TypeScript で関数を定義する方法

TypeScript の関数の概要を次に示します。💁‍♂️


パラメーターの後には、常に型の注釈を付ける必要があります。TypeScript は値自体から型を推測できるため、この規則の例外はデフォルト値です。パラメータはオプションですか? そうしない?と、関数を呼び出すときにリンターがエラーをスローします。

function printName(firstName: string, lastName = 'Smith', middleName?: string) { 
  if (middleName) console.log(firstName, middleName, lastName) 
  else console.log(firstName, lastName) 


TypeScript は関数の戻り値の型を推測できますが、自己文書化の形式として機能するため、明示的に記述することをお勧めします 👍 。型注釈は、括弧の、関数本体の前に記述されます。


function getNumber(): number { return 6 }
function sayHello(): void { console.log('Hello!') }



const isNumber5 = (value: number): boolean => value === 5 

ソース: https://dilshankelsen.com/define-functions-in-typescript/


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TypeScript で関数を定義する方法

The Definitive Guide to TypeScript & Possibly The Best TypeScript Book

TypeScript Deep Dive

I've been looking at the issues that turn up commonly when people start using TypeScript. This is based on the lessons from Stack Overflow / DefinitelyTyped and general engagement with the TypeScript community. You can follow for updates and don't forget to ★ on GitHub 🌹


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Get Started

If you are here to read the book online get started.


Book is completely free so you can copy paste whatever you want without requiring permission. If you have a translation you want me to link here. Send a PR.

Other Options

You can also download one of the Epub, Mobi, or PDF formats from the actions tab by clicking on the latest build run. You will find the files in the artifacts section.

Special Thanks

All the amazing contributors 🌹


Share URL: https://basarat.gitbook.io/typescript/

Author: Basarat
Source Code: https://github.com/basarat/typescript-book/ 
License: View license

#typescript #opensource 

Cayla  Erdman

Cayla Erdman


What’s New In Typescript 4.0?

Today I am going to talk about new features in Typescript 4.0.

TypeScript 4.0 comes with lots of new features to make JavaScript development easier.

Labeled Tuple Elements

You can label tuple elements.

You can write:

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

to restrict args to have a string and a number.

you can also write:

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

to have rest entries in your tuple.

If your tuple has type Foo, then the tuple starts with a number and a string.

Then the rest of the entries can be anything.

Labels don’t require you to name your variables differently when destructuring.

For example, if you have:

function foo(x: [first: string, second: number]) {
  const [a, b] = x;

then you can name the destructured variables anything you want.

#software-development #typescript-with-react #typescript #typescript-4 #react native

Christa  Stehr

Christa Stehr


Microsoft Releases TypeScript 4.0 With Speed Boosting Features

icrosoft recently announced the availability of TypeScript version 4.0. The developers at the tech giant claimed that this version of the language represents the next generation of TypeScript with more expressivity, productivity as well as scalability.

Developed by the tech giant, TypeScript is an open-source programming language that is built on top of JavaScript by adding syntax for static type definitions. The types in this language provide a way to describe the shape of an object, providing better documentation as well as allowing TypeScript to validate that the code is working correctly.

According to the latest Stack Overflow Developers survey 2020, it secured the second position as the most loved language and  9th position among 25 programming languages as the most commonly used programming language by the developers. In one of our articles, we discussed how TypeScript weighs over other programming languages.

It is one of the fastest-growing programming languages among the developers. The motive behind this language is that while writing down the types of values and where they are used, developers can use TypeScript to type-check the code and let them know about mistakes before they run the code. TypeScript compiler can be used to strip away types from the code, leaving them with clean, readable JavaScript that runs anywhere.

In the present scenario, TypeScript is a core part of many developer’s JavaScript stack. The language adds optional types to JavaScript that support tools for large-scale JavaScript applications for any browser, for any host and on any operating systems.

The program manager of TypeScript, Daniel Rosenwasser, said in a blog post, “In our past two major versions, we looked back at some highlights that shined over the years. For TypeScript 4.0, we’re going to keep up that tradition.”

What’s New?

Based on the feedback by the developer’s community, TypeScript 4.0 includes many intuitive features that are focussed on boosting the performance of this language. Some of them are mentioned below-

Speed Improvements in build mode with –noEmitOnError

According to Rosenwasser, previously, compiling a program after a previous compile with errors under incremental would result in extremely slow performance when using the –noEmitOnError flag. The reason is, none of the information from the last compilation would be cached in a .tsbuildinfo file based on the –noEmitOnError flag.

But now TypeScript 4.0 changes this. The new update provides a great speed boost in these scenarios, and in turn, improves the build mode scenarios, which imply both –incremental and –noEmitOnError.

#developers corner #microsoft #microsoft releases typescript 4.0 #programming language #programming language with high salary #typescript #typescript 4.0

Yoshiko  Jones

Yoshiko Jones


What is TypeScript? Why TypeScript? and Why Not TypeScript?

Maybe you’ve been using JavaScript for a while and you’ve been hearing the buzz around… TypeScript. In this 2 part series, we’ll take a look at Why TypeScript? and Why Not TypeScript?

What is TypeScript?

According to the TypeScript website “TypeScript extends JavaScript by adding Types… TypeScript code is transformed into JavaScript code via the TypeScript compiler or Babel. This JavaScript is clean, simple code which runs anywhere JavaScript runs…”. In short, many refer to TypeScript as a superset of JavaScript.

In my opinion, this is kind of true and kind of not. Technically, not all valid JavaScript code can also be TypeScript code. We’ll talk about that later on. But, at its core, TypeScript was designed to allow developers to turn JavaScript, a dynamic programming language (one that is interpreted at runtime) into a statically typed programming language (one that is compiled before running). Let’s dive in.

Reasons For TypeScript

Improved Code Quality (potentially)

TypeScript makes use of features similar to languages like C## or Java: it truly is object oriented. This allows for more robust code. It enables the developer to implement type checking and make use of interfaces, classes, inheritance, etc. This can up the game as far as clean code when looking at a JavaScript program and a TypeScript program.

_Why did you throw in the word _potentially? Well, most valid JavaScript code can become TypeScript code. This means you don’t have to use the object oriented or typed features of TypeScript. Remember, TypeScript is optionally typed. It is not required. If TypeScript is utilized merely like JavaScript then improved code quality may not actually be a thing.

#typescript #programming #javascript #web-development #developer

John David

John David


Write Safer JavaScript Applications With TypeScript!

Write Safer JavaScript Applications With TypeScript!

Learn how TypeScript can help you build safer and more resilient JavaScript applications. Expect some demos, some code and a lot of fun!

#typescript #typescript #programming #web-development #developer