typescript-4.0.md

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TypeScript 4.0	docs	/docs/handbook/release-notes/typescript-4-0.html	TypeScript 4.0 Release Notes

가변 인자 튜플 타입 (Variadic Tuple Types)

배열이나 튜플 타입 두 개를 결합하여 새로운 배열을 만드는 JavaScript의 concat 함수에 대해서 생각해봅시다.

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

그리고, 배열이나 튜플을 변수로 입력받아 첫 번째 원소를 제외한 나머지를 반환하는 tail 함수에 대해서도 생각해봅시다.

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

TypeScript에서는 이 두 함수의 타입을 어떻게 정의할 수 있을까요?

concat의 경우, 이전 버전에서는 여러 개의 오버로드를 작성하는 방법이 유일했습니다.

function concat(arr1: [], arr2: []): [];
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];)

음... 네, 이 오버로드들의 두 번째 배열은 전부 비어있습니다. 이때, arr2가 하나의 인자를 가지고 있는 경우를 추가해봅시다.

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];

이런 오버로딩 함수들은 분명 비합리적입니다. 안타깝게도, tail 함수를 타이핑할 때도 이와 비슷한 문제에 직면하게 됩니다.

이것은 "천 개의 오버로드로 인한 죽음(death by a thousand overloads)"의 하나의 경우이며, 심지어 대부분 문제를 해결하지도 못합니다. 우리가 작성하고자 하는 만큼의 오버로드에 한해서만 올바른 타입을 제공합니다. 포괄적인 케이스를 만들고 싶다면, 다음과 같은 오버로드가 필요합니다.

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

그러나 위 시그니처는 튜플을 사용할 때 입력 길이나 요소 순서에 대한 어떤 것도 처리하지 않습니다.

TypeScript 4.0은 타입 추론 개선을 포함한 두 가지 핵심적인 변화를 도입해 이러한 타이핑을 가능하도록 만들었습니다.

첫 번째 변화는 튜플 타입 구문의 스프레드 연산자에서 제네릭 타입을 사용할 수 있다는 점입니다. 우리가 작동하는 실제 타입을 모르더라도 튜플과 배열에 대한 고차함수를 표현할 수 있다는 뜻입니다. 이러한 튜플 타입에서 제네릭 스프레드 연산자가 인스턴스화(혹은, 실제 타입으로 대체)되면 또 다른 배열이나 튜플 타입 세트를 생산할 수 있습니다.

예를 들어, tail 같은 함수를 "천 개의 오버로드로 인한 죽음(death by a thousand overloads)"이슈 없이 타이핑 할 수 있게 됩니다.

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"];

const r1 = tail(myTuple);
//    ^ = const r1: [2, 3, 4]

const r2 = tail([...myTuple, ...myArray] as const);
//    ^ = const r2: [2, 3, 4, ...string[]]

두 번째 변화는 나머지 요소가 끝뿐만 아니라 튜플의 어느 곳에서도 발생할 수 있다는 것입니다.

type Strings = [string, string];
type Numbers = [number, number];

type StrStrNumNumBool = [...Strings, ...Numbers, boolean];
//   ^ = type StrStrNumNumBool = [string, string, number, number, boolean]

이전에는, TypeScript는 다음과 같은 오류를 생성했었습니다:

A rest element must be last in a tuple type.

TypeScript 4.0에서는 이러한 제한이 완화되었습니다.

길이가 정해지지 않은 타입을 확장하려고할 때, 결과의 타입은 제한되지 않으며, 다음 모든 요소가 결과의 나머지 요소 타입에 포함되는 점에 유의하시기 바랍니다.

type Strings = [string, string];
type Numbers = number[];

type Unbounded = [...Strings, ...Numbers, boolean];
//   ^ = type Unbounded = [string, string, ...(number | boolean)[]]

이 두 가지 동작을 함께 결합하여, concat에 대해 타입이 제대로 정의된 시그니처를 작성할 수 있습니다.

type Arr = readonly any[];

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

하나의 시그니처가 조금 길더라도, 반복할 필요가 없는 하나의 시그니처일 뿐이며, 모든 배열과 튜플에서 예측 가능한 행동을 제공합니다.

