promise chaining in javascript code example

Example 1: method chaining in javascript

class Arithmetic {
  constructor() {
    this.value = 0;
  }
  sum(...args) {
    this.value = args.reduce((sum, current) => sum + current, 0);
    return this;
  }
  add(value) {
    this.value = this.value + value;
    return this;
  }
  subtract(value) {
    this.value = this.value - value;
    return this;
  }
  average(...args) {
    this.value = args.length
      ? (this.sum(...args).value) / args.length
      : undefined;
    return this;
  }
}

a = new Arithmetic()
a.sum(1, 3, 6)   // => { value: 10 } 
  .subtract(3)   // => { value: 7 }
  .add(4)        // => { value: 11 }
  .value         // => 11 

// Console Output
// 11

Example 2: promise chaining

new Promise(function(resolve, reject) {

  setTimeout(() => resolve(1), 1000); // (*)

}).then(function(result) { // (**)

  alert(result); // 1
  return result * 2;

}).then(function(result) { // (***)

  alert(result); // 2
  return result * 2;

}).then(function(result) {

  alert(result); // 4
  return result * 2;

});