transform-array-of-arrays.js

//========== Solution 1: array.flatMap() and array.map() ==========

const prios = (arrayOfArrays) => arrayOfArrays.flatMap(
  (arr, index) => {
    return arr.map(elem => ({...elem, priority: index+1}))
  }
);

// .flatMap() both maps and flattens
// More information: https://exploringjs.com/impatient-js/ch_arrays.html#Array.prototype.flatMap

assert.deepEqual(
  prios([
    [
      { name: 'Jane' },
      { name: 'Tarzan' },
    ],
    [
      { name: 'Cheeta' },
    ],
  ]),
  [
    { name: 'Jane', priority: 1 },
    { name: 'Tarzan', priority: 1 },
    { name: 'Cheeta', priority: 2 },
  ]
);

//========== Solution 2: nested for-of loops ==========

const prios2 = (arrayOfArrays) => {
  const result = [];
  for (const [index, arr] of arrayOfArrays.entries()) {
    for (const elem of arr) {
      result.push({...elem, priority: index+1});
    }
  }
  return result;
};

assert.deepEqual(
  prios2([
    [
      { name: 'Jane' },
      { name: 'Tarzan' },
    ],
    [
      { name: 'Cheeta' },
    ],
  ]),
  [
    { name: 'Jane', priority: 1 },
    { name: 'Tarzan', priority: 1 },
    { name: 'Cheeta', priority: 2 },
  ]
);