promise.js

function m1(){
    let a, b;

    //Considering getValueFromFile1, getValueFromFile2 and getValueFromFile3
    //all return promises
    return getValueFromFile1();
    .then(function(value){
        a = value;
        return getValueFromFile2();
    })
    .then(function(value){
        b = value;
        let sum = a + b;
        return printSumToFile3(sum);
    })
    .then(function(){
        console.log('Success');
    })
}