Dynamically creating keys in JavaScript associative array

Dynamically creating keys in JavaScript associative array

How can I dynamically create keys in javascript associative arrays?
All the documentation I’ve found so far is to update keys that are already created:
arr[‘key’] = val;

I have a string like this ” name = oscar ”
And I want to end up with something like this:
{ name: ‘whatever’ }

That is I split the string and get the first element, and I want to put that in a dictionary.
var text = ‘ name = oscar ‘
var dict = new Array();
var keyValuePair = text.split(‘ = ‘);
dict[ keyValuePair[0] ] = ‘whatever’;
alert( dict ); // prints nothing.


Solution 1:

Use the first example. If the key doesn’t exist it will be added.

var a = new Array();
a['name'] = 'oscar';

Will pop up a message box containing ‘oscar’.


var text = 'name = oscar'
var dict = new Array()
var keyValuePair = text.replace(/ /g,'').split('=');
dict[ keyValuePair[0] ] = keyValuePair[1];
alert( dict[keyValuePair[0]] );

Solution 2:

Somehow all examples, while work well, are overcomplicated:

  • They use new Array(), which is an overkill (and an overhead) for a simple associative array (AKA dictionary).
  • The better ones use new Object(). Works fine, but why all this extra typing?

This question is tagged “beginner”, so let’s make it simple.

The uber-simple way to use a dictionary in JavaScript or “Why JavaScript doesn’t have a special dictionary object?”:

// create an empty associative array (in JavaScript it is called ... Object)
var dict = {};   // huh? {} is a shortcut for "new Object()"

// add a key named fred with value 42
dict.fred = 42;  // we can do that because "fred" is a constant
                 // and conforms to id rules

// add a key named 2bob2 with value "twins!"
dict["2bob2"] = "twins!";  // we use the subscript notation because
                           // the key is arbitrary (not id)

// add an arbitrary dynamic key with a dynamic value
var key = ..., // insanely complex calculations for the key
    val = ...; // insanely complex calculations for the value
dict[key] = val;

// read value of "fred"
val = dict.fred;

// read value of 2bob2
val = dict["2bob2"];

// read value of our cool secret key
val = dict[key];

Now let’s change values:

// change the value of fred
dict.fred = "astra";
// the assignment creates and/or replaces key-value pairs

// change value of 2bob2
dict["2bob2"] = [1, 2, 3];  // any legal value can be used

// change value of our secret key
dict[key] = undefined;
// contrary to popular beliefs assigning "undefined" does not remove the key

// go over all keys and values in our dictionary
for (key in dict) {
  // for-in loop goes over all properties including inherited properties
  // let's use only our own properties
  if (dict.hasOwnProperty(key)) {
    console.log("key = " + key + ", value = " + dict[key]);

Deleting values is easy too:

// let's delete fred
delete dict.fred;
// fred is removed, the rest is still intact

// let's delete 2bob2
delete dict["2bob2"];

// let's delete our secret key
delete dict[key];

// now dict is empty

// let's replace it, recreating all original data
dict = {
  fred:    42,
  "2bob2": "twins!"
  // we can't add the original secret key because it was dynamic,
  // we can only add static keys
  // ...
  // oh well
  temp1:   val
// let's rename temp1 into our secret key:
if (key != "temp1") {
  dict[key] = dict.temp1; // copy the value
  delete dict.temp1;      // kill the old key
} else {
  // do nothing, we are good ;-)

Solution 3:

Javascript does not have associative arrays, it has objects.

The following lines of code all do exactly the same thing – set the ‘name’ field on an object to ‘orion’.

var f = new Object(); f.name = 'orion';
var f = new Object(); f['name'] = 'orion';
var f = new Array(); f.name = 'orion';
var f = new Array(); f['name'] = 'orion';
var f = new XMLHttpRequest(); f['name'] = 'orion';

It looks like you have an associative array because an Array is also an Object – however you’re not actually adding things into the array at all, you’re setting fields on the object.

Related:  Concatenating strings with `if` statements in JavaScript

Now that that is cleared up, here is a working solution to your example

var text = '{ name = oscar }'
var dict = new Object();

// Remove {} and spaces
var cleaned = text.replace(/[{} ]/g, '');

// split into key and value
var kvp = cleaned.split('=');

// put in the object
dict[ kvp[0] ] = kvp[1];
alert( dict.name ); // prints oscar.

Solution 4:

In response to MK_Dev, one is able to iterate, but not consecutively. (For that obviously an array is needed)

Quick google search brings up hash tables in javascript

Example code for looping over values in a hash (from aforementioned link):

var myArray = new Array();
myArray['one'] = 1;
myArray['two'] = 2;
myArray['three'] = 3;

// show the values stored
for (var i in myArray) {
    alert('key is: ' + i + ', value is: ' + myArray[i]);

Solution 5:

Original Code (I added the line numbers so can refer to them):

1  var text = ' name = oscar '
2  var dict = new Array();
3  var keyValuePair = text.split(' = ');
4  dict[ keyValuePair[0] ] = 'whatever';
5  alert( dict ); // prints nothing.

Almost there…

  • line 1: you should do a trim on text so it is name = oscar.
  • line 3: okay as long as you ALWAYS have spaces around your equal.
    might be better to not trim in line 1, use = and trim each keyValuePair
  • add a line after 3 and before 4:

    key = keyValuePair[0];`
  • line 4: Now becomes:

    dict[key] = keyValuePair[1];
  • line 5: Change to:

    alert( dict['name'] );  // it will print out 'oscar'

What I’m trying to say is that the dict[keyValuePair[0]] does not work, you need to set a string to keyValuePair[0] and use that as the associative key. That is the only way I got mine to work. After you have set it up you can either refer to it with numeric index or key in quotes.

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Hope that helps.

Solution 6:

All modern browsers support a Map, which is a key/value data stricture. There are a couple of reasons that make using a Map better than Object:

  • An Object has a prototype, so there are default keys in the map.
  • The keys of an Object are Strings, where they can be any value for a Map.
  • You can get the size of a Map easily while you have to keep track of size for an Object.


var myMap = new Map();

var keyObj = {},
    keyFunc = function () {},
    keyString = "a string";

myMap.set(keyString, "value associated with 'a string'");
myMap.set(keyObj, "value associated with keyObj");
myMap.set(keyFunc, "value associated with keyFunc");

myMap.size; // 3

myMap.get(keyString);    // "value associated with 'a string'"
myMap.get(keyObj);       // "value associated with keyObj"
myMap.get(keyFunc);      // "value associated with keyFunc"

If you want keys that are not referenced from other objects to be garbage collected, consider using a WeakMap instead of a Map.