"variable:function(){}"是什么意思？

Question

在Node.JS示例代码中,我发现了一个代码块,如下所示.

var messageReceivedCallBack = {
    onMessageReceived: function (message) {
        console.log('Message received ' + message);
    }
};

这段代码意味着什么？具体来说,我无法理解带冒号的部分(':')

Answer 1

这是一个对象文字.这不是变量,它是属性名称:

var foo = {
   a: 1,
   b: 2
};

是相同的:

var foo = {};
foo.a = 1;
foo.b = 2;

同理:

var foo = {
    a: function () {}
};

是相同的:

var foo = {};
foo.a = function () {};

因此如下:

var messageReceivedCallBack = {
    onMessageReceived: function (message) {
        console.log('Message received ' + message);
    }
};

只是这样做:

var messageReceivedCallBack = {};

messageReceivedCallBack.onMessageReceived = function (message) {
    console.log('Message received ' + message);
};