이 기능은 그 자체만으로도 훌륭하지만, 조금 더 정교한 시나리오에서도 빛을 발합니다. 예를 들어,함수의 매개변수를 부분적으로 적용하여 새로운 함수를 반환하는 partialCall 함수가 있다고 생각해봅시다. partialCall은 다음과 같은 함수를 가집니다. - f가 예상하는 몇 가지 인수와 함께 f라고 지정하겠습니다. 그 후, f가 여전히 필요로 하는 다른 인수를 가지고, 그것을 받을 때 f를 호출하는 새로운 함수를 반환합니다.

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

TypeScript 4.0은 나머지 파라미터들과 튜플 원소들에 대한 추론 프로세스를 개선하여 타입을 지정할 수 있고 "그냥 동작"하도록 할 수 있습니다.

type Arr = readonly unknown[];

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

이 경우, partialCall은 처음에 취할 수 있는 파라미터와 할 수 없는 파라미터를 파악하고, 남은 것들은 적절히 수용하고 거부하는 함수들을 반환합니다.

// @errors: 2345 2554 2554 2345
type Arr = readonly unknown[];

function partialCall<T extends Arr, U extends Arr, R>(
  f: (...args: [...T, ...U]) => R,
  ...headArgs: T
) {
  return (...tailArgs: U) => f(...headArgs, ...tailArgs);
}
// ---cut---
const foo = (x: string, y: number, z: boolean) => {};

const f1 = partialCall(foo, 100);

const f2 = partialCall(foo, "hello", 100, true, "oops");

// 작동합니다!
const f3 = partialCall(foo, "hello");
//    ^ = const f3: (y: number, z: boolean) => void

// f3으로 뭘 할 수 있을까요?

// 작동합니다!
f3(123, true);

f3();

f3(123, "hello");

가변 인자 튜플 타입은 특히 기능 구성과 관련하여 많은 새로운 흥미로운 패턴을 가능하게 합니다. 우리는 JavaScript에 내장된 bind 메서드의 타입 체킹을 더 잘하기 위해 이를 활용할 수 있을 것이라고 기대합니다. 몇 가지 다른 추론 개선 및 패턴들도 여기에 포함되어 있으며, 가변 인자 튜플에 대해 더 알아보고 싶다면, the pull request를 참고해보세요.

Labeled Tuple Elements

Improving the experience around tuple types and parameter lists is important because it allows us to get strongly typed validation around common JavaScript idioms - really just slicing and dicing argument lists and passing them to other functions. The idea that we can use tuple types for rest parameters is one place where this is crucial.

For example, the following function that uses a tuple type as a rest parameter...

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

...should appear no different from the following function...

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

...for any caller of foo.

// @errors: 2554
function foo(arg0: string, arg1: number): void {
  // ...
}
// ---cut---
foo("hello", 42);

foo("hello", 42, true);
foo("hello");

There is one place where the differences begin to become observable though: readability. In the first example, we have no parameter names for the first and second elements. While these have no impact on type-checking, the lack of labels on tuple positions can make them harder to use - harder to communicate our intent.

That's why in TypeScript 4.0, tuples types can now provide labels.

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

To deepen the connection between parameter lists and tuple types, the syntax for rest elements and optional elements mirrors the syntax for parameter lists.

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

There are a few rules when using labeled tuples. For one, when labeling a tuple element, all other elements in the tuple must also be labeled.

// @errors: 5084
type Bar = [first: string, number];

It's worth noting - labels don't require us to name our variables differently when destructuring. They're purely there for documentation and tooling.

function foo(x: [first: string, second: number]) {
    // ...

    // note: we didn't need to name these 'first' and 'second'
    const [a, b] = x;
    a
//  ^?
    b
//  ^?
}

Overall, labeled tuples are handy when taking advantage of patterns around tuples and argument lists, along with implementing overloads in a type-safe way. In fact, TypeScript's editor support will try to display them as overloads when possible.

