Suggestion: Union types and string constants

[omidkrad]

It is great to know that the next TypeScript will come with Union Types. This will allow to tighten up those previously any parameter definitions that accept more than one type. One thing that I wanted to bring up which is very common in JavaScript is parameters or objects that take documented string constants. For example assume we have the following function that takes a width and a color:

changeStyle(width, color);

My suggestion is let us include string constants in Union Types, so that we can have:

type Width = number /* pixels */ | "small" | "medium" | "large";
type Color = number | string | "white" | "black" | "red" | "green" | "blue"; 
changeStyle(width: Width, color: Color);

Then we can have tools suggest those especial strings in tooltips or intellisense, and catch more errors. For example:

shape.changeStyle(120, 0xFFFFFF); // OK
shape.changeStyle("small", "magenta"); // OK
shape.changeStyle("16pt", "yellow"); // error on width
shape.changeStyle(stringObj, "red"); // OK, if string variable is passed, just allow it

Thanks!

[DanielRosenwasser]

I think we're warm on this idea, and a similar proposal can be found on #1003. I think we should continue discussion on there.

[omidkrad]

That's great, I didn't know it was suggested 282 issues back! :)