Format Array : JavaScript

Input:
[ { x: 'a', order: 10}, { x: 'a', order: 3}, { x: 'a', order: 4}, { x: 'b', order: 3}, { x: 'b', order: 2}, { x: 'b', order: 1} ]

Output :
{ “a":[{"x":"a","order":10},{"x":"a","order":3},{"x":"a","order":4}], “b":[{"x":"b","order":3},{"x":"b","order":2},{"x":"b","order":1}] }

Solution:

const arr = [
  { x: 'a', order: 10},
  { x: 'a', order: 3},
  { x: 'a', order: 4},
  { x: 'b', order: 3},
  { x: 'b', order: 2},
  { x: 'b', order: 1}
];

function formatArray(input){
  output = {};
  input.forEach( (value)=>{
    const xValue = value.x;
    if(Object.keys(output).includes(xValue+'')){
      output[xValue].push(value);
    }else{
      output[xValue] = [value];
    }    
  });
  return output;
}

console.log("output",formatArray(arr));