Loop through a 'Hashmap' in JavaScript

Question

I'm using this method to make artificial 'hashmaps' in javascript. All I am aiming for is key|value pairs, the actual run time is not important. The method below works fine.

Are there any other ways to loop through this?

for (var i in a_hashMap[i]) {
    console.log('Key is: ' + i + '. Value is: ' + a_hashMap[i]);
}

I run into a problem where this outputs a bunch of undefined keys after the first key, when the array only contains one entry. I have a feeling it is because the code is within a loop which uses i, even though when I follow in debug it shouldn't be happening. I also cannot change i as the for loop seems to not understand the replaced var at all.

Anyone any ideas?

Just watch this thread: [Javascript Hashmap Equivalent][1] Hope it helps you. [1]: stackoverflow.com/questions/368280/… — elvenbyte
– elvenbyte, Commented Jul 19, 2011 at 14:35

jhurshman · Accepted Answer · 2011-07-19 14:30:03Z

65

for (var i in a_hashmap[i])

is not correct. It should be

for (var i in a_hashmap)

which means "loop over the properties of a_hashmap, assigning each property name in turn to i"

answered Jul 19, 2011 at 14:30

jhurshman

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Comments

Raynos · Accepted Answer · 2011-07-19 14:31:12Z

12

for (var i = 0, keys = Object.keys(a_hashmap), ii = keys.length; i < ii; i++) {
  console.log('key : ' + keys[i] + ' val : ' + a_hashmap[keys[i]]);
}

answered Jul 19, 2011 at 14:31

Raynos

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1 Comment

abhy Over a year ago

This is neat way to loop through keys and values.

spraff · Accepted Answer · 2011-07-19 16:13:20Z

7

Do you mean

for (var i in a_hashmap) { // Or `let` if you're a language pedant :-)
   ...
}

i is undefined when the for-loop gets set up.

edited Jul 19, 2011 at 16:13

answered Jul 19, 2011 at 14:29

spraff

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4 Comments

spraff Over a year ago

Didn't think to check. I suppose you're right.

spraff Over a year ago

Downvote? let aside, this is the same as the accepted answer :-/

Nivas Over a year ago

+1 because this does not deserve a -1. @spraff, you might want to add an update (edit the answer) stating what you have stated in comments.

SJ00 Over a year ago

+1 for mentioning 'let'. This is best use case of let. Anyone down voting, probably ignored the fact, JS is used on servers as well.

GothamGirl · Accepted Answer · 2014-09-15 11:55:54Z

7

You can use JQuery function

$.each( hashMap, function(index,value){
 console.log("Index = " + index + " value = " + value); 
})

edited Sep 15, 2014 at 11:55

GothamGirl

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answered Jul 19, 2011 at 14:33

dchhetri

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Comments

Atmaram · Accepted Answer · 2015-06-30 10:33:41Z

6

Try this in order to print console correctly...

for(var i in a_hashMap) {
    if (a_hashMap.hasOwnProperty(i)) {
        console.log('Key is: ' + i + '. Value is: ' + a_hashMap[i]);
    }
}

answered Jun 30, 2015 at 10:33

Atmaram

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Comments

Binita Bharati · Accepted Answer · 2013-09-27 08:34:06Z

4

Iterating through a map in vanilla Javacsript is simple .

var map = {...};//your map defined here
for(var index in map)
 {
       var mapKey = index;//This is the map's key.
       for(i = 0 ; i < map[mapKey].length ; i++)
        {
              var mapKeyVal = map[mapKey];//This is the value part for the map's key.


          }
  }

edited Sep 27, 2013 at 8:34

answered Sep 27, 2013 at 5:56

Binita Bharati

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Comments

Ketu · Accepted Answer · 2018-04-20 13:27:42Z

2

This is an old post, but one way I can think of is

const someMap = { a: 1, b: 2, c: 3 };
Object.keys(someMap)
.map(key => 'key is ' + key + ' value is ' + someMap[key]);

Should this way of iterating be used? Are there any issues with this approach?

answered Apr 20, 2018 at 13:27

Ketu

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Comments

Matt · Accepted Answer · 2021-07-22 16:19:14Z

2

var a_hashMap = {a:1,b:2,c:3};

for (var key in a_hashMap) {
    console.log('Key: ' + key + '. Value: ' + a_hashMap[key]);
}

answered Jul 22, 2021 at 16:19

Matt

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Comments

Khushal Vyas · Accepted Answer · 2021-06-16 12:28:55Z

1

For lopping through a Hashmap you need to fetch the keys and values.

const new_Map = new Map();

for (const [key, value] of new_Map.entries()) {
   console.log(`The key is ${key} and value is ${value}`);
}

It should work with keys and values of hashmap in key and value.

answered Jun 16, 2021 at 12:28

Khushal Vyas

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Comments

bhuvnesh kumar sharma · Accepted Answer · 2023-05-03 11:44:44Z

1

many of the answer are using for-in but it didn't work for me in nodejs. there are below methods which you can use to iterate over the Hashmap. Injavascript hashmap store key-value pair in series so what key-value stored first will come first and last will come in the last when iterating

let map = new Map();
map.set(2,"two");
map.set(1,"one");
map.set(3,"three");

// in this map when you iterate keys will will in order 2,1,3

below methods will work in iteration of a Hashmap

// iterate by using keys of map

let map = new Map();
map.set(1,true);
map.set(2,false);
map.set(3,true);
map.set(4,true);

let keys = map.keys();

    for(let key of keys) console.log(`for key ${key} the value is : ${map.get(key)}`)

// to print value only without key

     let map = new Map();
     map.set(1,"one");
     map.set(2,"two");
     map.set(3,"three");
 
     for(let value of map.values()) console.log(`value is ${value}`)

 // by usign key value pairs using entries 

    let map = new Map();
    map.set(1,true);
    map.set(2,false);
    map.set(3,true);
    map.set(4,true);
    // Object.entries() return a array of pair and pair is array of key and value
    for(let pair of map.entries()) console.log(`for key ${pair[0]} the value is ${pair[1]}`)
    /* you can use [key,value] insted of pair by destructuring pair*/

// by using forEach for keys and value

      let map = new Map();
      map.set(1,true);
      map.set(2,false);
      map.set(3,true);
      map.set(4,true);
      
      map.forEach((val,key)=>{ console.log(`for the key ${key} the value is ${val}`)})

edited May 3, 2023 at 11:44

answered May 3, 2023 at 3:45

bhuvnesh kumar sharma

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1 Comment

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