TypeScript 5.0 Betaがリリースされた

Decorators

クラスやメソッドなどの上に@xxxxと書いて機能拡張できる機能

デコレータ

function logged(originalMethod: any, _context: any) {
    function replacementMethod(this: any, ...args: any[]) {
        console.log("LOG: Entering method.")
        const result = originalMethod.call(this, ...args);
        console.log("LOG: Exiting method.")
        return result;
    }
    return replacementMethod;
}

使用方法

class Person {
  @logged
  greet() {
    console.log(`Hi, my name is ${this.name}`)
  }
}

const person = new Person('john smith')
person.greet()
// LOG: Entering method.
// Hi, my name is john smith
// LOG: Exiting method.

const type Parameters

Before

type HasNames = { readonly names: string[] };
function getNamesExactly1<T extends HasNames>(arg: T): T["names"] {
  return arg.names;
}
// string[]
const names1 = getNamesExactly1({ names: ["Alice", "Bob", "Eve"]});

After

function getNamesExactly2<const T extends HasNames>(arg: T): T["names"] {
  return arg.names;
}
// readonly ["Alice", "Bob", "Eve"]
const names2 = getNamesExactly2({ names: ["Alice", "Bob", "Eve"]} as const);