Typescript study notes | Byte Youth Training Notes

Introduction to TypeScript

TypeScript is a free and open source programming language *developed by Microsoft*.It is a superset of JavaScript, and in essence TypeScript extends JavaScript syntax, which means that it supports all JavaScript syntax.TS adds type support on top of JavaScript and can be run anywhere JavaScript is running.
TypeScript It is a *static type checker* that checks the program for errors before execution (compiles), can display labelsUnexpected behavior in the code, and checking based on the kind of value, reduces the likelihood of errors.

TypeScript Features:

- Enhanced readability: parsing TSDos based on syntax, enhanced ide
- Enhanced maintainability: exposed most errors at compile stage.(In large-scale projects with multi-person cooperation, better stability and development efficiency are obtained)
- Included in compatible with all JS features, support coexistence
- Support progressive introduction and upgrade

Object Type

Features: Object types are more refined in TS, and each object has its own type language.

**Interface**: In object-oriented programming, an interface is a normative definition, which defines the specification of behavior and actions. In programming, the interface acts as a restrictionand normative role.

> **NOTE**: Defining an interface with the interface keyword generally capitalizes the first letter and prefixes the interface name with a capital I.

- **optional attribute**: define that this attribute may not exist.(Add a ? sign after the attribute).
- **Read-only properties**: Constraint properties cannot be assigned outside of object initialization.(Add the keyword **readonly** before one or some properties of the object, you can set the property to be read-only).
- **Any Attribute**: Constraints that all object attributes must be subtypes of this attribute.

###Array type

**Two ways**:

```1.const numbers: number[] = [a,b,c]2.const strings: Array = ['a', 'b', 'c']```

What if there are both numbers and strings in the array?

**Union type (a type composed of two or more other types, which can be any of these types)**:

```const arr: (number | string)[] = [1,'a',2,'b']```

Function Type

In TypeScript, there are two common ways to define functions - Function Declaration and Function Expression.
1. Specify parameters individually, return value type:

// The first and second numbers add numeric types to the parameters num1 and num2 respectively, and the third number add numeric types to the return valuefunction plus(num1: number, num2: number): number {return num1+ num2}plus(1,2) 

1. Specify parameters individually, return value type:

```const add = (num1: number, num2: number): number => {return num1 + num2}add(2,3)```

**NOTE**: Do not confuse `=>` in TypeScript with `=>` in ES6.

- In the type definition of TypeScript, `=>` is used to represent the definition of the function, **the left side is the input type, which needs to be enclosed in parentheses, and the right side is the output type**.
- In ES6, `=>` is called arrow function, which is widely used.