プログラミング言語「TypeScript 4.3」をMicrosoftが公開：インポート文の入力補完が利用可能に

function bar(s: string): `hello ${string}` {
    return `hello ${s}`;  // Now ok, previously was error
}
declare let s: string;
declare function g1<T>(x: T): T;
declare function g2<T extends string>(x: T): T;
let x1 = g1(`xyz-${s}`);  // string
let x2 = g2(`xyz-${s}`);  // `xyz-${string}`, previously was string

class Foo {
    #someMethod() {
        //...
    }
    get #someValue() {
        return 100;
    }
    publicMethod() {
        // These work.
        // We can access private-named members inside this class.
        this.#someMethod();
        return this.#someValue;
    }
}
new Foo().#someMethod();
//        ~~~~~~~~~~~
// error!
// Property '#someMethod' is not accessible
// outside class 'Foo' because it has a private identifier.
new Foo().#someValue;
//        ~~~~~~~~~~
// error!
// Property '#someValue' is not accessible
// outside class 'Foo' because it has a private identifier.

class Foo {
    static #someMethod() {
        // ...
    }
}
Foo.#someMethod();
//  ~~~~~~~~~~~
// error!
// Property '#someMethod' is not accessible
// outside class 'Foo' because it has a private identifier.

function f1<T extends string | undefined>(x: T): string {
    if (x) {
        x.length;  // x narrowed to string
        return x;  // x narrowed to string (error before this PR)
    }
}

declare function takeA(a: 'a'): void;
declare function takeB(b: 'b'); void;
function f2<T extends 'a' | 'b'>(x: T) {
    if (x === 'a') {
        takeA(x);  // x narrowed to 'a' (error before this PR)
    }
    else {
        takeB(x);  // x narrowed to 'b' (error before this PR)
    }
}
type A = { kind: 'a', value: string };
type B = { kind: 'b', value: number };
function f3<T extends A | B>(x: T) {
    if (x.kind === 'a') {
        x.value;  // Narrowed to string (wasn't narrowed before this PR)
    }
    else {
        x.value;  // Narrowed to number (wasn't narrowed before this PR)
    }
}

class Foo {
    hello = "hello";
    world = 1234;
    // This is an index signature:
    [propName: string]: string | number | undefined;
}
let instance = new Foo();
// Valid assigment
instance["whatever"] = 42;
// Has type 'string | number | undefined'.
let x = instance["something"];

class Foo {
    static hello = "hello";
    static world = 1234;
    static [propName: string]: string | number | undefined;
}
// Valid.
Foo["whatever"] = 42;
// Has type 'string | number | undefined'
let x = Foo["something"];

class Foo {
    static prop = true;
    //     ~~~~
    // Error! Property 'prop' of type 'boolean'
    // is not assignable to string index type
    // 'string | number | undefined'.
    static [propName: string]: string | number | undefined;
}

import { func } from "./module.js";
　次のような記述であればよいのだが、そうではない。
from "./module.js" import { func };

/**
 * To be called 70 to 80 days after {@link plantCarrot}.
 */
function harvestCarrot(carrot: Carrot) {
}
/**
 * Call early in spring for best results. Added in v2.1.0.
 * @param seed Make sure it's a carrot seed!
 */
function plantCarrot(seed: Seed) {
    // TODO: some gardening
}