100

Is there a way I can dynamically add data to a map in javascript. A map.put(key,value)? I am using the yui libraries for javascript, but didn't see anything there to support this.

Brian Tompsett - 汤莱恩
  • 5,753
  • 72
  • 57
  • 129
stevebot
  • 23,275
  • 29
  • 119
  • 181

4 Answers4

154

Well any Javascript object functions sort-of like a "map"

randomObject['hello'] = 'world';

Typically people build simple objects for the purpose:

var myMap = {};

// ...

myMap[newKey] = newValue;

edit — well the problem with having an explicit "put" function is that you'd then have to go to pains to avoid having the function itself look like part of the map. It's not really a Javascripty thing to do.

13 Feb 2014 — modern JavaScript has facilities for creating object properties that aren't enumerable, and it's pretty easy to do. However, it's still the case that a "put" property, enumerable or not, would claim the property name "put" and make it unavailable. That is, there's still only one namespace per object.

Pointy
  • 405,095
  • 59
  • 585
  • 614
  • 1
    Right, I mean a javascript "map". how would you build a myMap.put() function? – stevebot Jun 03 '10 at 14:49
  • 2
    @stevebot: Doesn't the last line in his post do exactly that? – Matti Virkkunen Jun 03 '10 at 14:58
  • Okay, I hear you. I can just do myMap[anyKey] = anyValue and this works for me. thanks! – stevebot Jun 03 '10 at 15:06
  • How to put/add in the beginning of the map? – master_dodo Dec 01 '16 at 10:00
  • 2
    @user3241111 maps don't have a "beginning" – Pointy Dec 01 '16 at 14:01
  • @Pointy So, currently I am doing like this: mapA having ele1, ele2, ele3, and I've to add ele4 in the beginning. I am creating another map(mapB) having just ele4 and then creating 3rd map which is actually using mapB and mapA is the same order (not mapA and mapB).. I don't know if map has order or not but while iterating they always give the same order of elements, which makes me doubt if they really don't have order. – master_dodo Dec 04 '16 at 11:36
  • @user3241111 object properties have no guaranteed order when you iterate with `for ... in` or `Object.keys()`. JavaScript runtimes don't randomize property order for no reason, but it's risky to write code that relies on the ordering remaining consistent because it can definitely change if properties are added/removed. You can always keep your own array of property keys and maintain its order however you want, and then use it to iterate through property values as necessary. – Pointy Dec 04 '16 at 13:33
83

Javascript now has a specific built in object called Map, you can call as follows :

   var myMap = new Map()

You can update it with .set :

   myMap.set("key0","value")

This has the advantage of methods you can use to handle look ups, like the boolean .has

  myMap.has("key1"); // evaluates to false

You can use this before calling .get on your Map object to handle looking up non-existent keys

NiallJG
  • 1,881
  • 19
  • 22
  • 15
    An additional note: `myMap["key0"] = "value"` is NOT an alternative syntax for `myMap.set("key0","value")`, it "works", but probably does something most poeple are not looking for. – Akavall Sep 12 '18 at 05:25
  • 6
    @Akavall, one hour trying to figure out why Map.delete doesn't work and why it's size shows 0 even though the console output clearly shows the items are in there and I realize I put in the items just as you've mentioned. As you said, it 'works', but it's not what you want. – pwilcox Sep 30 '18 at 23:51
  • 1
    This is a proper map object in javascript. Please vote this – yue you Nov 26 '19 at 19:47
  • how would you set a map of map? do you have to get one then set the sub map? i guess – mike01010 Sep 02 '22 at 19:09
1

I like this way to achieve this

const M = new Map(Object.entries({ 
  language: "JavaScript" 
}));

console.log(M.size); // 1 
console.log(...M); // ["language", "JavaScript"]

// (1) Add and update some map entries 
M.set("year", 1991); 
M.set("language", "Python");

console.log(M.size); // 2 
console.log(...M); // \["language", "Python"\] ["year", 1991]
Slava Vasylenko
  • 838
  • 1
  • 9
  • 14
1

In Typescript 

let ar = [1, 2, 3, 4, 5, 6];

 let map = new Map<number, string>();
    ar.forEach(value => {
      map.set(value, 'value'+ value);
    });
console.log(map, 'map data');
Kalana Weerarathne
  • 321
  • 4
  • 11