Small knowledge, big challenge! This article is participating in the creation activity of “Essential Tips for Programmers”

At the same time, I participated in the Digitalstar project to win the creative gift package and challenge the creative incentive money

preface

Bugs in our system programs are called bugs. The world’s first bug, is 1946 Hope discovered the first computer bug, turned out to be a moth “bedbug”. The process of solving these problems is called bug catching, debugging, or debugging.

Preparing for development Environment

There are many detailed installation tutorials available online!

Install VSCode

Installation Node. Js

Create a project

Initialize a TypeScript project

Create a [first-ts-project] folder, go to the folder, and execute on terminal

npm init -y
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You can create one in your folderpackage.jsonfileThe latest stable version of typescript

npm install -- save-dev typescript
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The latest typescript

npm install --save-dev typescript@next 
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Create tsconfig. Json

Method 1: Create an index.ts file in the VScode editor and open it with VScode. Click on the TypeScript version number at the bottom of VScode and click create tsconfg in the pop-up box.

{
    "compilerOptions": {
        "module": "commonjs",
        "target": "es2020",
        "jsx": "preserve",
        "strictFunctionTypes": true,
        "sourceMap": true
    },
    "exclude": [
        "node_modules",
        "**/node_modules/*"
    ]
}
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Method 2: Use the command line

node_modules/.bin/tsc   --init --locale zh-CN
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Created this waytsconfig.jsonContains all compiler parameters and parameter descriptions

tool

TS playground

TS Playground

Equal<X, Y>

Preliminary knowledge

TS and JS

TypeScript is a superset of JavaScript
TypeScript provides all JavaScript features. And adds the TypeScript type system on top of it.
The type system is designed to be “optional,” which means that all legitimate JavaScript code is legitimate TypeScript code.

TS compilation process

‘TS type checking’ and ‘JS generation’ are two separate processes;
Type checking errors do not affect generating JavaScript code;

The type system

Structural Type systems determine whether two types are equal or compatible by their actual structure

TypeScript, Haskell, OCaml, Go,...
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Nominal Type System Determines whether two types are equal based on the Type name

C, C++, Java, C#, Rust,...
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TS uses the structure type system

Structure type system

// struct type system: OK class Foo {public hi() {console.log('hi'); } } class Bar() { public hi() { console.log('Hello'); } // assign an instance of Foo to a const a: Foo = new Foo(); // assign a to b const b: Bar = a; // call the hi() method on b. //Works!Copy the code

Nominal type system

Class Foo {public: void hi() {STD ::cout << "hi" << STD ::endl; }}; class Bar { public: void hi() { std::cout << "Hello" << std::endl; }}; int main() { Foo foo; Foo &fooRef = foo; //Error: Type 'Bar' cannot // Bind to a value of 'Foo' // Assign the variable Foo to the Bar reference, and during the assignment, the compiler will report an Error because the two types are completely unrelated. Bar & Bar = foo; return 0; }Copy the code

Type annotations

Contrast: type annotation syntax in different languages

C++ first

int printf(const char*,...) ;Copy the code

Objc comes first, parentheses

- (id)initWithInt:(int)value;
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Julia at the end, double colon

commission(sale::Int,rate::Float64)::Float64
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The type annotation for TS follows with a single colon

function log(message: string): void
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The relationship between types and collections

Empty Set (Empty Type)

Never = ∅ = {}Copy the code

Set of elements (Unit Type)

Null = {Null} Undefined = {Undefined} Literal Type(Literal Type,"a",1,true)Copy the code

Collection co., LTD.

Boolean = {true, false}
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Infinite set (approximation)

String = {a','b',... ,'hello',... } Symbol = {Symbol(...) . } BigInt = {... ,-1n, 0n, 1n,... } Number = {-Infinity,-(2^53 - 1.),... 0,... ,+2^ 253-1,Infinity} and NaNCopy the code

Universal Set

Type union and crossover

Type the alias

Analogy:

In JS: you can declare variables or constants using let, const, var
In TS: You can use type to declare aliases for types

Can be interpreted as the name of “something”, which is easy to remember and more descriptive.

