merge sub-array if other key-value pair match

April 25, 2022

I have this simple array, which I want to merge key b value if key a in the whole array matches.

const array = [
  {a: 1, b: ['Foo']},
  {a: 1, b: ['Bar']},
  {a: 1, b: ['Baz']},
  {a: 2, b: ['Foo']},
  {a: 3, b: ['Foo']},
]

into

const array = [
  {a: 1, b: ['Foo','Bar','Baz']},
  {a: 2, b: ['Foo']},
  {a: 3, b: ['Foo']},
]

is there a way to do it? Any help would be appreciated!

EDIT 1 : What I am lacking here is the logic to grab all similar key a

>Solution :

Use reduce to iterate over the array of objects to create an object where the keys match the object’s a value, adding the various b values to their arrays. Then use Object.values to create a new array.

const arr = [
  {a: 1, b: ['Foo']},
  {a: 1, b: ['Bar']},
  {a: 1, b: ['Baz']},
  {a: 2, b: ['Foo']},
  {a: 3, b: ['Foo']},
];

// For each iteration pass in the accumulator
// and the current object
const out = arr.reduce((acc, c) => {

  // Assign the value of `a` to `key`
  const key = c.a;
  
  // If the key doesn't exist on the accumulator
  // set a new object
  acc[key] = acc[key] || { a: key, b: [] };

  // And then push the first element of `b`
  // into the object's array
  acc[key].b.push(c.b[0]);

  // Return the accumulator
  return acc;

}, {});

// And then finally get the Object.values from
// your returned object to make an array
console.log(Object.values(out));