To learn more, check out the pull request for labeled tuple elements.

Class Property Inference from Constructors

TypeScript 4.0 can now use control flow analysis to determine the types of properties in classes when noImplicitAny is enabled.

class Square {
  // Previously both of these were any
  area;
// ^?
  sideLength;
// ^?
  constructor(sideLength: number) {
    this.sideLength = sideLength;
    this.area = sideLength ** 2;
  }
}

In cases where not all paths of a constructor assign to an instance member, the property is considered to potentially be undefined.

// @errors: 2532
class Square {
  sideLength;
// ^?

  constructor(sideLength: number) {
    if (Math.random()) {
      this.sideLength = sideLength;
    }
  }

  get area() {
    return this.sideLength ** 2;
  }
}

In cases where you know better (e.g. you have an initialize method of some sort), you'll still need an explicit type annotation along with a definite assignment assertion (!) if you're in strictPropertyInitialization.

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;
  }
}

For more details, see the implementing pull request.

Short-Circuiting Assignment Operators

JavaScript, and a lot of other languages, support a set of operators called compound assignment operators. Compound assignment operators apply an operator to two arguments, and then assign the result to the left side. You may have seen these before:

// 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;

So many operators in JavaScript have a corresponding assignment operator! Up until recently, however, there were three notable exceptions: logical and (&&), logical or (||), and nullish coalescing (??).

That's why TypeScript 4.0 supports a new ECMAScript feature to add three new assignment operators: &&=, ||=, and ??=.

These operators are great for substituting any example where a user might write code like the following:

a = a && b;
a = a || b;
a = a ?? b;

Or a similar if block like

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

There are even some patterns we've seen (or, uh, written ourselves) to lazily initialize values, only if they'll be needed.

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

// After
(values ??= []).push("hello");

(look, we're not proud of all the code we write...)

On the rare case that you use getters or setters with side-effects, it's worth noting that these operators only perform assignments if necessary. In that sense, not only is the right side of the operator "short-circuited" - the assignment itself is too.

obj.prop ||= foo();

// roughly equivalent to either of the following

obj.prop || (obj.prop = foo());

if (!obj.prop) {
    obj.prop = foo();
}

Try running the following example to see how that differs from always performing the assignment.

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();

We'd like to extend a big thanks to community member Wenlu Wang for this contribution!

For more details, you can take a look at the pull request here. You can also check out TC39's proposal repository for this feature.

unknown on catch Clause Bindings

Since the beginning days of TypeScript, catch clause variables have always been typed as any. This meant that TypeScript allowed you to do anything you wanted with them.

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

The above has some undesirable behavior if we're trying to prevent more errors from happening in our error-handling code! Because these variables have the type any by default, they lack any type-safety which could have errored on invalid operations.

That's why TypeScript 4.0 now lets you specify the type of catch clause variables as unknown instead. unknown is safer than any because it reminds us that we need to perform some sorts of type-checks before operating on our values.

// @errors: 2571
try {
  // ...
} catch (e: unknown) {
  // Can't access values on unknowns
  console.log(e.toUpperCase());

  if (typeof e === "string") {
    // We've narrowed 'e' down to the type 'string'.
    console.log(e.toUpperCase());
  }
}

While the types of catch variables won't change by default, we might consider a new --strict mode flag in the future so that users can opt in to this behavior. In the meantime, it should be possible to write a lint rule to force catch variables to have an explicit annotation of either : any or : unknown.

For more details you can peek at the changes for this feature.

Custom JSX Factories

When using JSX, a fragment is a type of JSX element that allows us to return multiple child elements. When we first implemented fragments in TypeScript, we didn't have a great idea about how other libraries would utilize them. Nowadays most other libraries that encourage using JSX and support fragments have a similar API shape.

In TypeScript 4.0, users can customize the fragment factory through the new jsxFragmentFactory option.

