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TypeScript Generics

A generic function is a function that names a placeholder type. The compiler fills the placeholder with a real type at each call.

A generic identity function

identity returns the value it receives. The type of the value becomes T at each call.

function identity<T>(value: T): T {
  return value;
}
console.log(identity("hello"));
console.log(identity(42));

An explicit type argument

A type argument names T directly at the call. The argument must match that type.

function identity<T>(value: T): T {
  return value;
}
console.log(identity<string>("hello"));
console.log(identity<number>(7));

A mismatched type argument

The call names number as T. The string argument does not match the named type.

function identity<T>(value: T): T {
  return value;
}
console.log(identity<number>("five"));

A generic array function

first reads the first item of an array. The array item type becomes T at each call.

function first<T>(items: T[]): T {
  return items[0];
}
console.log(first([10, 20, 30]));
console.log(first(["red", "blue"]));

Example

function identity<T>(value: T): T {
  return value;
}
function first<T>(items: T[]): T {
  return items[0];
}
console.log(identity("done"));
console.log(identity(true));
console.log(first([5, 9, 2]));
console.log(first(["a", "b", "c"]));

Key points

  • The letter T is a common name for a placeholder type.
  • The placeholder is written inside angle brackets after the function name.
  • The compiler infers T from the argument when no type argument is given.
  • A type argument can be written in angle brackets at the call.
  • One generic function can work with many types without copies.

In this example, identity runs with a string and then with a boolean. Each call fills T with a different type. first then reads the first item from a number array and from a string array.

Note

TypeScript erases generic types when it emits JavaScript.

Try it Yourself

Edit the code, then press Run.

Running is simulated · TypeScript Playground

Exercise

What does T stand for in a generic function?