TypeScript 4.0正式发布！现在是开始使用它的最佳时机

npm install -D typescript

我们还没有提到关于语言服务、基础架构、网站和其他核心项目中的那些工作，这些工作对于 TypeScript 的体验非常关键。核心团队的项目之外，我们在生态系统中还有非常出色的贡献者社区，他们推动了体验的不断改进，并通过 DefinitelyTyped 甚至 TypeScript 本身提供了帮助。在 2012 年刚开始时，DefinitelyTyped 仅有 80 个拉取请求。在 2019 年，它有超过 8300 个拉取请求，非常震撼人心。这些贡献是 TypeScript 体验的基础，这样一个繁忙而热情的社区在不断改善我们的生态系统，并推动我们不断改进，我们对此表示感谢。

function concat(arr1, arr2) {
    return [...arr1, ...arr2];
}

function tail(arg) {
    const [_, ...result] = arg;
    return result
}

function concat<>(arr1: [], arr2: []): [A];
function concat<A>(arr1: [A], arr2: []): [A];
function concat<A, B>(arr1: [A, B], arr2: []): [A, B];
function concat<A, B, C>(arr1: [A, B, C], arr2: []): [A, B, C];
function concat<A, B, C, D>(arr1: [A, B, C, D], arr2: []): [A, B, C, D];
function concat<A, B, C, D, E>(arr1: [A, B, C, D, E], arr2: []): [A, B, C, D, E];
function concat<A, B, C, D, E, F>(arr1: [A, B, C, D, E, F], arr2: []): [A, B, C, D, E, F];)

function concat<A2>(arr1: [], arr2: [A2]): [A2];
function concat<A1, A2>(arr1: [A1], arr2: [A2]): [A1, A2];
function concat<A1, B1, A2>(arr1: [A1, B1], arr2: [A2]): [A1, B1, A2];
function concat<A1, B1, C1, A2>(arr1: [A1, B1, C1], arr2: [A2]): [A1, B1, C1, A2];
function concat<A1, B1, C1, D1, A2>(arr1: [A1, B1, C1, D1], arr2: [A2]): [A1, B1, C1, D1, A2];
function concat<A1, B1, C1, D1, E1, A2>(arr1: [A1, B1, C1, D1, E1], arr2: [A2]): [A1, B1, C1, D1, E1, A2];
function concat<A1, B1, C1, D1, E1, F1, A2>(arr1: [A1, B1, C1, D1, E1, F1], arr2: [A2]): [A1, B1, C1, D1, E1, F1, A2];

function concat<T, U>(arr1: T[], arr2, U[]): Array<T | U>;

function tail<T extends any[]>(arr: readonly [any, ...T]) {
    const [_ignored, ...rest] = arr;
    return rest;
}
const myTuple = [1, 2, 3, 4] as const;
const myArray = ["hello", "world"];
// type [2, 3, 4]
const r1 = tail(myTuple);
// type [2, 3, ...string[]]
const r2 = tail([...myTuple, ...myArray] as const);

type Strings = [string, string];
type Numbers = [number, number];
// [string, string, number, number]
type StrStrNumNum = [...Strings, ...Numbers];

A rest element must be last in a tuple type.

type Strings = [string, string];
type Numbers = number[]
// [string, string, ...Array]
type Unbounded = [...Strings, ...Numbers, boolean];

type Arr = readonly any[];
function concat<T extends Arr, U extends Arr>(arr1: T, arr2: U): [...T, ...U] {
    return [...arr1, ...arr2];
}

function partialCall(f, ...headArgs) {
    return (...tailArgs) => f(...headArgs, ...tailArgs)
}

type Arr = readonly unknown[];
function partialCall<T extends Arr, U extends Arr, R>(f: (...args: [...T, ...U]) => R, ...headArgs: T) {
    return (...b: U) => f(...headArgs, ...b)
}

const foo = (x: string, y: number, z: boolean) => {}
// This doesn't work because we're feeding in the wrong type for 'x'.
const f1 = partialCall(foo, 100);
// ~~~
// error! Argument of type 'number' is not assignable to parameter of type 'string'.

// This doesn't work because we're passing in too many arguments.
const f2 = partialCall(foo, "hello", 100, true, "oops")
// ~~~~~~
// error! Expected 4 arguments, but got 5.

