Typescript enumeration

One , Preface

1.enum Is in typescript New legal types in , It's right JavaScript A supplement to standard data types .

2.enum Its main function is to give friendly names to a set of values within a limited range .

Two ,enum Introduce

1.enmu The basic form is as follows

enum Animals {
    
	Dog,
	Cat,
	Pig,
}

2. According to the value and definition form, it can be divided into three types ： Enumeration of numbers , String Enum , Constant enumeration

3、 ... and , Enumeration of numbers

1. Enumerations with numeric values are numeric enumerations

enum Animals {
    
	Dog=1,
	Cat=2,
	Pig=3,
}

2. By default , If you do not assign values to enumeration members manually , Will be defined as numeric enumeration , And from No 0 Start incrementally assigning values to enumeration members

enum Animals {
    
	Dog,
	Cat,
	Pig,
}

console.log(Animals.Dog)  //0
console.log(Animals.Cat)  //1
console.log(Animals.Pig)  //2

3. We can for one or more of them Manual assignment , The subsequent sequence is incremented by the value assigned

enum Animals {
    
	Dog,
	Cow=5,
	Cat=2,
	Pig,
}

console.log(Animals.Dog)  //0
console.log(Animals.Cow)  //5
console.log(Animals.Cat)  //2
console.log(Animals.Pig)  //3

4. There are two types of enumerations ： Constant term and Calculated item , All the above are constant terms , It should be noted that , When one of the items is the calculated item , Then its subordinate must be given the initial value , Otherwise, an error will be reported , Due to the use of calculated items , The automatic calculation of the lower position is invalid .

enum Animals {
    
	Dog,
	Cow=Math.random(), //Cow Constant term 
	Cat=2, // Next, you need to assign an initial value , Otherwise, an error will be reported 
	Pig,
}

Two , String Enum

1. If the enumeration value is string, it is string enumeration

enum Animals {
    
	Dog="dog",
	Cat="cat",
	Pig="pig",
}

2. The string enumeration value requires that the value of each field must be a string literal , Calculated items cannot be used

3、 ... and , Constant enumeration

1. Use const enmu The enumeration created is a constant enumeration

const enum Animals {
    
	Dog="dog",
	Cat="cat",
	Pig="pig",
}

2. The difference between constant enumeration and ordinary enumeration is that constant enumeration itself will be deleted during compilation , Only in the place of use , Get the corresponding value and replace .

Four , Reverse mapping

1. When we define enumeration, we can use enmu.key perhaps enmu[key] To get the value

const enum Animals {
    
	Dog="dog",
	Cat="cat",
	Pig="pig",
}

console.log(Animals["Dog"]) //dog
console.log(Animals.Cat) //cat

2. Not only that, we can get the enumeration item according to the value key

const enum Animals {
    
	Dog,
	Cat,
	Pig,
}
console.log(Animals[0]) //Dog
console.log(Animals[2]) //Pig

5、 ... and , Enumeration type

1. Enumeration itself and enumeration members can be used as a type to declare variables

enum Animals {
    
	Dog,
	Cat,
	Pig,
}

let test:Animals = Animals.Dog // Enumeration itself as a type , Only the enumeration items in the enumeration can be assigned 
let test:Animals = 12 // The statement is Animals type , Assigning other values will result in an error 


let test:Animals.Dog = Animals.Dog // Enumerate members as types , Only the corresponding enumeration value can be assigned 
let test:Animals.Dog = Animals.Cat // Report errors