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

javascript arrays hashmap

2022-08-29 23:08:06

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.

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

答案 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"}