Javascript map 带复合键

Question

In JavaScript I want to store values to compound keys, similar to a C# dictionary with a tuple as key. This is where I came across the Map class. However, it does not seem to work quite as well as I would like it to. Here's my current approach:

var test = new Map();

test.set({a: 1, b: 1}, 'Bla');
test.set({a: 5, b: 7}, 'Blub');

test.get({a: 1, b: 1}); // ==> Returns undefined; would expect 'Bla'

I guess, that this has something to do that both objects with {a: 1, b: 1} have a different memory address and therefore are the same, but not identical. The Dictionary class in c# uses a hash function in background. Is there something similar in JS? Or a much easier approach?

My real key object consistst of three strings.

Answer 1

{} this operator will create a new object every time; and a new object will have a different object refenece each time; if you save the object reference and use that for multiple operation its ok; but since you are trying to use a new object refence every time it won't work; you may either use a primitive type as key, or same object reference like the snippet below

 //approach 1 using same object reference var test = new Map(); var obj = {a: 1, b: 1}; test.set(obj, 'Bla'); test.set({a: 5, b: 7}, 'Blub'); let result = test.get(obj); console.log(result); // aproach 2 using JSON.stringify test = new Map(); test.set(JSON.stringify({a: 1, b: 1}), 'Bla'); test.set({a: 5, b: 7}, 'Blub'); result = test.get(JSON.stringify({a: 1, b: 1})); console.log(result)

Answer 2

Your analysis is correct. It works like this because in Javascript you usually operate primitive objects that don't have all this hashing behavior attached to them out of the box. Nothing stops you from implementing your own Dictionary with hash function in background though

 class Dictionary { map = {} constructor(hashFunction) { this.hashFunction = hashFunction } set(key, item) { this.map[this.hashFunction(key)] = item } get(key) { return this.map[this.hashFunction(key)] } delete(key) { delete this.map[this.hashFunction(key)] } } const dict = new Dictionary((keyObject) => JSON.stringify(keyObject)) dict.set({ a: 1, b: 2 }, 'hello') console.log(dict.get({ a: 1, b: 2 })) // hello

As to what to use, Map or object, the difference between Map and object is simply that object only supports string keys (also Symbols but irrelevant right now) while Map supports any value at a cost of using more resources, less compatibility with old browsers, and it's generally less handy to use than object (and also stops GC from cleaning out those objects you use as keys). That said, object is your choice here