how do I define a number->string dictionary in Typescript?

Question

I'm playing with Typescript and converting over a small library over to it from Javascript. In one area of the code there was a statically defined mapping of friendly key name to their keycode. The original code looked like:

keys: { "Backspace": 8, "Tab": 9, "Enter": 13, ....etc.. };

I defined this in typescript via:

static keys: { [name: string]: number; } = { "Backspace": 8, "Tab": 9, "Enter": 13, ... etc.. };

This seems to work fine, however another part of the code uses the opposite mapping:

    chars: {8:"Backspace",9:"Tab",13:"Enter", ...etc... };

So I tried to do the same type of definition in typescript that I did previously:

chars: { [code: number]: string; } = { 8: "Backspace", 9: "Tab", 13: "Enter", ...etc.. };

This fails to compile with the following error:

Cannot convert '{ 0: string; 1: string; 2: string; 4: string; 8: string; 9: string; : string; }' to '{ [name: number]: string; }': Index signatures of types '{ 0: string; 1: string; 2: string; 4: string; 8: string; 9: string; : string; }' and '{ [name: number]: string; }' are incompatible

How do I define this mapping in Typescript?

Answer 1

The problem is that in JavaScript - and hence in TypeScript - the keys of an object literal are always strings, and so it cannot be assigned to a variable with a number indexer.

An object with a number indexer is an array - and so you have to do this:

var chars: string[] = [];
chars[8]= "Backspace";
chars[9]= "Tab";
chars[13] = "Enter";

Answer 2

Indeed js internally only has string based indices (with Array being different from {}) but string / number conversion is stable in js :

var x = {0 : "asdf",1:"bb"} 
alert(x[0])
alert(x[1])

I think typescript should support it as well and created a work item you can vote on: http://typescript.codeplex.com/workitem/832