How can I cast custom type to primitive type?

Question

I have a type

type Rating = 0 | 1 | 2 | 3 | 4 | 5 | number

Now I want to do something like this.

let myRating:Rating = 4
let rate:number = myRating as number

How can I cast my myRating into number primitive type?

It is giving me error as:

Conversion of type 'Rating' to type 'number' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.ts(2352)

I have been through this, but what I want is its reverse

Edit:

tsconfig.json

{
  "compilerOptions": {
    "noImplicitAny": false,
    "target": "es6",
    "allowJs": true,
    "skipLibCheck": false,
    "esModuleInterop": true,
    "allowSyntheticDefaultImports": true,
    "forceConsistentCasingInFileNames": true,
    "module": "esnext",
    "moduleResolution": "node",
    "resolveJsonModule": true,
    "isolatedModules": true,
    "noEmit": true,
    "jsx": "preserve",
    "strict": true
  },
  "include": [
    "src"
  ]
}

tsc version: 3.2.1

Solution 1

Try:

myRating as unknown as number

Also, remove | number from your declaration.

Explanation: You cannot cast from a custom to a primitive without erasing the type first. unknown erases the type checking.

Solution 2

Update 2020

TS 3.8 update

now no need to cast using as, it is supported implicitly except in some cases. Where you can do type conversion as given in the accepted answer. Here is a good explanation of type conversion on the typescript.

type Rating = 0 | 1 | 2 | 3 | 4 | 5;
let myRating:Rating = 4
let rate:number = myRating;

TS Playground

---

Original Answer

I think it is fixed in the typescript update TS 3.5.1

type Rating = 0 | 1 | 2 | 3 | 4 | 5;
let myRating:Rating = 4

Now

let rate:number = myRating;

and

let rate:number = myRating as number;

both working fine.

TS Playground

Comments

[Jason Foglia]

I ran into this recently when trying covariance. This solution just doesn't seem right. But it works.

[Steven Spungin]

You can't cast from a custom to a primitive without 'erasing' the type first. unknown erases the type checking.