AJP February 2016

Access object values using "non-string keys" and define all as having a certain type

Not a big deal but I was wondering if it's possible to access an object using a non-string key AND define that it will return the same type whatever is accessed. I can figure out how to do both separately or can "laboriously" do both together.

I guess I want something like:

var DEFINITIONS: {[key: 'non-string']: Definition} = { ... }

or

var DEFINITIONS: {[any]: Definition} = { ... }

I guess this syntax doesn't exist (yet? ;) ).

Define return type of object DEFINITIONS but have to access using a string key:

interface Definition {
    id: number;
    type: string;
}

var DEFINITIONS: {[key: string]: Definition} = {
    v1: {
        id: 4,
        type: 'abc'
    }
}

DEFINITIONS['v1']

Can access DEFINITIONS using a non-string key but not define return type:

interface Definition {
    id: number;
    type: string;
}

var DEFINITIONS = {
    v1: {
        id: 4,
        type: 'abc'
    }
}

DEFINITIONS.v1  // Rightly does not show as having interface of `Definition`

Or laboriously:

interface Definition {
    id: number;
    type: string;
}

var DEFINITIONS: {v1: Definition, v2: Definition, v3: Definition, etc...} = {
    v1: {
        id: 4,
        type: 'abc'
    },
    v2: { ... },
    v3: { ... },
    etc ...
}

DEFINITIONS.v1   // Great! Has explicit interface of Definition

// or:

var v1: Definition = {id: 4, type: 'abc'};
var DEFINITIONS2 = {v1};

p.s. I don't know what the correct technical terms for access using keys like ['v1'] or v1 is.

Answers


C Snover February 2016

The indexer property of a type establishes the object as being a hash map. Intentionally, this means that you can only use the bracket notation to access properties. This is normally actually a good thing for readability because it means that there is a clear syntactic difference between a hash map created using Object versus other non-dynamic objects. However, there is also an open enhancement request to do what you are asking.

p.s. I don't know what the correct technical terms for access using keys like ['v1'] or v1 is.

The EcmaScript specification refers to these as the “bracket notation” and “dot notation”, respectively; colloquially they are often referred to as “array accessor” and “dot accessor”.

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Asked in February 2016
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