Best way to store a key=>value array in JavaScript?

Question

That's just what a JavaScript object is:

var myArray = {id1: 100, id2: 200, "tag with spaces": 300};
myArray.id3 = 400;
myArray["id4"] = 500;

You can loop through it using for..in loop:

for (var key in myArray) {
  console.log("key " + key + " has value " + myArray[key]);
}

See also: Working with objects (MDN).

In ECMAScript6 there is also Map (see the browser compatibility table there):

• An Object has a prototype, so there are default keys in the map. This could be bypassed by using map = Object.create(null) since ES5, but was seldomly done.

• The keys of an Object are Strings and Symbols, where they can be any value for a Map.

• You can get the size of a Map easily while you have to manually keep track of size for an Object.

If I understood you correctly:

var hash = {};
hash['bob'] = 123;
hash['joe'] = 456;

var sum = 0;
for (var name in hash) {
    sum += hash[name];
}
alert(sum); // 579

You can use Map.

• A new data structure introduced in JavaScript ES6.
• Alternative to JavaScript Object for storing key/value pairs.
• Has useful methods for iteration over the key/value pairs.

var map = new Map();
map.set('name', 'John');
map.set('id', 11);

// Get the full content of the Map
console.log(map); // Map { 'name' => 'John', 'id' => 11 }

Get value of the Map using key

console.log(map.get('name')); // John 
console.log(map.get('id')); // 11

Get size of the Map

console.log(map.size); // 2

Check key exists in Map

console.log(map.has('name')); // true
console.log(map.has('age')); // false

Get keys

console.log(map.keys()); // MapIterator { 'name', 'id' }

Get values

console.log(map.values()); // MapIterator { 'John', 11 }

Get elements of the Map

for (let element of map) {
  console.log(element);
}

// Output:
// [ 'name', 'John' ]
// [ 'id', 11 ]

Print key value pairs

for (let [key, value] of map) {
  console.log(key + " - " + value);
}

// Output: 
// name - John
// id - 11

Print only keys of the Map

for (let key of map.keys()) {
  console.log(key);
}

// Output:
// name
// id

Print only values of the Map

for (let value of map.values()) {
  console.log(value);
}

// Output:
// John
// 11

In javascript a key value array is stored as an object. There are such things as arrays in javascript, but they are also somewhat considered objects still, check this guys answer - Why can I add named properties to an array as if it were an object?

Arrays are typically seen using square bracket syntax, and objects ("key=>value" arrays) using curly bracket syntax, though you can access and set object properties using square bracket syntax as Alexey Romanov has shown.

Arrays in javascript are typically used only with numeric, auto incremented keys, but javascript objects can hold named key value pairs, functions and even other objects as well.

Simple Array eg.

$(document).ready(function(){

    var countries = ['Canada','Us','France','Italy'];
    console.log('I am from '+countries[0]);
    $.each(countries, function(key, value) {
        console.log(key, value);
    });

});

Output -

0 "Canada"

1 "Us"

2 "France"

3 "Italy"

We see above that we can loop a numerical array using the jQuery.each function and access info outside of the loop using square brackets with numerical keys.

Simple Object (json)

$(document).ready(function(){

    var person = {
        name: "James",
        occupation: "programmer",
        height: {
            feet: 6,
            inches: 1
        },
    }

    console.log("My name is "+person.name+" and I am a "+person.height.feet+" ft "+person.height.inches+" "+person.occupation);

    $.each(person, function(key, value) {
        console.log(key, value);
    });

});

Output -

My name is James and I am a 6 ft 1 programmer

name James

occupation programmer

height Object {feet: 6, inches: 1}

In a language like php this would be considered a multidimensional array with key value pairs, or an array within an array. I'm assuming because you asked about how to loop through a key value array you would want to know how to get an object (key=>value array) like the person object above to have, let's say, more than one person.

Well, now that we know javascript arrays are used typically for numeric indexing and objects more flexibly for associative indexing, we will use them together to create an array of objects that we can loop through, like so -

JSON array (array of objects) -

$(document).ready(function(){

    var people = [
        {
            name: "James",
            occupation: "programmer",
            height: {
                feet: 6,
                inches: 1
            }
        }, {
            name: "Peter",
            occupation: "designer",
            height: {
                feet: 4,
                inches: 10
            }
        }, {
            name: "Joshua",
            occupation: "CEO",
            height: {
                feet: 5,
                inches: 11
            }
        }
    ];

    console.log("My name is "+people[2].name+" and I am a "+people[2].height.feet+" ft "+people[2].height.inches+" "+people[2].occupation+"\n");

    $.each(people, function(key, person) {
        console.log("My name is "+person.name+" and I am a "+person.height.feet+" ft "+person.height.inches+" "+person.occupation+"\n");
    });

});

Output -

My name is Joshua and I am a 5 ft 11 CEO

My name is James and I am a 6 ft 1 programmer

My name is Peter and I am a 4 ft 10 designer

My name is Joshua and I am a 5 ft 11 CEO

Note that outside the loop I have to use the square bracket syntax with a numeric key because this is now an numerically indexed array of objects, and of course inside the loop the numeric key is implied.