TypeScript 4.2 正式发布：更智能的类型别名保留，声明缺失的帮助函数，还有许多破坏性更新

npm install typescript

type BasicPrimitive = number | string | boolean;

export type BasicPrimitive = number | string | boolean;
export function doStuff(value: BasicPrimitive) {
    let x = value;
    return x;
}

export type BasicPrimitive = number | string | boolean;
export function doStuff(value: BasicPrimitive) {
    if (Math.random() < 0.5) {
        return undefined;
    }
    return value;
}

// A tuple that stores a pair of numbers
let a: [number, number] = [1, 2];
// A tuple that stores a string, a number, and a boolean
let b: [string, number, boolean] = ["hello", 42, true];

// A tuple that has either one or two strings.
let c: [string, string?] = ["hello"];
c = ["hello", "world"];
// A labeled tuple that has either one or two strings.
let d: [first: string, second?: string] = ["hello"];
d = ["hello", "world"];
// A tuple with a *rest element* - holds at least 2 strings at the front,
// and any number of booleans at the back.
let e: [string, string, ...boolean[]];
e = ["hello", "world"];
e = ["hello", "world", false];
e = ["hello", "world", true, false, true];

let foo: [...string[], number];
foo = [123];
foo = ["hello", 123];
foo = ["hello!", "hello!", "hello!", 123];
let bar: [boolean, ...string[], boolean];
bar = [true, false];
bar = [true, "some text", false];
bar = [true, "some", "separated", "text", false];

interface Clown { /*...*/ }
interface Joker { /*...*/ }
let StealersWheel: [...Clown[], "me", ...Joker[]];
// ~~~~~~~~~~ Error!
// A rest element cannot follow another rest element.
let StringsAndMaybeBoolean: [...string[], boolean?];
// ~~~~~~~~ Error!
// An optional element cannot follow a rest element.

declare function doStuff(...args: [...names: string[], shouldCapitalize: boolean]): void;
doStuff(/*shouldCapitalize:*/ false)
doStuff("fee", "fi", "fo", "fum", /*shouldCapitalize:*/ true);

"foo" in 42
// ~~
// error! The right-hand side of an 'in' expression must not be a primitive.

interface SomeType {
    /** This is an index signature. */
    [propName: string]: any;
}
function doStuff(value: SomeType) {
    let x = value["someProperty"];
}

interface Options {
    /** File patterns to be excluded. */
    exclude?: string[];
    /**
     * It handles any extra properties that we haven't declared as type 'any'.
     */
    [x: string]: any;
}
function processOptions(opts: Options) {
    // Notice we're *intentionally* accessing `excludes`, not `exclude`
    if (opts.excludes) {
        console.error("The option `excludes` is not valid. Did you mean `exclude`?");
    }
}

function processOptions(opts: Options) {
    // ...
    // Notice we're *accidentally* accessing `excludes` this time.
    // Oops! Totally valid.
    for (const excludePattern of opts.excludes) {
        // ...
    }
}

abstract class Shape {
    abstract getArea(): number;
}
// Error! Can't instantiate an abstract class.
new Shape();
class Square extends Shape {
    #sideLength: number;
    constructor(sideLength: number) {
        this.#sideLength = sideLength;
    }
    getArea() {
        return this.#sideLength ** 2;
    }
}
// Works fine.
new Square(42);

interface HasArea {
    getArea(): number;
}
// Error! Cannot assign an abstract constructor type to a non-abstract constructor type.
let Ctor: new () => HasArea = Shape;

functon makeSubclassWithArea(Ctor: new () => HasArea) {
    return class extends Ctor {
        getArea() {
            // ...
        }
    }
}
let MyShape = makeSubclassWithArea(Shape);

// Error!
// Type 'typeof Shape' does not satisfy the constraint 'new (...args: any) => any'.
// Cannot assign an abstract constructor type to a non-abstract constructor type.
type MyInstance = InstanceType<typeof Shape>;

interface HasArea {
    getArea(): number;
}
// Works!
let Ctor: abstract new () => HasArea = Shape;
// ^^^^^^^^

abstract class SuperClass {
    abstract someMethod(): void;
    badda() {}
}
type AbstractConstructor<T> = abstract new (...args: any[]) => T
function withStyles<T extends AbstractConstructor<object>>(Ctor: T) {
    abstract class StyledClass extends Ctor {
        getStyles() {
            // ...
        }
    }
    return StyledClass;
}
class SubClass extends withStyles(SuperClass) {
    someMethod() {
        this.someMethod()
    }
}

tsc --explainFiles

# Forward output to a text file
tsc --explainFiles > expanation.txt
# Pipe output to a utility program like `less`, or an editor like VS Code
tsc --explainFiles | less
tsc --explainFiles | code -

