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.

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.

• javascript
• functional-programming
• map-function

Share Improve this question Follow edited Mar 6, 2022 at 22:19 Sebastian Simon 19.5k88 gold badges6161 silver badges8484 bronze badges asked Nov 18, 2013 at 18:29 Kevin MeredithKevin Meredith 41.9k6767 gold badges220220 silver badges390390 bronze badges 3

• Note, I updated to include var i = 0; per a user's ment. Thanks to that user, but still getting no output. jsfiddle/FgdSj/1 – Kevin Meredith Commented Nov 18, 2013 at 18:32
• 3 You never call map() in your fiddle – carter Commented Nov 18, 2013 at 18:39
• As an aside - I meant to originally create this - jsfiddle/9U2GK/1 – Kevin Meredith Commented Nov 18, 2013 at 20:00

Add a ment  | 

4 Answers 4

Sorted by: Reset to default 3

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/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/FgdSj/5/

arr.prototype.map(makeKv);

should be

arr.map(makeKv);

Now you have another issue since it will return

[ { item : 0}, { item : 1} ]

If you change the mapped function to

function makeKv(item) {
    var x = {}
    x[item.name] = i++;        
    return x;
};

it would give you what you want.

JSFiddle

Just call .map directly

arr.map(makeKv)

I, for whatever reason (maybe map is overridden), you want to use the Array.prototype's method

[].map.call(arr, makeKv);

---

Here's, it's all fixed up for you to match your desired output

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

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

    return arr.map(makeKv);
}

var result = map([{name: "Kevin"}, {name: "Bob"}], "name");
console.log(result);
// [{"Kevin" : 0}, {"Bob" : 1}];

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

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

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

map();