Convert object array to hash map, indexed by an attribute value of the Object

Question

Use Case

The use case is to convert an array of objects into a hash map based on string or function provided to evaluate and use as the key in the hash map and value as an object itself. A common case of using this is converting an array of objects into a hash map of objects.

Code

The following is a small snippet in JavaScript to convert an array of objects to a hash map, indexed by the attribute value of object. You can provide a function to evaluate the key of hash map dynamically (run time).

function isFunction(func) {     return Object.prototype.toString.call(func) === '[object Function]'; }  /**  * This function converts an array to hash map  * @param {String | function} key describes the key to be evaluated in each object to use as key for hashmap  * @returns Object  * @Example   *      [{id:123, name:'naveen'}, {id:345, name:"kumar"}].toHashMap("id")  *      Returns :- Object {123: Object, 345: Object}  *  *      [{id:123, name:'naveen'}, {id:345, name:"kumar"}].toHashMap(function(obj){return obj.id+1})  *      Returns :- Object {124: Object, 346: Object}  */ Array.prototype.toHashMap = function(key) {     var _hashMap = {}, getKey = isFunction(key)?key: function(_obj){return _obj[key];};     this.forEach(function (obj){         _hashMap[getKey(obj)] = obj;     });     return _hashMap; }; 

You can find the gist here: Converts Array of Objects to HashMap.

Answer 1

This is fairly trivial to do with Array.prototype.reduce:

var arr = [      { key: 'foo', val: 'bar' },      { key: 'hello', val: 'world' }  ];    var result = arr.reduce(function(map, obj) {      map[obj.key] = obj.val;      return map;  }, {});    console.log(result);  // { foo:'bar', hello:'world' }

Note: Array.prototype.reduce() is IE9+, so if you need to support older browsers you will need to polyfill it.

Answer 2

Using ES6 Map (pretty well supported), you can try this:

var arr = [      { key: 'foo', val: 'bar' },      { key: 'hello', val: 'world' }  ];    var result = new Map(arr.map(i => [i.key, i.val]));    // When using TypeScript, need to specify type:  // var result = arr.map((i): [string, string] => [i.key, i.val])    // Unfortunately maps don't stringify well.  This is the contents in array form.  console.log("Result is: " + JSON.stringify([...result]));   // Map {"foo" => "bar", "hello" => "world"}