text: // addition of two numbers
  export const add = function( a, b ) {
      return a + b;
  }
  
  // division two numbers
  export const divide = function( a, b ) {
      return b === 0 ? null : a / b;
  }
  
  // multiply two numbers
  export const multiply = function( a, b ) {
      return new Promise( resolve => {
          setTimeout( () => {
              resolve( a * b );
          }, 1000 ); // 1s
      } );
  }