Struggling to convert an Object to have keys within one of the sub objects

You can use reduce() on things array

const obj = { 
 "id": "575802", 
 "things": [{ 
         "clientId": "123456", 
         "val": "file1" 
     }, 
     { 
         "clientId": "1234", 
         "val": "file2" 
     } 
 ] 
} 
obj.things = obj.things.reduce((ac,{clientId,...rest}) => (ac[clientId] = rest, ac), {}) 
console.log(obj)

You can use reduce to create the things key in the required object. In reduce callback function return an object

let data = { 
  "id": "575802", 
  "things": [{ 
      "clientId": "123456", 
      "val": "file1" 
    }, 
    { 
      "clientId": "1234", 
      "val": "file2" 
    } 
  ] 
} 
 
let newData = { 
  id: data.id, 
  things: data.things.reduce(function(acc, curr) { 
    acc[curr.clientId] = { 
      val: curr.val 
    } 
    return acc; 
  }, {}) 
}; 
 
console.log(newData)

This might not be an ideal solution (based on how dynamic you need the solution to be).

But here is the fastest solution I came up with.

const data = {
  id: '575802',
  things: [{
    clientId: '123456',
    val: 'file1'
  },
    {
      clientId: '1234',
      val: 'file2'
    }
  ]
};
const resultObj = {};
resultObj['id'] = data['id'];
resultObj['things'] = {};
const things = data['things'];
for (const thing of things) {
  const clientId = thing['clientId'];
  resultObj['things'][clientId] = {val: thing['val']};
}
console.log(resultObj);

All I am doing here is iterating over the 'things' property and flattening it into the desired object.