JS: Object Type

By Xah Lee. Date: . Last updated: .

What is object

A JavaScript object is one of the Value Types.

JavaScript spec defines object as: “a collection of key-and-value pairs”. (each pair is called a Property of the object.)

For example, the following are all objects: array, function, date, regex.

Test if a value is object type

Data object (object object)

The data object, e.g. {a:1, b:2} , is the best example of a collection of key-and-value pairs. We often call this data object or just object. JavaScript spec calls it object object.

Special purpose objects

All objects other than the “data object” have special purposes and or hold internal data for that purpose.

For example, array, function, date, regex.

For many more, see

You can add properties to any value of object type

You can add properties to function, date, regexp, etc., even though they are not usually used for holding data.

// example of adding properties to different objects

let xx;

// array
xx = [3, 4];
xx.cats = 2;
console.log(xx);
// [ 3, 4, cats: 2 ]

// s------------------------------
// arrow function

xx = () => 3;
xx.cats = 2;
console.log(xx);
// [Function: xx] { cats: 2 }

// s------------------------------
// date

xx = new Date();
xx.cats = 2;
console.log(xx);
// 2026-01-18T07:16:41.722Z { cats: 2 }

// s------------------------------
// RegExp
xx = /\d+/;
xx.cats = 2;
console.log(xx);
// /\d+/ { cats: 2 }