TypeScript : Type 'number' is not assignable to type '0'

Question

I declared a variable x as

public x : 0;

Instead of initialising it to 0

Thinking that I had declared it correctly, I initialised it like

this.x = 5;

I was getting error on the above line, which read

Type '5' is not assignable to type '0'.ts(2322)

Can anybody tell, why is this happening?

Answer 1

This is a feature of Typescript, after the : the possible type(s) are declared and that could be specific values also.

For example you could also limit variables to specific strings:

var action : "email" | "sms";

In this case action = "fax" will give a compile error.

With strings this is called "String Literal Types". With numbers this is called "Numeric Literal Types".

So in you case you declared it a numeric literal type with 0 as allowed value.

See also https://www.typescriptlang.org/docs/handbook/advanced-types.html#string-literal-types and https://www.typescriptlang.org/docs/handbook/advanced-types.html#numeric-literal-types

So beware of mixing = and : in Typescript, as both are correct Typescript in this case but have different behavior ;)