JavaScript set object key by variable

You need to make the object first, then use [] to set it.

var key = "happyCount";
var obj = {};

obj[key] = someValueArray;
myArray.push(obj);

UPDATE 2021:

Computed property names feature was introduced in ECMAScript 2015 (ES6) that allows you to dynamically compute the names of the object properties in JavaScript object literal notation.

const yourKeyVariable = "happyCount";
const someValueArray= [...];

const obj = {
    [yourKeyVariable]: someValueArray,
}

In ES6, you can do like this.

var key = "name";
var person = {[key]:"John"}; // same as var person = {"name" : "John"}
console.log(person); // should print  Object { name="John"}

    var key = "name";
    var person = {[key]:"John"};
    console.log(person); // should print  Object { name="John"}

Its called Computed Property Names, its implemented using bracket notation( square brackets) []

Example: { [variableName] : someValue }

Starting with ECMAScript 2015, the object initializer syntax also supports computed property names. That allows you to put an expression in brackets [], that will be computed and used as the property name.

For ES5, try something like this

var yourObject = {};

yourObject[yourKey] = "yourValue";

console.log(yourObject );

example:

var person = {};
var key = "name";

person[key] /* this is same as person.name */ = "John";

console.log(person); // should print  Object { name="John"}

    var person = {};
    var key = "name";
    
    person[key] /* this is same as person.name */ = "John";
    
    console.log(person); // should print  Object { name="John"}