Attempting to grasp the concept of Typescript generics

Question

Attempting to grasp the concept of Typescript generics

I have a function defined as follows:

interface ExtraModels extends Model {
  unknown: string
}

const write = async (data: ExtraModels[]) => {
  console.log(data[0].unknown)
}

This function is currently working. However, I want to modify it to:

const write = async <T = ExtraModels>(data: T[]) => {
  console.log(data[0].unknown)
}

Unfortunately, when I make this change, I encounter the following error:

Property 'unknown' does not exist on type 'T'.ts(2339)

I'm confused as to why this error is occurring. Based on my understanding, this should be a valid modification.

typescript generics

Answer 1

Answer №1

Although technically correct, the code snippet may not behave as expected. In this scenario, using = ExtraModels indicates that T will default to ExtraModels if its type cannot be inferred from the arguments. However, it's important to note that T can actually be any type, even one that does not inherit from

ExtraModels</code. This explains why the compiler prohibits access to properties of <code>ExtraModels

.

If your goal is to restrict T to only allow types that extend ExtraModels, you should use the syntax T extends ExtraModels. By specifying this constraint, typescript will enable access to properties of ExtraModels.

const write = async <T extends  ExtraModels>(data: T[]) => {
  console.log(data[0].unknown)
}

Answer 2

Although technically correct, the code snippet may not behave as expected. In this scenario, using = ExtraModels indicates that T will default to ExtraModels if its type cannot be inferred from the arguments. However, it's important to note that T can actually be any type, even one that does not inherit from

ExtraModels</code. This explains why the compiler prohibits access to properties of <code>ExtraModels

.

If your goal is to restrict T to only allow types that extend ExtraModels, you should use the syntax T extends ExtraModels. By specifying this constraint, typescript will enable access to properties of ExtraModels.

const write = async <T extends  ExtraModels>(data: T[]) => {
  console.log(data[0].unknown)
}

Attempting to grasp the concept of Typescript generics

Answer №1

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