function identity<String>(arg: String): String {
  return arg;
}
 
let myIdentity = identity([2]);
console.log()

Hi could someone help me to understand why this doesn't throw any type error even if i am passing a array of numbers ?

1. Is it because the type is "String" instead of "string" , which looks for objects than primitive ?
2. If answer is "yes" , if i change everything to string i get error saying string is never used

function identity<string>(arg: string): string {
  return arg;
}
 
let myIdentity = identity([2]);
console.log(myIdentity )

'string' is declared but its value is never read.

CodePudding user response：

You are shadowing the builtin types String and string by using their names as your generic parameter. Just use a parameter name that is not a built-in to solve it:

TS Playground link

function identity<T extends string>(value: T): T {
  return value;
}
 
identity([2]); // error [2] doesn't extend string
identity('2'); // ok

If you want a generic identity function without any kind of constraint, you can use an unconstrained type parameter:

TS Playground link

function identity<T>(value: T): T {
  return value;
}
 
const numberArrayValue = identity([2]); // number[]
const objectValue = identity({b: 2}); // { b: number; }
const stringLiteralValue = identity('2'); // "2"
const numberLiteralValue = identity(2); // 2