Typescript Type 'string' is not assignable to type

Question

Here's what I have in fruit.ts

export type Fruit = "Orange" | "Apple" | "Banana"

Now I'm importing fruit.ts in another typescript file. Here's what I have

myString:string = "Banana";

myFruit:Fruit = myString;

When I do

myFruit = myString;

I get an error:

Type 'string' is not assignable to type '"Orange" | "Apple" | "Banana"'

How can I assign a string to a variable of custom type Fruit?

Answer 1

Update

As mentioned in @Simon_Weaver's answer, since TypeScript version 3.4 it's possible to assert it to const:

let fruit = "Banana" as const;

Old answer

You'll need to cast it:

export type Fruit = "Orange" | "Apple" | "Banana";
let myString: string = "Banana";

let myFruit: Fruit = myString as Fruit;

Also notice that when using string literals you need to use only one |

Answer 2

Typescript 3.4 introduced the new 'const' assertion

You can now prevent literal types (eg. 'orange' or 'red') being 'widened' to type string with a so-called const assertion.

You will be able to do:

let fruit = 'orange' as const;  // or...
let fruit = <const> 'orange';

And then it won't turn itself into a string anymore - which is the root of the problem in the question.

You can also do it on a whole object:

let animals = [ { species: 'dog' }, { species: 'cat' } ] as const;

type firstAnimal = (typeof animals)[0]['species'];  // string literal 'dog'

Bonus Tip: You can also use <const> false or <const> true to represent a boolean that must be true or false. This can be useful in discriminated unions sometimes. You'll know it when you see it.