Typescript declaration merge

1. Interface merge

interface TestInterface {
    
    name:string;
}
interface TestInterface {
    
    age:number;
}
// It's equivalent to 
interface TestInterface {
    
    name:string;
    age:number;
}
class Person implements TestInterface{
    
    name:string;
    age:number;
}

1.1 If the interface with the same name has the same property name , Then the attribute types must be consistent

interface TestInterface {
    
    name:string;
}
interface TestInterface {
    
    name:number;// Report errors 
}

1.2 If a function with the same name appears in the interface with the same name , Then it will become a function overload

interface TestInterface {
    
    getValue(value:number):number;
}
interface TestInterface {
    
    getValue(value:string):number;
}
let obj:TestInterface = {
    
    getValue(value:any):number{
    
        if(typeof value === 'string'){
    
            return value.length;
        }else{
    
            return value.toFixed();
        }
    }
}
console.log(obj.getValue("abcdef"));
console.log(obj.getValue(3.14));

2. Namespace

namespace Validation{
    
    export let name:string = 'lnj';
}
namespace Validation{
    
    export let age:number = 18;
}
console.log(Validation.name);
console.log(Validation.age);

2.1 A variable with the same name cannot appear in a namespace with the same name , Such method

namespace Validation{
    
    export let name:string = 'lnj';
    export let say = ()=> "abc";
}
namespace Validation{
    
    export let name:string = 'zs';// Report errors 
    export let say = ()=> "abc";// Report errors 
}

2.2 Other namespaces in the namespace with the same name did not pass export The exported content cannot be obtained

namespace Validation{
    
    let name:string = 'lnj';// Output name =  Can't get name
    // export let name:string = 'lnj';  Output name =lnj
    
}
namespace Validation{
    
    export let say = ()=> {
    
        console.log(`name = ${
      name}`);
    };
}
Validation.say();

3. Namespace and class merge

 Be careful :  Class must be defined before namespace 
 The method exported from the namespace will be merged into the class as a static method 
class Person {
    
    say():void{
    
        console.log('hello world');
    }
}
namespace Person{
    
    export const hi = ():void=>{
    
        console.log('hi');
    }
}
console.dir(Person);

4. Namespace and function merge

 Be careful :  The function must be defined before the namespace 
function getCounter() {
    
    getCounter.count++;
    console.log(getCounter.count);
}
namespace getCounter{
    
    export let count:number = 0;
}

5. Namespace and enumeration merge

 Be careful :  There is no requirement of order 
enum Gender {
    
    Male,
    Female
}
namespace Gender{
    
    export const Yao:number = 666;
}
console.log(Gender);