Utilize TypeScript to match patterns against either a string or Blob (type union = string | Blob)

I have created a union type called DataType,

type TextData = string
type BinaryData = Blob
type DataType = TextData | BinaryData

Now, I want to implement it in a function

function processData(data: DataType): void {
    if (data instanceof TextData)
      // encountering an error when using the type as a value

    if (typeof data === 'Blob')
      // getting an error because typeof data is 'object'

    if (data instanceof Blob)
      // this works, but I prefer not to use a type alias
}

Is there a way to make this work, or should I consider redesigning the approach?

typescripttypes

Answer №1

I understand the issue with the type alias being compiled away after compilation. Is there a different approach that can be taken?

In order to perform runtime checks, it is necessary to utilize variables that are available at runtime.

Implementing at Runtime

One possible solution is to export BinaryData as a runtime variable.

type TextData = string
type BinaryData = Blob
const BinaryData = Blob;
type DataType = TextData | BinaryData

function doSomethingWithData(data: DataType): void {
    if (data instanceof BinaryData) {
      // application logic here
    }
}

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