Typescript Type 'string' is not assignable to type

When you do this:

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

...you are creating a type called Fruit that can only contain the literals "Orange", "Apple" and "Banana". This type extends String, hence it can be assigned to String. However, String does NOT extend "Orange" | "Apple" | "Banana", so it cannot be assigned to it. String is less specific. It can be any string.

When you do this:

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

const myString = "Banana";

const myFruit: Fruit = myString;

...it works. Why? Because the actual type of myString in this example is "Banana". Yes, "Banana" is the type. It is extends String so it's assignable to String. Additionally, a type extends a Union Type when it extends any of its components. In this case, "Banana", the type, extends "Orange" | "Apple" | "Banana" because it extends one of its components. Hence, "Banana" is assignable to "Orange" | "Apple" | "Banana" or Fruit.

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 |

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.