Real use case dynamic (computed) property

Question

A dynamic property:

var obj = {
  // Computed (dynamic) property names
  [ 'prop_' + (() => 42)() ]: 42
};

This is of course very fancy. But where could someone use this without adding unnecessary complexity?

Answer 1

If you have a property name as a constant:

var obj = { [SOME_CONSTANT]: 42 };

Answer 2

One case where I wanted it was where property names for JSON were defined in generated files, based off Java classes.

// Generated
var SomeJsonBodyParams = {NAME: 'name', ID: 'id', ETA, 'estimatedTimeOfArrival'};

// Using it
sendAjax('some/url', {
    [SomeJsonBodyParams.NAME] = userData.name,
    ...
});

We even had a method so we could kind of do it

function makeObj() {
  var obj = {};
  for (var i=0; i < arguments.length; i+=2) {
      obj[i] = obj[i+i];
  }
  return obj;
}

sendAjax('some/url', makeObj(
    SomeJsonBodyParams.NAME, userData.name,
    ...
));