JavaScript“地图"功能 [英] JavaScript "map" Function

问题描述

查看地图功能在JavaScript中，我在这里做什么错了?

Looking at the map function in JavaScript, what am I doing wrong here?

// input: [{name: "Kevin"}, {name: "Bob"}];
// output: [{"Kevin" : 0}, {"Bob" : 1}];
var map = function(arr, property) { 

    var i = 0;        
    var m = arr.prototype.map(makeKv);

    // input: {name: "Kevin"}
    // output: {"Kevin" = i} // GLOBAL
    function makeKv(item) {
        return {item: i++};
    };

    console.log("m : " + m);
}

http://jsfiddle.net/FgdSj/2

此外，如果您能帮助我，请也摆脱 global .

Also, if you could help please me get rid of the global too.

推荐答案

这里有一些问题:

首先

var m = arr.prototype.map(makeKv);

您在这里不需要 prototype .仅在使用构造函数时使用它，例如 Array.prototype.map .在这里，您只需要执行 arr.map .

You don't need prototype here. You only use that when you are using the constructor, like Array.prototype.map. Here, you just need to do arr.map.

第二，

function makeKv(item) {
    return {item: i++};
};

您永远不会在任何地方声明 i .如何将一个添加到不存在的对象中.在此之前，您需要具有 var i = 0; .

You never declare i anywhere. How can you add one to something that doesn't exist. You need to have var i = 0; before this.

最后， return {item:i ++}; 将创建一个名为 literally "item" 的键.您需要先声明对象( var ret = {}; )，然后使用 [item] 设置值.

Finally, return {item: i++}; will make a key called literally "item". You need to declare the object first (var ret = {};), then use [item] to set the value.

Array.map 的回调函数将数组中的元素作为第一个参数传递，因此 item 将是一个对象.您需要执行 item [property] 来获取所需的值.

Array.map's callback is passed the element in the array as the 1st parameter, so item will be an object. You need to do item[property] to get the value you want.

P.S.不要在您的 console.log 中执行"m:" + m ，这会连接字符串，从而将 m 转换为字符串.使用，代替: console.log("m:"，m);

P.S. Don't do "m : " + m in your console.log, that will concat strings, thus converting m to a string. Use , instead: console.log("m : ", m);

因此，一起尝试:

var map = function(arr, property) { 
    var i = 0;        
    var m = arr.map(makeKv);

    function makeKv(item) {
        var ret = {};
        ret[item[property]] = i++;
        return ret;
    };

    console.log("m : ", m);
}

演示: http://jsfiddle.net/FgdSj/3/

编辑: Array.map 的回调传递了数组中的索引作为第二个参数，因此 var i = 0; 这里不需要:

EDIT: Array.map's callback is passed the index in the array as the 2nd parameter, so var i = 0; isn't needed here:

var map = function(arr, property) {      
    var m = arr.map(makeKv);

    function makeKv(item, index) {
        var ret = {};
        ret[item[property]] = index;
        return ret;
    };

    console.log("m : ", m);
}

演示: http://jsfiddle.net/FgdSj/5/

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