Join two objects by key in JavaScript

Suppose, we have two child and parent JSON arrays of objects like these −

const child = [{
   id: 1,
   name: 'somename',
   parent: {
      id: 2
   },
},
{
   id: 2,
   name: 'some child name',
   parent: {
      id: 4
   }
}];
const parent = [{
   id: 1,
   parentName: 'The first',
   child: {}
},
{
   id: 2,
   parentName: 'The second',
   child: {}
},
{
   id: 3,
   parentName: 'The third',
   child: {}
},
{
   id: 4,
   parentName: 'The fourth',
   child: {}
}];

We are required to write a JavaScript function that takes in these two arrays.

And our function should merge the child array objects into corresponding parent objects.

Therefore, the final output should look something like this −

const output = [
   {
      id: 1,
      parentName: The first,
      child: {}
   },
   {
      id: 2,
      parentName: The second,
      child: {
         id: 1,
         name: somename,
      }
   },
   {
      id: 3,
      parentName: The third,
      child: {}
   },
   {
      id: 4,
      parentName: The fourth,
      child: {
         id: 2
         name: some child name,
      }
   },
];

Example

The code for this will be −

const child = [{
   id: 1,
   name: 'somename',
   parent: {
      id: 2
   },
},
{
   id: 2,
   name: 'some child name',
   parent: {
      id: 4
   }
}];
const parent = [{
   id: 1,
   parentName: 'The first',
   child: {}
},
{
   id: 2,
   parentName: 'The second',
   child: {}
},
{
   id: 3,
   parentName: 'The third',
   child: {}
},
{
   id: 4,
   parentName: 'The fourth',
   child: {}
}];
const combineParentChild = (parent, child) => {
   const combined = [];
   for (let i = 0; i < parent.length; i++) {
      for (let j = 0; j < child.length; j++) {
         if (child[j].parent.id === parent[i].id) {
            parent[i].child.id = child[j].id;
            parent[i].child.name = child[j].name;
            break;
         };
      };
      combined.push(parent[i])
   };
   return combined;
};
console.log(combineParentChild(parent, child));

Output

And the output in the console will be −

[
   { id: 1, parentName: 'The first', child: {} },
   {
      id: 2,
      parentName: 'The second',
      child: { id: 1, name: 'somename' }
   },
   { id: 3, parentName: 'The third', child: {} },
   {
      id: 4,
      parentName: 'The fourth',
      child: { id: 2, name: 'some child name' }
   }
]