As an example, the following tsconfig.json file tells TypeScript to transform JSX in a way compatible with React, but switches each factory invocation to h instead of React.createElement, and uses Fragment instead of React.Fragment.

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

In cases where you need to have a different JSX factory on a per-file basis, you can take advantage of the new /** @jsxFrag */ pragma comment. For example, the following...

// @noErrors
// 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";

export const Header = (
  <>
    <h1>Welcome</h1>
  </>
);

...will get transformed to this output JavaScript...

// @noErrors
// @showEmit
// 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";

export const Header = (
  <>
    <h1>Welcome</h1>
  </>
);

We'd like to extend a big thanks to community member Noj Vek for sending this pull request and patiently working with our team on it.

You can see that the pull request for more details!

Speed Improvements in build mode with --noEmitOnError

Previously, compiling a program after a previous compile with errors under --incremental would be extremely slow when using the --noEmitOnError flag. This is because none of the information from the last compilation would be cached in a .tsbuildinfo file based on the --noEmitOnError flag.

TypeScript 4.0 changes this which gives a great speed boost in these scenarios, and in turn improves --build mode scenarios (which imply both --incremental and --noEmitOnError).

For details, read up more on the pull request.

--incremental with --noEmit

TypeScript 4.0 allows us to use the --noEmit flag when while still leveraging --incremental compiles. This was previously not allowed, as --incremental needs to emit a .tsbuildinfo files; however, the use-case to enable faster incremental builds is important enough to enable for all users.

For more details, you can see the implementing pull request.

Editor Improvements

The TypeScript compiler doesn't only power the editing experience for TypeScript itself in most major editors - it also powers the JavaScript experience in the Visual Studio family of editors and more. For that reason, much of our work focuses on improving editor scenarios - the place you spend most of your time as a developer.

Using new TypeScript/JavaScript functionality in your editor will differ depending on your editor, but

• Visual Studio Code supports selecting different versions of TypeScript. Alternatively, there's the JavaScript/TypeScript Nightly Extension to stay on the bleeding edge (which is typically very stable).
• Visual Studio 2017/2019 have [the SDK installers above] and MSBuild installs.
• Sublime Text 3 supports selecting different versions of TypeScript

You can check out a partial list of editors that have support for TypeScript to learn more about whether your favorite editor has support to use new versions.

Convert to Optional Chaining

Optional chaining is a recent feature that's received a lot of love. That's why TypeScript 4.0 brings a new refactoring to convert common patterns to take advantage of optional chaining and nullish coalescing!

Keep in mind that while this refactoring doesn't perfectly capture the same behavior due to subtleties with truthiness/falsiness in JavaScript, we believe it should capture the intent for most use-cases, especially when TypeScript has more precise knowledge of your types.

For more details, check out the pull request for this feature.

/** @deprecated */ Support

TypeScript's editing support now recognizes when a declaration has been marked with a /** @deprecated * JSDoc comment. That information is surfaced in completion lists and as a suggestion diagnostic that editors can handle specially. In an editor like VS Code, deprecated values are typically displayed a strike-though style like this.

This new functionality is available thanks to Wenlu Wang. See the pull request for more details.

Partial Semantic Mode at Startup

We've heard a lot from users suffering from long startup times, especially on bigger projects. The culprit is usually a process called program construction. This is the process of starting with an initial set of root files, parsing them, finding their dependencies, parsing those dependencies, finding those dependencies' dependencies, and so on. The bigger your project is, the longer you'll have to wait before you can get basic editor operations like go-to-definition or quick info.

That's why we've been working on a new mode for editors to provide a partial experience until the full language service experience has loaded up. The core idea is that editors can run a lightweight partial server that only looks at the current files that the editor has open.

It's hard to say precisely what sorts of improvements you'll see, but anecdotally, it used to take anywhere between 20 seconds to a minute before TypeScript would become fully responsive on the Visual Studio Code codebase. In contrast, our new partial semantic mode seems to bring that delay down to just a few seconds. As an example, in the following video, you can see two side-by-side editors with TypeScript 3.9 running on the left and TypeScript 4.0 running on the right.

