JS: Constructor

By Xah Lee. Date: 2013-11-11. Last updated: 2024-05-20.

What is a Constructor

Constructor is meant to create a object that is specific to certain type of data. (e.g. create a date object, array object, etc.)

In practice, when a function is designed to be called with the Operator “new” , the function f is called constructor. (There are some exceptions. For example, the constructor Symbol cannot be called with new 〔see Symbol Tutorial〕)

Constructor is one of the way to create object. 〔see Create Object〕

Builtin Constructors

The most common use of Operator “new” is to create a builtin object instance, such as Date object.

const x = new Date();

Here is example of standard builtin constructors.

Common Builtin Construtors
Construtor	Literal Expression	Constructor Syntax
Object	`{properties}`	`new Object(args)`
Array	`[items]`	`new Array(args)`
RegExp	`/pattern/flag`	`new RegExp(args)`
Date	none	`new Date(args)`

There are many more constructors. Builtin objects almost always have a constructor. 〔see JavaScript Object Reference〕

Primitive Value Wrapper Constructors

The following are constructors for Primitive Value 's wrapper objects. (when you call a method on a primitive value, such as "a,b".split(","), the string is temporarily converted to a string object. That's why it has methods.)

User Defined Constructor

Two common ways to define your own constructor:

Use the keyword class. 〔see Class〕
Create a function using the function keyword. Make sure the function does not have return statement. 〔see Operator “new”〕

Parenthesis Optional for Constructor Call with No Args

When a function is used as a constructor and without argument, the parenthesis are optional. e.g. new F() and new F are equivalent.

Do Not Confuse with Property Name “constructor”

Do not confuse constructor with property key "constructor".

〔see Property Key "constructor"〕

💡 TIP: Use Literal Expression When Possible

For builtin objects, you should use the literal expression to create object whenever possible. Because:

The constructor functions are often very complex, involves many arguments. Arg type and number of args may change its behavior.
Some constructor can be called with Operator “new”, some not. e.g. the Symbol constructor cannot. 〔see Symbol Tutorial〕
For some constructors, the behavior of calling it with new or without, is the same. For example, {Object, Array, Function, RegExp } are like that, but not Date.

💡 TIP: Constructor Name Start with Capital Letter

By convention, functions designed to be used as constructor starts with a capital letter.