JS: Constructor

What is a Constructor

Constructor is a function that is designed to create a object that is of specific type. (e.g. create a date object, array object, etc.)

Constructor is typically called with new (operator)

Builtin Constructors

example of standard builtin constructors.

• JS: Object Constructor
• JS: Array Constructor
• JS: RegExp Constructor
• JS: Date Constructor

Primitive Value Wrapper Constructors

The following are constructors for Primitive Value 's wrapper objects.

• JS: Number Constructor
• JS: String Constructor
• JS: Boolean Constructor

User Defined Constructor

• JS: class (keyword)

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. equivalent:

• new F()
• new F

Do Not Confuse with Property Name “constructor”

Do not confuse constructor with property key "constructor".

• JS: constructor (property)

💡 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 new (operator), 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.