为什么说xxx不是函数

Question

why is this not ok?

aContract = function(){};
aContract.prototype = {
    someFunction: function() {
        alert('yo');
    },
    someOtherFunction: some$Other$Function
};

var some$Other$Function = function() {
    alert('Yo yo yo');
};

var c = new aContract();
c.someFunction();
c.someOtherFunction();

Firebug says c.someOtherFunction is not a function

But this works just fine

aContract = function(){};
aContract.prototype = {
    someFunction: function() {
        alert('yo');
    },
    someOtherFunction: some$Other$Function
};

function some$Other$Function() {
    alert('Yo yo yo');
};

var c = new aContract();
c.someFunction();
c.someOtherFunction();

What am I missing here??? I prefer to code in javascript using the first method, which usually works fine, but doesn't seem to work correctly when I prototype.

Thanks, ~ck in Sandy Eggo

Answer 1

At the time this is evaluated:

 aContract.prototype = { ... }

this has not yet been evaluated:

var some$Other$Function = function() { ... }

Thus aContract.prototype.someOtherFunction is set to undefined .

The reason the second works is because function declarations (which the second is, the first is an expression) are evaluated before any other statements. There are more details here: Named function expressions demystified

Answer 2

You have assigned some$Other$Function to aContract.prototype.someOtherFunction before you actually create some$Other$Function . The order of statements matters. If you switch the order of things you'll be good:

var some$Other$Function = function() {
    alert('Yo yo yo');
};

aContract = function(){};
aContract.prototype = {
    someFunction: function() {
        alert('yo');
    },
    someOtherFunction: some$Other$Function
};

Answer 3

It's due to hoisting. Function statements are moved to the top of their scope.

Edit: Verification Crockford's JavaScript: The Good Parts page 113.

Answer 4

It appears that, in the global scope, var works differently than it does in function-local scopes. In function-local scopes, var -declared variables are lifted to the top of the function (ie, any variable declared anywhere within a function with the keyword var is available anywhere else within that function). But in the global scope, perhaps only use of the keyword function to declare a variable yields the same result (ie, that a variable declared with the keyword function will be available anywhere within that global scope, even in lines preceding that declaration).