Higher Order Functions ES6

function mapConsecutive(values, fn) { 

  let result = []; 

  for(let i=0; i < values.length -1; i++) { 

    result.push(fn(values[i], values[i+1])); 

  } 

  return result; 

} 

const letters = ['a','b','c','d','e','f','g']; 

let twoByTwo = mapConsecutive(letters, (x,y) => [x,y]); 

console.log(twoByTwo); // [[a,b], [b,c], [c,d], [d,e], [e,f], [f,g]]