I want to create a map object in javascript. I came to the following idea:
var a = new Array();
a["key1"] = "value1";
a["key2"] = "value2";
but then how I can find if a particular key exists or not?
This question is related to
javascript
dictionary
Use the in
operator: e.g. "key1" in a
.
if( a['desiredKey'] !== undefined )
{
// it exists
}
A built-in Map type is now available in JavaScript. It can be used instead of simply using Object. It is supported by current versions of all major browsers.
Maps do not support the [subscript]
notation used by Objects. That syntax implicitly casts the subscript
value to a primitive string or symbol. Maps support any values as keys, so you must use the methods .get(key)
, .set(key, value)
and .has(key)
.
var m = new Map();_x000D_
var key1 = 'key1';_x000D_
var key2 = {};_x000D_
var key3 = {};_x000D_
_x000D_
m.set(key1, 'value1');_x000D_
m.set(key2, 'value2');_x000D_
_x000D_
console.assert(m.has(key2), "m should contain key2.");_x000D_
console.assert(!m.has(key3), "m should not contain key3.");
_x000D_
Objects only supports primitive strings and symbols as keys, because the values are stored as properties. If you were using Object, it wouldn't be able to to distinguish key2
and key3
because their string representations would be the same:
var o = new Object();_x000D_
var key1 = 'key1';_x000D_
var key2 = {};_x000D_
var key3 = {};_x000D_
_x000D_
o[key1] = 'value1';_x000D_
o[key2] = 'value2';_x000D_
_x000D_
console.assert(o.hasOwnProperty(key2), "o should contain key2.");_x000D_
console.assert(!o.hasOwnProperty(key3), "o should not contain key3."); // Fails!
_x000D_
You want to create an Object, not an Array.
Like so,
var Map = {};
Map['key1'] = 'value1';
Map['key2'] = 'value2';
You can check if the key exists in multiple ways:
Map.hasOwnProperty(key);
Map[key] != undefined // For illustration // Edit, remove null check
if (key in Map) ...
Source: Stackoverflow.com