Typechecking of Array Types in TypeScript

Question

In TypeScript can declare strongly-typed Array Types

But errors (from compiller) for the next snippet very strange. Could someone explain me error cases?

interface SomeArray1 {
    [index: number]: string;    
}

var my1: SomeArray1;
my1["ab"] = 12;  // <-- why this line is OK?
my1["ab"] = "ab"; // <-- why this line is OK?
my1[12] = "ab";
my1[12] = 12; // <-- error

//-----------------------------

interface SomeArray2 {
    [index: string]: number;    
}

var my2: SomeArray2;

my2["ab"] = 12;
my2["ab"] = "ab"; // <-- error
my2[12] = "ab"; // <-- error
my2[12] = 12;

//-----------------------------

interface SomeArray3 {
    [index: string]: string;
}

var my3: SomeArray3;

my3["ab"] = 12; // <-- error
my3["ab"] = "ab";
my3[12] = "ab";
my3[12] = 12; // <-- error

Open sample in playground

Community · Accepted Answer · 2017-05-23 12:32:42Z

In my opinion, the documentation is confusing because the array type it describes does not constrain exclusively to an array. For example, the following would be allowed:

var my1: SomeArray1 = {};

What's being talked about, is more like a key-value/dictionary-like type where, in the case of SomeArray1, the key is a number and the value is a string.

It would make more sense if it were instead defined as an array type with a string value constraint:

var my1: Array<string> = [];

Or even a type alias:

type SomeArray1 = Array<string>;
var my1: SomeArray1 = [];

In both these cases, assigning {} to the variable would not be allowed.

Value type constraint errors

All the errors are very similar. The main reason they are happening is because either there is a string value constraint and it is being assigned a number or vice-versa.

Index type constraint... no errors?

It is weird because the type constraint doesn't seem to be working on the index/key. I am certain this used to work. This is either a bug in the compiler or the typescript team decided to change this. I've opened up an issue to check.

And the response:

We changed the behavior here because (at runtime) numbers are implicitly converted to strings during index operations. Basically, a string indexer is a "superset" of a number indexer.

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