TS_Compiler_Directory/4.2.2/lib/lib.es5.d.ts
  Library referenced via 'es5' from file 'TS_Compiler_Directory/4.2.2/lib/lib.es2015.d.ts'
TS_Compiler_Directory/4.2.2/lib/lib.es2015.d.ts
  Library referenced via 'es2015' from file 'TS_Compiler_Directory/4.2.2/lib/lib.es2016.d.ts'
TS_Compiler_Directory/4.2.2/lib/lib.es2016.d.ts
  Library referenced via 'es2016' from file 'TS_Compiler_Directory/4.2.2/lib/lib.es2017.d.ts'
TS_Compiler_Directory/4.2.2/lib/lib.es2017.d.ts
  Library referenced via 'es2017' from file 'TS_Compiler_Directory/4.2.2/lib/lib.es2018.d.ts'
TS_Compiler_Directory/4.2.2/lib/lib.es2018.d.ts
  Library referenced via 'es2018' from file 'TS_Compiler_Directory/4.2.2/lib/lib.es2019.d.ts'
TS_Compiler_Directory/4.2.2/lib/lib.es2019.d.ts
  Library referenced via 'es2019' from file 'TS_Compiler_Directory/4.2.2/lib/lib.es2020.d.ts'
TS_Compiler_Directory/4.2.2/lib/lib.es2020.d.ts
  Library referenced via 'es2020' from file 'TS_Compiler_Directory/4.2.2/lib/lib.esnext.d.ts'
TS_Compiler_Directory/4.2.2/lib/lib.esnext.d.ts
  Library 'lib.esnext.d.ts' specified in compilerOptions
... More Library References...
foo.ts
  Matched by include pattern '**/*' in 'tsconfig.json'

function shouldDisplayElement(element: Element) {
    // ...
    return true;
}
function getVisibleItems(elements: Element[]) {
    return elements.filter(e => shouldDisplayElement && e.children.length)
    // ~~~~~~~~~~~~~~~~~~~~
    // This condition will always return true since the function is always defined.
    // Did you mean to call it instead.
}

let [_first, second] = getValues();

const movieWatchCount: { [key: string]: number } = {};
function watchMovie(title: string) {
  movieWatchCount[title] = (movieWatchCount[title] ?? 0) + 1;
}

type WesAndersonWatchCount = {
  "Fantastic Mr. Fox"?: number;
  "The Royal Tenenbaums"?: number;
  "Moonrise Kingdom"?: number;
  "The Grand Budapest Hotel"?: number;
};
declare const wesAndersonWatchCount: WesAndersonWatchCount;
const movieWatchCount: { [key: string]: number } = wesAndersonWatchCount;
// ~~~~~~~~~~~~~~~ error!
// Type 'WesAndersonWatchCount' is not assignable to type '{ [key: string]: number; }'.
// Property '"Fantastic Mr. Fox"' is incompatible with index signature.
// Type 'number | undefined' is not assignable to type 'number'.
// Type 'undefined' is not assignable to type 'number'. (2322)

type BatmanWatchCount = {
  "Batman Begins": number | undefined;
  "The Dark Knight": number | undefined;
  "The Dark Knight Rises": number | undefined;
};
declare const batmanWatchCount: BatmanWatchCount;
// Still an error in TypeScript 4.2.
// `undefined` is only ignored when properties are marked optional.
const movieWatchCount: { [key: string]: number } = batmanWatchCount;
// Still an error in TypeScript 4.2.
// Index signatures don't implicitly allow explicit `undefined`.
movieWatchCount["It's the Great Pumpkin, Charlie Brown"] = undefined;

declare let sortOfArrayish: { [key: number]: string };
declare let numberKeys: { 42?: string };
// Error! Type '{ 42?: string | undefined; }' is not assignable to type '{ [key: number]: string; }'.
sortOfArrayish = numberKeys;

function* g1() {
  const value = yield 1;
  // ~~~~~~~
  // Error!
  // 'yield' expression implicitly results in an 'any' type
  // because its containing generator lacks a return-type annotation.
}
function* g2() {
  // No error.
  // The result of `yield 1` is unused.
  yield 1;
}
function* g3() {
  // No error.
  // `yield 1` is contextually typed by 'string'.
  const value: string = yield 1;
}
function* g3(): Generator<number, void, string> {
  // No error.
  // TypeScript can figure out the type of `yield 1`
  // from the explicit return type of `g3`.
  const value = yield 1;
}

f<T>(100)

(f < T) > (100)

"foo" in 42
// ~~
// error! The right-hand side of an 'in' expression must not be a primitive.

// Tuple types with spread elements
type NumStr = [number, string];
type NumStrNumStr = [...NumStr, ...NumStr];
// Array spread expressions
const numStr = [123, "hello"] as const;
const numStrNumStr = [...numStr, ...numStr] as const;

// must be changed something like
// - "./foo"
// - "./foo.js"
import { Foo } from "./foo.d.ts";

import { Foo } from "./foo";
import { Foo } from "./foo.js";
import { Foo } from "./foo/index.js";

declare const yourName: string;
// 'bar' is constant.
// It has type '`hello ${string}`'.
const bar = `hello ${yourName}`;

// 'baz' is mutable.
// It has type 'string'.
let baz = `hello ${yourName}`;

// 'bar' has type '"hello"'.
const bar = "hello";
// 'baz' has type 'string'.
let baz = "hello";

declare const yourName: string;
// 'bar' has type '`hello ${string}`'.
const bar = `hello ${yourName}` as const;
// ^^^^^^^^
// 'baz' has type 'string'.
const baz = `hello ${yourName}`;