how to use string indexed interface of typescript?

Question

I'd like to use string index of interface.

interface IDictionary {
     [index: string]: string;
}
var params: IDictionary;
params = [{ "heart": "HP" }, { "mana": "MP" }];
console.log(params);

I thought this might be like MAP such as a pair of key and value. I'm not sure this is correct approach.

Daryl · Accepted Answer · 2016-09-22 17:47:52Z

51

Using the indexer limits what can be put or fetched for the object using the index syntax. E.g. foo is inferred to be of type string:

interface IDictionary {
     [index: string]: string;
}
var params = {} as IDictionary;

params['heart'] = 'HP'; // okay
var foo = params['heart']; // foo:string

The following on the other hand is an error as it will not type check:

var bar:number = params['heart']; // ERROR
params['heart'] = 234; // ERROR

Complete demo:

interface IDictionary {
     [index: string]: string;
}
var params = {} as IDictionary;

params['heart'] = 'HP'; // okay
var foo = params['heart']; // foo:string


var bar:number = params['heart']; // ERROR
params['heart'] = 234; // ERROR

edited Sep 22, 2016 at 17:47

Daryl

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answered Mar 13, 2015 at 23:22

basarat

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2 Comments

mejdev Over a year ago

Don't forget you need to initialize params.. params = {};

Prosper Ablordeppey Over a year ago

instead of hard defining the value as a string, using "any" will remove the limitation.

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