Dynamically creating keys in a JavaScript associative array

Question

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, split the string and get the first element, and then put that in a dictionary.

Code

var text = ' name = oscar '
var dict = new Array();
var keyValuePair = text.split(' = ');
dict[ keyValuePair[0] ] = 'whatever';
alert( dict ); // Prints nothing.

Fenton

Jump link to Eugene's answer for convenience – Salsbury

Answer 1

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

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

Will pop up a message box containing 'oscar'.

Try:

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

Petey

I ran that as a sample in Firefox just to be sure. Did you make sure to put 'name' in quotes? – Petey

Uhmm nope, because, I'm creating the key "dynamically" not statically. Let me doublecheck anyway :) – Fenton

Please refer to Danny's more complete explanation. You won't be able to refer to the array values in a for loop with an index (such as myarray[i]). Hope that's not too confusing. – Bellanca

Even better is to use an Object (bracket {} notation) to avoid the overhead of having .length, .slice(), etc which are included in the Array prototype – Toilette

Answer 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(). It works fine, but why all this extra typing?

This question is tagged "beginner", so let's make it simple.

The über-simple way to use a dictionary in JavaScript or "Why doesn't JavaScript 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 the 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) {
  // A 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, but 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, but
  // 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 ;-)
}

Twelfthtide

Hello, I know I am replying to old answer, but it ranks high on Google, so I will ask anyway. I am kind of confused at what "we can't add the original secret key because it was dynamic, we can only add static keys" means in your example. – Clearheaded

It means precisely what it says: we don't know its value, so we cannot represent it as a constant, which is required when specifying a key in an object literal. – Twelfthtide

However, "we can't add the original secret key because it was dynamic" is not correct by itself, even though you cannot use a variable as a key directly in {}, or as a key with dot-notation. We can still add a dynamic key through "dict[key] = val", as you show early in the example. The limitation is with using the {}-notation, rather than the key itself. – Unwieldy

This looks like Sheldon Cooper's answer :) – Heck

Answer 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.

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.

Canaliculus

Assuming the text string actually does have the curly braces, you could more or less treat it as JSON.. replace the = sign with a : and you've got an object to eval.. – Osteoporosis

Ooops, the string isn't delimited properly. Nothing regex can't fix. – Osteoporosis

Answer 4

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

A quick Google search brings up hash tables in JavaScript.

Example code for looping over values in a hash (from the 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]);
}

Virilism

Answer 5

The 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. It 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'
  

I'm trying to say 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.

Dekameter

Answer 6

All modern browsers support a Map, which is a key/value data structure. 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.

Example:

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.

Snook

Answer 7

I think it is better if you just created it like this:

var arr = [];

arr = {
   key1: 'value1',
   key2:'value2'
};

For more info, take a look at this:

JavaScript Data Structures - Associative Array

Tune

Answer 8

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]);
}

This is ok, but it iterates through every property of the array object.

If you want to only iterate through the properties myArray.one, myArray.two... you try like this:

myArray['one'] = 1;
myArray['two'] = 2;
myArray['three'] = 3;
myArray.push("one");
myArray.push("two");
myArray.push("three");
for(var i=0;i<maArray.length;i++){
    console.log(myArray[myArray[i]])
}

Now you can access both by myArray["one"] and iterate only through these properties.

Lactam

Have you count the count of mistypes in your example? :-) maArray, forgotten closed ')'... – Whin

Thanks for the example. We could unite Array and Object and work only with Object trees! Wonderful insight! It is very useful to make Object.getOwnPropertyNames(obj/array)! – Whin

This syntax is invalid: for(i=0;i<maArray.length;i++{ – Gobang

Answer 9

var obj = {};

for (i = 0; i < data.length; i++) {
    if(i%2==0) {
        var left = data[i].substring(data[i].indexOf('.') + 1);
        var right = data[i + 1].substring(data[i + 1].indexOf('.') + 1);

        obj[left] = right;
        count++;
    }
}

console.log("obj");
console.log(obj);

// Show the values stored
for (var i in obj) {
    console.log('key is: ' + i + ', value is: ' + obj[i]);
}


}
};
}

Soften

This is unbalanced. Where is the rest (there is a surplus of three }s)? – Gobang

Answer 10

const arrayValues = [
                    "yPnPQpdVgzvSFdxRoyiwMxcx",
                    "yPnPQpdVgzvSFdxRoyiwMxcx",
                    "a96b3Z-rqt6U3QV_1032fxcsa",
                    "iNeoJfVnF7dXqARwnDOhj233dsd"
                ];
                
                const newArr = arrayValues.map((x)=>{
                    return {
                        "_id":x
                    }
                });
     console.log(newArr)

Telstar