Typescript type protection

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Type protection
For union type variables , How to tell the compiler exactly what type it is
Through type assertion or type protection

let getRandomValue = ():(string | number)=>{
    
    let num = Math.random();
    return (num >= 0.5) ? 'abc' : 123.123;
}
 let value = getRandomValue(); 
 console.log(value);
 // Because I don't know what type to return , Then use .length and .toFixed() Will report a mistake 
if(value.length){
    
    console.log((value as string).length);
}else{
    
    console.log(value.toFixed());
}

Solution
1. Use type assertion

 Although type assertions can tell the compiler exactly what type the current variable is ,
 But every time you use it, you need to tell the compiler manually ,  It's a little bit of a hassle ,  There are many redundant codes 
if((value as string).length){
    
    console.log((value as string).length);
}else{
    
    console.log((value as number).toFixed());
}

2. Type protection function

//  Defines a type protection function ,  Of this function ' Return type ' Is a boolean type 
//  The return value type of this function is ,  Incoming parameter  + is  The specific type 
function isString(value:(string | number)): value is string {
    
    return typeof value === 'string';
}
if(isString(value)){
    
    console.log(value.length);
}else{
    
    console.log(value.toFixed());
}

3. Use typeof To implement type protection

 Be careful :
 If you use typeof To implement type protection ,  Then only use  === / !==
 If you use typeof To implement type protection ,  Then it can only protect  number/string/boolean/symbol type 
if(typeof value === 'string'){
    
    console.log(value.length);
}else{
    
    console.log(value.toFixed());
}

4. because typeof Can only protect number/string/boolean/symbol class , Objects can pass through instanceof To implement type protection

class Person {
    
    name:string = 'lnj';
}
class Animal {
    
    age: number = 18;
}
let getRandomObject = ():(Person | Animal)=>{
    
    let num = Math.random();
    return (num >= 0.5) ? new Person() : new Animal();
};
let obj = getRandomObject();
console.log(obj);
if(obj instanceof Person){
    
    console.log(obj.name);
}else{
    
    console.log(obj.age);
}