// let ID= 123; Cconst PI = 3.14; var myPI = PI; //TS alias for the type declaration type ID = string; type User = { id:ID; name:string; }Copy the code

Knowledge:

Type aliases are similar to let variables in that they have block-level scope. Therefore, the same name cannot be in the same scope.

type User = { 
    id:ID;
    name:string;
}

type User = { }
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The inner type alias hides the outer type of the same name

type User = { id:ID; name:string; } // block level scope {type User = {id: id; nname:string; }}Copy the code

The types are broadened and narrowed

4) Type broadening (Type KP)

When you assign a literal to a let or var variable, TS does not use a literal type (a collection of elements) as the type of the variable. It broadens from literal types to correspondingly broader types. This process is called type broadening.

Type Narrowing

In some cases, TS can be more specific about the type of the variable, in which case it Narrows it down.

Knowledge:

TS tries to strike a balance between type certainty and flexibility
TS provides a number of methods to help narrow types to improve type determinism:

Null Check, as const, Instanceof, Typeof, attribute checking, Tagged Union, User-defined Type Guard, code flow analysis

When a variable is declared with let var, TS believes that the variable will change in the future, so the type is inferred as a relatively broad type
When a const is declared as a constant, TS knows that the constant will not change and extrapolates the type to the narrowest literal type

On the left isType to broadenOn the right is theType narrowing

Value space and type space

The type space is created by the TS compiler, TSC, and contains various types. TSC instantiates various types based on code.
The value space is created after JSEngine and contains various runtime values.

How do you tell which space the sign is in?

Symbols that disappear after translation → type space
Symbols as type annotations, aliases → type space (type T = typeof Person; T is in the type space const p: Person; T is in type space.
Symbol after type assertion → type space (Target as/is HTMLElement; The symbol after as/is is in the type space.
Const, let, var symbol → value space
Class, enum, symbol after namespace → value space + type space

In addition, there are some operators that exist in both Spaces but have completely different meanings:

typeof

In the value space, Typeof returns a string representation of the JavaScript type corresponding to the following expression:

'string', 'number', 'bigint', 'boolean', 'symbol', 'undefined', 'object', 'function'
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In type space, Typeof returns identifiers corresponding toTypeScript type

[](Indexed Access Operator Indexed Access Operator)

In the value space, val[field] or val.field returns the value of the property corresponding to val
In Type space, Type[T] returns the TS Type corresponding to T

Symbol and Template string pattern were added after TS4.4

This keyword

In value space, this refers to… More complicated
In type space, this can be used as the return value of a class method to implement chained calls

& | operator

Represents Bitwise AND AND Bitwise OR in value space
Represents the intersection and union of types in type space

const

Used to declare constants in the value space
In type space with as, that is, “as const” constant assertion, narrowing the type

extends

Used to define subclasses in value space (class A extends B)
Used in the type space for type constraints (T extends Number) or interface inheritance (interface A extends B)

In value space for loop (for (key in object) {… }) and check whether the attribute exists (‘name’ in person)
Declarations (Mapped Types) for keys used to map types in Type space

conclusion

If there is any mistake above, please point out in the comment section, thank you!

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Get started with TypeScript development environment and preparation

preface

Preparing for development Environment

Create a project

Initialize a TypeScript project

Create tsconfig. Json

tool

TS playground

Equal<X, Y>

Preliminary knowledge

TS and JS

TS compilation process

The type system

Structure type system

Nominal type system

Type annotations

The relationship between types and collections

Type union and crossover

Type the alias

The types are broadened and narrowed

4) Type broadening (Type KP)

Type Narrowing

Value space and type space

How do you tell which space the sign is in?

conclusion

Get started with TypeScript development environment and preparation

preface

Preparing for development Environment

Create a project

Initialize a TypeScript project

Create tsconfig. Json

tool

TS playground

Equal<X, Y>

Preliminary knowledge

TS and JS

TS compilation process

The type system

Structure type system

Nominal type system

Type annotations

The relationship between types and collections

Type union and crossover

Type the alias

The types are broadened and narrowed

4) Type broadening (Type KP)

Type Narrowing

Value space and type space

How do you tell which space the sign is in?

conclusion

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