ways to convert to a specific subtype within a union

Question

ways to convert to a specific subtype within a union

type example = {
    name : string | number | null
}
type processNumber = {
    name : number
}

const a : example = {
    name : 123
} as const
function PROCESS (input : processNumber) {
    console.log(input.name)
}

PROCESS(a);

I am facing an issue where the error message states:

Argument of type 'example' is not assignable to parameter of type 'processNumber'.
  Types of property 'name' are incompatible.
    Type 'string | number | null' is not assignable to type 'number'.
      Type 'null' is not assignable to type 'number'.

How can I convert name to be strictly of type number? It should be noted that there are multiple properties that require coercion, so any quick and inline method for coercion would be greatly appreciated.

typescript

Answer 1

Answer №1

An easy solution is available:

type example = {
    name: string | number | null
}
type processNumber = {
    name: number
}

const a = {
    name: 123
} as const satisfies example

function PROCESS(input: processNumber) {
    console.log(input.name)
}

PROCESS(a);

Playground

To achieve this, simply make use of the satisfies operator. It enables checking if it fulfills a specific type and aids in type inference.

Note that the feature satisfies was introduced in the latest TypeScript version 4.9.

Moreover, when utilizing as const assertion, you can eliminate satisfies, but remember to remove explicit type annotation as shown below:

const a = {
    name: 123
} as const

Answer 2

An easy solution is available:

type example = {
    name: string | number | null
}
type processNumber = {
    name: number
}

const a = {
    name: 123
} as const satisfies example

function PROCESS(input: processNumber) {
    console.log(input.name)
}

PROCESS(a);

Playground

To achieve this, simply make use of the satisfies operator. It enables checking if it fulfills a specific type and aids in type inference.

Note that the feature satisfies was introduced in the latest TypeScript version 4.9.

Moreover, when utilizing as const assertion, you can eliminate satisfies, but remember to remove explicit type annotation as shown below:

const a = {
    name: 123
} as const

ways to convert to a specific subtype within a union

Answer №1

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