When I'm building a JavaScript object dynamically I often want dynamic keys, or better said, keys that are dynamic at build-time of the object. It's actually simple, but I always forget the syntax, so if you're just looking for that it looks like this, assuming you have a javascript object already declared called obj:
obj['key' + someVal] = 'some value ' + someVal

Example Usage

I've found myself receiving an API response from a back-end and needing to build a JavaScript object based on that API response but needing validation on the response to make sure it is correctly shaped. You might do this with something like:

const dynamicObject = {}

// apiResponse would normally be done with something like const apiResponse = fetch(url) but statically setting for illustration purposes
const apiResponse = {
  someKey1: 'someValue1',
  someKey2: 2,
  someKey3: 'someValue3',
  someKey4: 'someValue4',
}
 
const someValidationFunction = (value) => typeof value === 'string'

Object.keys(apiResponse).forEach((key) => {
  if (someValidationFunction(apiResponse[key])) {
    dynamicObject[key] = apiResponse[key]
  }
});

console.log(dynamicObject)

// Expected output:
// {
//   someKey1 = 'someValue1',
//   someKey3 = 'someValue3',
//   someKey4 = 'someValue4',
// }

To try it yourself check out this codepen. You'll need to click the "Console" button on the bottom left to see the result.