Typescript does not know about Array.map

Question

I suspect I have made a mistake, but I can't get the following to pass type checking. I end up with Property 'map' does not exist for type 'Restos'. I have set up the interface according to the TS tutorials, so it feels as though map is not included as default (I have "target": "es6" in tsconfig)

interface Resto {
    rname:String,
    qname:String,
    tel:String
}

interface Restos {
    [index:number]:Resto;
}

class MainController {
    headline: String;
    rnames: [String];

    constructor($http : ng.IHttpService) {
        this.headline = "hello world";

        $http.get('http://afbackend.herokuapp.com/api/restos')
        .success( (res:Restos) => {
            console.log(res);
            this.rnames = res.map(r => r.rname)
                              ^^^ 
        });
    }
}

In your interface for Restos where is the map method? Maybe just use Resto[] instead of Restos. — Martin
– Martin, Commented Aug 10, 2015 at 18:36

Ryan Cavanaugh · Accepted Answer · 2015-08-10 18:29:40Z

4

The only thing the compiler knows is true about Restos is that when you index it by a number, you get a Resto. Just because something has a numeric index signature doesn't mean it's an array (for example, the object {0: 'hello', 1: 'world'} can be indexed by numbers to produce strings, but you can't call .map on it).

What you probably want to write is type Restos = Array<Resto> instead of the interface.

edited Aug 10, 2015 at 18:29

answered Aug 10, 2015 at 18:20

Ryan Cavanaugh

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1 Comment

Simon H Over a year ago

Cheers - I was making it too complex with Interface, type does it. And type Restos = Resto[] worked too. (Yes, I know [String] was wrong too). In my defence I would note that the type syntax is not mentioned on typescriptlang.org/Handbook

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