Initialize array interface

Question

As said in the Handbook, you can declare a custom array interface with string or number index:

interface StringArray {
    [index: number]: string;
}

So I made this:

interface EventRegistrations {
    [index:string]:ListenerEntry[];
}

problem is simple: how can I initialize such a thing?

Here is what I tried and the error message provided (using PhpStorm 2016.1 with tsc 1.8.7):

foo:EventRegistrations = [];

Type 'undefined[]' is not assignable to type 'EventRegistrations'

foo:EventRegistrations = [[]];

Type 'undefined[][]' is not assignable to type 'EventRegistrations'

foo:EventRegistrations = [index:string]:ListenerEntry[];

Syntax error (expected , where first : is)

foo:EventRegistrations = [string]:ListenerEntry[];

Syntax error (expected ; where : is)

foo:EventRegistrations = new EventRegistrations();

Syntax error (cannot find name 'EventRegistrations')

Matthew Layton · Accepted Answer · 2016-03-30 11:05:22Z

1

In this case, you are not actually defining an array [], you are defining a dictionary {}, since string indexers are used for dictionary style lookup.

In the following example, you will see that the object is declared using {} rather than [], because in JavaScript, all objects are dictionaries (even arrays!) Arrays just have special semantics and a length property that allows them to behave like arrays (numerically indexed)

interface ListenerEntry {
    foo: string;
    bar: string;
}

interface EventRegistrations {
    [index:string]:ListenerEntry[];
}

var x: EventRegistrations = {};

x["one"][0].foo = "Hello";
x["two"][1].bar = "World";

answered Mar 30, 2016 at 11:05

Matthew Layton

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