不了解Javascript中的范围

Question

I don't quite understand this bit of code.

$('div').click((function () { 
    var number = 0;
    return function () {
        alert(++number);
    };
})());

My understanding is:

• An anonymous function is defined and assigned to the click handler.

• When I click on div, this function is invoked.

What the function does is:

• Define a variable number = 0

• Return ++number

So why does the number in alert increment every time I click? Shouldn't number be reset to 0 every time I click?

Answer 1

Here you've got a self-invoking function , which returns a function. Watch out for the brackets at the end:

(function () { 
    var number = 0;
    return function () {
        alert(++number);
    };
})()

So the callback of the click handler is only the returned inner function :

function () {
    alert(++number);
};

This inner function has access to the variable number , which is in the scope of the outer function.

So your code can also be written as follows:

function outerFunction() {
    var number = 0;
    return function () {
        alert(++number);
    };
};

var innerFunction = outerFunction();

$('div').click(innerFunction);

Answer 2

If we used (ugly) names for the anonymous functions, your code could be rewritten as:

$('div').click((function makeIncrementer() {
    var number = 0;
    return function incrementAndAlert() {
        alert(++number);
    };
})());

More verbose code retaining similar semantics would be:

var makeIncrementer = function() {
    var number = 0;
    return function() { alert(++number); };
};

var incrementAndAlert = makeIncrementer(); // function() { alert(++number); }

$('div').click(incrementAndAlert);

makeIncrementer is a function that, when called, defines a number variable in its scope, and returns a function - note that makeIncrementer doesn't increment, nor alert the number variable, instead it returns another function that does just that.

Now incrementAndAlert is bound to this returned function

function() { alert(++number); }

that captures makeIncrementer 's number variable, which enables it to keep number 's state between incrementAndAlert calls triggered by $('div') clicks.

Answer 3

This is not an "answer", but hopefully it will show a different way of looking at the problem.

---

First off, note that JavaScript functions are just objects and are thus just values that can be bound to variables. As such,

$('div').click((function () { 
    var number = 0;
    return function () {
        alert(++number);
    };
})());

can be, with a simple expression substitution, rewritten as

var f = function () { 
    var number = 0;
    return function () {
        alert(++number);
    };
};
// $('div').click((f)()); and when removing extra parenthesis ->
$('div').click(f());

Then, after one more simple expression substitution

var g = f()
$('div').click(g);

it should be clear that the (outer) function, f , is invoked first and it is the resulting (inner function) value, g , that is used as the handler.

Aside from introducing variables, the above substitutions are semantically equivalent to the original code.