TypeScript hashmap/dictionary interface

Issue

I'm trying to implement a hashmap/dictionary interface. So far I have:

export interface IHash {
    [details: string] : string;
} 

I'm having some trouble understanding what exactly this syntax means. If I were to do var x : IHash = {}; how would I go about adding/accessing the data?

Solution

Just as a normal js object:

let myhash: IHash = {};   

myhash["somestring"] = "value"; //set

let value = myhash["somestring"]; //get

There are two things you're doing with [indexer: string] : string

• tell TypeScript that the object can have any string-based key
• that for all key entries the value MUST be a string type.

You can make a general dictionary with explicitly typed fields by using [key: string]: any;

e.g. age must be number, while name must be a string - both are required. Any implicit field can be any type of value.

As an alternative, there is a Map class:

let map = new Map<object, string>(); 

let key = new Object();

map.set(key, "value");
map.get(key); // return "value"

This allows you have any Object instance (not just number/string) as the key.

Although its relatively new so you may have to polyfill it if you target old systems.

Answered By - Meirion Hughes