When restarting both editors on a particularly large codebase, the one with TypeScript 3.9 can't provide completions or quick info at all. On the other hand, the editor with TypeScript 4.0 can immediately give us a rich experience in the current file we're editing, despite loading the full project in the background.

Currently the only editor that supports this mode is Visual Studio Code which has some UX improvements coming up in Visual Studio Code Insiders. We recognize that this experience may still have room for polish in UX and functionality, and we have a list of improvements in mind. We're looking for more feedback on what you think might be useful.

For more information, you can see the original proposal, the implementing pull request, along with the follow-up meta issue.

Smarter Auto-Imports

Auto-import is a fantastic feature that makes coding a lot easier; however, every time auto-import doesn't seem to work, it can throw users off a lot. One specific issue that we heard from users was that auto-imports didn't work on dependencies that were written in TypeScript - that is, until they wrote at least one explicit import somewhere else in their project.

Why would auto-imports work for @types packages, but not for packages that ship their own types? It turns out that auto-imports only work on packages your project already includes. Because TypeScript has some quirky defaults that automatically add packages in node_modules/@types to your project, those packages would be auto-imported. On the other hand, other packages were excluded because crawling through all your node_modules packages can be really expensive.

All of this leads to a pretty lousy getting started experience for when you're trying to auto-import something that you've just installed but haven't used yet.

TypeScript 4.0 now does a little extra work in editor scenarios to include the packages you've listed in your package.json's dependencies (and peerDependencies) fields. The information from these packages is only used to improve auto-imports, and doesn't change anything else like type-checking. This allows us to provide auto-imports for all of your dependencies that have types, without incurring the cost of a complete node_modules search.

In the rare cases when your package.json lists more than ten typed dependencies that haven't been imported yet, this feature automatically disables itself to prevent slow project loading. To force the feature to work, or to disable it entirely, you should be able to configure your editor. For Visual Studio Code, this is the "Include Package JSON Auto Imports" (or typescript.preferences.includePackageJsonAutoImports) setting.

For more details, you can see the proposal issue along with the implementing pull request.

Our New Website

The TypeScript website has recently been rewritten from the ground up and rolled out!

We already wrote a bit about our new site, so you can read up more there; but it's worth mentioning that we're still looking to hear what you think! If you have questions, comments, or suggestions, you can file them over on the website's issue tracker.

Breaking Changes

lib.d.ts Changes

Our lib.d.ts declarations have changed - most specifically, types for the DOM have changed. The most notable change may be the removal of document.origin which only worked in old versions of IE and Safari MDN recommends moving to self.origin.

Properties Overriding Accessors (and vice versa) is an Error

Previously, it was only an error for properties to override accessors, or accessors to override properties, when using useDefineForClassFields; however, TypeScript now always issues an error when declaring a property in a derived class that would override a getter or setter in the base class.

// @errors: 1049 2610
class Base {
  get foo() {
    return 100;
  }
  set foo(value) {
    // ...
  }
}

class Derived extends Base {
  foo = 10;
}

// @errors: 2611
class Base {
  prop = 10;
}

class Derived extends Base {
  get prop() {
    return 100;
  }
}

See more details on the implementing pull request.

Operands for delete must be optional

When using the delete operator in strictNullChecks, the operand must now be any, unknown, never, or be optional (in that it contains undefined in the type). Otherwise, use of the delete operator is an error.

// @errors: 2790
interface Thing {
  prop: string;
}

function f(x: Thing) {
  delete x.prop;
}

See more details on the implementing pull request.

Usage of TypeScript's Node Factory is Deprecated

Today TypeScript provides a set of "factory" functions for producing AST Nodes; however, TypeScript 4.0 provides a new node factory API. As a result, for TypeScript 4.0 we've made the decision to deprecate these older functions in favor of the new ones.

For more details, read up on the relevant pull request for this change.