javascript - "variable : function(){}"是什么意思？

Question

在 Node.JS 示例代码中，我发现了如下代码块。

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

这段代码是什么意思？具体来说，我无法理解带有冒号(':')的部分

Answer 1

这是一个对象字面量。这不是一个变量，它是一个属性名称:

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

等同于:

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

同样:

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

等同于:

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);
};