JS constructors & Examples

Two types: inbuilt, custom

• Inbuilt
  • Part of JS
  • Provide objects in various predetermined formats, like Date, Error objects, and Objects for each data type
• Custom
  • Constructors that we design ourselves for our own purposes

Useful inbuilt constructors

• These exist but are rarely used
  • String()
    • Strings
  • Number()
  • Array()
  • Object() (except this, this is used)
  • Boolean()

Date() constructor

• We instantiate a new Date object

const dateObject = new Date();
console.log(dateObject); //2025-10-11T09:15:51.762Z
console.log(dateObject.toString()); // Sat Oct 11 2025 18:17:19 GMT+0900 (Korean Standard Time)
console.log(dateObject.getFullYear()); // 2025

• Alternatives like luxon exist because Date can be pain

Error() constructor

• creates new Error objects Syntax:

new Error()
new Error(message)
new Error(message, options)
new Error(message, fileName)
new Error(message, fileName, lineNumber)
 
Error()
Error(message)
Error(message, options)
Error(message, fileName)
Error(message, fileName, lineNumber)

Examples

const x = Error("I was created using a function call!");
 
// above has the same functionality as following
const y = new Error('I was constructed via the "new" keyword!');
 
// in try catch
try {
  frameworkThatCanThrow();
} catch (err) {
  throw new Error("New error message", { cause: err });
}

• the throw keyword ends the execution