"2022年TypeScript手册PDF：基础知识、函数和对象类型详解"

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The TypeScript Handbook is a comprehensive guide to learning and understanding TypeScript, a popular programming language that is a superset of JavaScript. This particular copy of the handbook was created on Monday, January 10, 2022, using TypeScript 4.5 and is in PDF format for personal reference. The handbook starts with the basics, focusing on the fundamental types in TypeScript. It covers everyday types, which are the language primitives that are commonly used in TypeScript programming. The handbook also delves into narrowing, which is a concept that explains how TypeScript leverages JavaScript knowledge to reduce the amount of type syntax needed in projects. One of the key sections in the handbook is about functions in TypeScript. Readers can learn about how functions work in TypeScript and how they can be effectively used in their projects. The handbook also explores object types in TypeScript, providing insights into how TypeScript describes different types of objects in a program. In addition to these topics, the handbook offers a wealth of information on various other aspects of TypeScript programming. Readers can find detailed explanations on advanced concepts, best practices, and practical examples to help them enhance their understanding and proficiency in TypeScript development. Overall, the TypeScript Handbook serves as a valuable resource for both beginners and experienced developers who want to master TypeScript and leverage its powerful features for building robust and scalable applications. With its clear explanations, comprehensive coverage, and practical insights, this handbook is an essential companion for anyone looking to dive deep into the world of TypeScript programming.

While the default target is ES3, the great majority of current browsers support ES2015. Most developers

can therefore safely specify ES2015 or above as a target, unless compatibility with certain ancient

browsers is important.

Strictness

Different users come to TypeScript looking for different things in a type-checker. Some people are

looking for a more loose opt-in experience which can help validate only some parts of their

program, and still have decent tooling. This is the default experience with TypeScript, where types

are optional, inference takes the most lenient types, and there's no checking for potentially

null / undefined values. Much like how tsc emits in the face of errors, these defaults are put in

place to stay out of your way. If you're migrating existing JavaScript, that might be a desirable first

step.

In contrast, a lot of users prefer to have TypeScript validate as much as it can straight away, and

that's why the language provides strictness settings as well. These strictness settings turn static

type-checking from a switch (either your code is checked or not) into something closer to a dial. The

further you turn this dial up, the more TypeScript will check for you. This can require a little extra

work, but generally speaking it pays for itself in the long run, and enables more thorough checks

and more accurate tooling. When possible, a new codebase should always turn these strictness

checks on.

TypeScript has several type-checking strictness flags that can be turned on or off, and all of our

examples will be written with all of them enabled unless otherwise stated. The strict flag in the

CLI, or "strict": true in a tsconfig.json toggles them all on simultaneously, but we can

opt out of them individually. The two biggest ones you should know about are noImplicitAny

and strictNullChecks.

noImplicitAny

Recall that in some places, TypeScript doesn't try to infer types for us and instead falls back to the

most lenient type: any . This isn't the worst thing that can happen - after all, falling back to any is

just the plain JavaScript experience anyway.

function greet(person, date) {

console.log(`Hello ${person}, today is ${date.toDateString()}!`);

}

greet("Maddison", new Date());

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