JavaScript “map” function

Question

Looking at the map method 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);
};

JSFiddle

Please also help me get rid of the global, too.

Answer 1

There are a few issues here:

First,

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

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.

Second,

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

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.

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'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. 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);

So, all together, try:

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);
}

DEMO: http://jsfiddle.net/FgdSj/3/

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);
}

DEMO: http://jsfiddle.net/FgdSj/5/