope. You actually only have one variable that every loop iteration uses. So when the timeouts fire, they all use the same value, the last value set by the loop.

var i, colors = ['green', 'blue', 'red'];

for (i = 0; i < colors.length; i++) {
    (function(color) {
        setTimeout(function() {
            alert(color);
        }, i * 1000);
    })(colors[i]);
}

// green
// blue
// red

This one captures the value at each iteration into an argument to a function, which does create a scope. Now each function gets it's own version of a color variable which won't change when functions created within that loop are later executed.

You're almost there. It makes only sense in your snippet, if you pass in the attr value into your self invoking function as an argument. That way, it can store that variable within its very own scope object

(function( attr ) {
    var colorAttr = colors[attr];

    // do something with colorAttr
})( attr );

Now, the activation object respectively lexical environment record (which are ES3 and ES5 scope objects) will have an entry for whatever value is behind attr and therefore, its closured.

本文标签： using anonymous function in javascript for loopsStack Overflow