// This works! It has the type '(y: number, z: boolean) => void'
const f3 = partialCall(foo, "hello");
// What can we do with f3 now?
f3(123, true); // works!
f3();
// error! Expected 2 arguments, but got 0.
f3(123, "hello");
// ~~~~~~~
// error! Argument of type '"hello"' is not assignable to parameter of type 'boolean'.

function foo(...args: [string, number]): void {
    // ...
}

function foo(arg0: string, arg1: number): void {
    // ...
}

foo("hello", 42); // works
foo("hello", 42, true); // error
foo("hello"); // error

type Range = [start: number, end: number];

type Foo = [first: number, second?: string, ...rest: any[]];

type Bar = [first: string, number];
// ~~~~~~
// error! Tuple members must all have names or all not have names.

function foo(x: [first: string, second: number]) {
    // ...
    // note: we didn't need to name these 'first' and 'second'
    let [a, b] = x;
    // ...
}

class Square {
    // Previously: implicit any!
    // Now: inferred to `number`!
    area;
    sideLength;
    constructor(sideLength: number) {
        this.sideLength = sideLength;
        this.area = sideLength ** 2;
    }
}

class Square {
    sideLength;
    constructor(sideLength: number) {
        if (Math.random()) {
            this.sideLength = sideLength;
        }
    }
    get area() {
        return this.sideLength ** 2;
        // ~~~~~~~~~~~~~~~
        // error! Object is possibly 'undefined'.
    }
}

class Square {
    // definite assignment assertion
    // v
    sideLength!: number;
    // ^^^^^^^^
    // type annotation
    constructor(sideLength: number) {
        this.initialize(sideLength)
    }
    initialize(sideLength: number) {
        this.sideLength = sideLength;
    }
    get area() {
        return this.sideLength ** 2;
    }
}

// Addition
// a = a + b
a += b;
// Subtraction
// a = a - b
a -= b;
// Multiplication
// a = a * b
a *= b;
// Division
// a = a / b
a /= b;
// Exponentiation
// a = a ** b
a **= b;
// Left Bit Shift
// a = a << b
a <<= b;

a = a && b;
a = a || b;
a = a ?? b;

// could be 'a ||= b'
if (!a) {
    a = b;
}

let values: string[];
// Before
(values ?? (values = [])).push("hello");
// After
(values ??= []).push("hello");

obj.prop ||= foo();
// roughly equivalent to either of the following
obj.prop || (obj.prop = foo());
if (!obj.prop) {
    obj.prop = foo();
}

const obj = {
    get prop() {
        console.log("getter has run");
        // Replace me!
        return Math.random() < 0.5;
    },
    set prop(_val: boolean) {
        console.log("setter has run");
    }
};
function foo() {
    console.log("right side evaluated");
    return true;
}
console.log("This one always runs the setter");
obj.prop = obj.prop || foo();
console.log("This one *sometimes* runs the setter");
obj.prop ||= foo();

try {
    // ...
}
catch (x) {
    // x has type 'any' - have fun!
    console.log(x.message);
    console.log(x.toUpperCase());
    x++;
    x.yadda.yadda.yadda();
}

try {
    // ...
}
catch (e: unknown) {
    // error!
    // Property 'toUpperCase' does not exist on type 'unknown'.
    console.log(e.toUpperCase());
    if (typeof e === "string") {
        // works!
        // We've narrowed 'e' down to the type 'string'.
        console.log(e.toUpperCase());
    }
}

{
  "compilerOptions": {
    "target": "esnext",
    "module": "commonjs",
    "jsx": "react",
    "jsxFactory": "h",
    "jsxFragmentFactory": "Fragment"
  }
}

// Note: these pragma comments need to be written
// with a JSDoc-style multiline syntax to take effect.
/** @jsx h */
/** @jsxFrag Fragment */
import { h, Fragment } from "preact";
let stuff = <>
    <div>Hellodiv>
;

// Note: these pragma comments need to be written
// with a JSDoc-style multiline syntax to take effect.
/** @jsx h */
/** @jsxFrag Fragment */
import { h, Fragment } from "preact";
let stuff = h(Fragment, null,
    h("div", null, "Hello"));

class Base {
    get foo() {
        return 100;
    }
    set foo() {
        // ...
    }
}
class Derived extends Base {
    foo = 10;
//  ~~~
// error!
// 'foo' is defined as an accessor in class 'Base',
// but is overridden here in 'Derived' as an instance property.
}

class Base {
    prop = 10;
}
class Derived extends Base {
    get prop() {
    // ~~~~
    // error!
    // 'prop' is defined as a property in class 'Base', but is overridden here in 'Derived' as an accessor.
        return 100;
    }
}

interface Thing {
    prop: string;
}
function f(x: Thing) {
    delete x.prop;
    // ~~~~~~
    // error! The operand of a 'delete' operator must be optional.
}