JS: Object Type

What is Object

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

Object is any value that is NOT any of { • undefined • null • true • false • string • number (including NaN, Infinity) • symbol }. 〔see Value Types〕

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

Test If a Value is Object Type

• Test If a Value is Object Type 📜

Data Object (aka Object Object)

The data object, e.g. {a:1, b:2} , is the best example of a collection of key-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,

• A function object function f (x) { return x+1; } is a function.
• A regex object /a+/ is for matching string. 〔see JS: RegExp (class)〕
• A Date Object represents date. 〔see JS: Date (class)〕

They are all objects, but they are usually called by other names, such as function, array, regex, etc. For a list of all builtin objects, see JavaScript Object Reference

You Can Add Properties to Any Object

You can add properties to function, date, regexp, etc., even though they are not usually used as key-value pairs.

// example of adding properties to different objects

// array
const aa = [3, 4];
aa.kk = 2;
console.log(aa.hasOwnProperty("kk"));

// function
const f1 = function () {
  return 3;
};

f1.kk = 2;
console.log(f1.hasOwnProperty("kk"));

// arrow function
const f2 = () => 3;
f2.kk = 2;
console.log(f2.hasOwnProperty("kk"));

// date
const dd = new Date();
dd.kk = 2;
console.log(dd.hasOwnProperty("kk"));

// RegExp
const rx = /\d+/;
rx.kk = 2;
console.log(rx.hasOwnProperty("kk"));