SOURCE

let str1 = 'str1 ';


const func1=(str1)=>new Promise((resolve,reject)=>{
    resolve(str1 + 'str2 ');
})
const func2=(str2)=>new Promise((resolve,reject)=>{
    resolve(str2 + 'str3 ');
})
const func3=(str3)=>new Promise((resolve,reject)=>{
    resolve(str3 + 'str4 ');
})


func1(str1)
.then(res=>{

    func2(res)
    .then(res=>{

        func3(res)
        .then(res=>{

            console.log(res);
            console.log("==================");
        })
    })
})

func1(str1)
.then(res=>func2(res))
.then(res=>func3(res))
.then(res=>{
     console.log(res);
     console.log("==================");
})