Why is it possible to declare a variable of 'anyTextString' type in TypeScript?

Question

Why tsc doesn't complain for this line of code:

let a: 'my text string';

and allows a to be of type 'my text string'?

And... Isn't this bug-prone if, let's say, someone takes advantage of implicit type inference, and just puts a ':' instead of '='?!

This is called literal type. – Aleksey L. Dec 15 '16 at 08:25 — Aleksey L., Dec 15 '16 at 08:25

Paleo · Accepted Answer · 2016-12-15T10:43:50.273

This is a literal type. The documentation is here. An example:

type Color = 'blue' | 'red'

function showColor(c: Color) {
  console.log(c)
}

showColor('blue') // OK
showColor('other') // Error

Notice: Since TypeScript 2.0, literal types are expanded to numbers and booleans (not only strings). Then, with TypeScript 2.1, literal types are better inferred.

And... Isn't this bug-prone if, let's say, someone takes advantage of implicit type inference, and just puts a ':' instead of '='?!

In TypeScript, it's required to spot the :. The following code:

let a: 'my text string';

... is compiled to (here with the target ES6):

let a;

1 Answers1