What is the JavaScript equivalent to a C# HashSet?

Question

I have a list of a few thousand integer keys. The only thing I need to do with this list is say whether or not a given value is in the list.

For C# I would use a HashSet to make that look-up fast. What's the JavaScript equivalent?

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Minimal support level: IE 9+, jQuery (current)

Answer 1

Actually JavaScript provides a Set object, fairly simple to use:

var set = new Set();
set.add(1);
set.add(2);

set.has(1)    // true

Unfortunately, it is not compatible with IE9.

Answer 2

Under the hood, the JavaScript Object is implemented with a hash table. So, your Key:Value pair would be (your integer):true

A constant-time lookup function could be implemented as:

var hash = {
  1:true,
  2:true,
  7:true
  //etc...
};

var checkValue = function(value){
  return hash[value] === true;
};


checkValue(7); // => true
checkValue(3); // => false

Answer 3

Use an object. To add a key to the set, do:

object[key] = true;

To test whether a key is in the set, do:

if (object.hasOwnProperty(key)) { ... }

To remove a key from the set, do:

delete object[key]

Answer 4

You can use just a regular JavaScript object and the 'in' keyword to see if that object has a certain key.

var myObj = {
  name: true,
  age: true
}

'name' in myObj //returns true;
'height' in myObj // returns false;

Or if you know you're going to have keys in your object that might be built in JavaScript object properties use...

var myObj = {
  name: true,
  age: true
}

myObj.hasOwnProperty('name') //returns true;
myObj.hasOwnProperty('height') // returns false;

Answer 5

I've read the solutions and I tried some. After trying to use the object[key] method I realized that it wasn't going to work. I wanted a HashSet that could store HTML elements. When adding these objects the key was translated to a string, so I came up with my own set based on jQuery. It supports add, remove, contains and clear.

var HashSet = function () {

    var set = [];

    this.add = function (obj) {
        if (!this.contains(obj)) {
            set.push(obj);
        }
    };

    this.remove = function (obj) {
        set = jQuery.grep(set, function (value) {
            return value !== obj;
        });
    };

    this.clear = function () {
        set = [];
    };

    this.contains = function (obj) {
        return $.inArray(obj, set) > -1;
    };

    this.isEmpty = function () {
        return set.length === 0;
    };
};

Note
When adding something like $('#myElement') to the set, one should add the real HTML element $('#myElement')[0]. Oh... and if you want to keep a list of changed controls - use the name of the element (gave me a problem with :radio controls).

Note2
I think the object[key] might be faster for your integers.

Note3
If you are only going to store numbers or string, this set will be faster:

var HashSet = function () {

    var set = {};

    this.add = function (key) {
        set[key] = true;
    };

    this.remove = function (key) {
        delete set[key];
    };

    this.clear = function () {
        set = {};
    };

    this.contains = function (key) {
        return set.hasOwnProperty(key);
    };

    this.isEmpty = function () {
        return jQuery.isEmptyObject(set);
    };
};

Answer 6

Map or if no need to iterate WeakMap

let m1=new Map();

m1.set('isClosed',false);
m1.set('isInitialized',false);

m1.set('isClosed',true);

m1.forEach(function(v,k)
{
    console.log(`${k}=${v}`);
});

Answer 7

Only Map in Javascript has faster look ups https://jsben.ch/RbLvM . if you want only for look ups you can create a lookup map out of the array you have like the below and use it

function createLookUpMap(arr = []) {
  return new Map(arr.map(item => [item, true]));
}

const lookupMap = createLookUpMap(['apple', 'orange', 'banana']);

lookupMap.has('banana'); // O(1)