JAVASCRIPT

js object using variable as key

let yourKeyVariable = "Fire";
{
    [yourKeyVariable]: someValue,
}

/* object will resolve to { Fire : someValue } */

/*
 This is similar to adding k-v pair to an object using the other syntax 
ie. object[yourKeyVariable] = someValue; 
*/

object key as variable

var key = "happyCount";
var obj = {};
obj[key] = someValueArray;
myArray.push(obj);

javascript variable as object key

var key = "happyCount";
var obj = {};
obj[key] = someValueArray;
myArray.push(obj);

javascript object as key

var fighters = new WeakMap();
var bruce = {name: 'Bruce Lee'};
var chuck = {name: 'Chuck Norris'};

fighters.set(bruce, 'Jeet Kune Do');
fighters.set(chuck, 'Karate');

console.log(fighters.get(bruce)); // Jeet Kune Do
console.log(fighters.get(chuck)); // Karate