#lookups

keyBy takes an array of object and a property name then creates an object map of that property name to the corresponding array element. This is useful to make checks for various properties in arrays easier and faster. After doing the keyBy direct property tests can be used instead of find loops or other tactics. For example:

const cars = [ { id: 'miata', hp: 155, weight: 2400 }, { id: '4c', hp: 237, weight: 2465 }, { id: 'elise', hp: 217, weight: 2000 } ]; const carIds = keyBy(cars, 'id'); console.log('Cars found (keyBy)'); console.log('miata:', 'miata' in carIds); console.log('elise:', 'elise' in carIds); console.log('ferrari:', 'ferrari' in carIds); // miata: true // elise: true // ferrari: false

The only disadvantage is that the array must be looped through once to create the key-value object:

function keyBy(arr, propertyName) { const obj = {}; for (const ele of arr) { obj[ele[propertyName]] = ele; } return obj; }

keyBy is available in utility libraries like Lodash: https://lodash.com/docs/4.17.15#keyBy

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.