alex’s blog

obscurejavascript:

A simple invert function can do this. Here is how it works:

const obj = { a: 1, b: 2, c: 3 };
console.log(invert(obj));
// { '1': 'a', '2': 'b', '3': 'c' }

This is useful when there is a list of names to ids. Sometimes you will need to check if an id exists in that list. If this is done in a loop, it is usually much faster to check an inverted id to name object that is created before the loop:

function hasIds(obj, ids) {
  const byId = invert(obj);
  return ids.map((id) => byId[id] !== undefined);
}

const cars = { 
  'miata': 1,
  'elise': 2,
  'elan': 3
};

console.log(hasIds(cars, [1, 2, 5]));
// [ true, true, false ]

The implementation for invert is simple:

function invert(obj) {
  const newObj = {};

  for (const keyName in obj) {
    if (obj.hasOwnProperty(keyName)) {
      newObj[obj[keyName]] = keyName;
    }
  }

  return newObj;
}

It is also available in many utility libraries like Lodash.

Github Location: https://github.com/Jacob-Friesen/obscurejs/blob/master/2019/invert.js