JavaScript Built-in Objects (part 1)

JavaScript has several built-in objects that provide functionality to perform various tasks - they are essential to the language and can be used to make development easier and faster.

Here are some of the most commonly used built-in objects in JavaScript:

1. Object

The Object object is the base object for all JavaScript objects and provides generic methods for creating and manipulating objects:

// Create an object using the Object constructor
var person = new Object();

// Add properties to the object
person.name = "Charlie";
person.location = "Rome";

// Access the properties using dot notation
console.log(person.name);      // prints: "Charlie"
console.log(person.location);  // prints: "Rome"

// Use the Object.keys() method to get an array of property names
var propertyNames = Object.keys(person);
console.log(propertyNames); // prints: ["name", "location"]

// Use the Object.values() method to get an array of property values
var propertyValues = Object.values(person);
console.log(propertyValues); // Output: ["Charlie", "Rome"]

// Use the Object.entries() method to get an array of [key, value] pairs
var entries = Object.entries(person);
console.log(entries); // Output: [["name", "Charlie"], ["location", "Rome"]]

2. Array

The Array object is used to store multiple values in a single variable and provides methods for adding, removing, and manipulating elements.

const myArray = new Array('a', 42, 3.14);

For more, read our JavaScript Arrays article.

3. String

Represents and manipulates strings of text - here is how to create a string using the String constructor:

const firstName = new String("Charlie");

For more, read our JavaScript String Data Type article.

4. Number

Represents and manipulates numeric values, and provide utilitarian method, such as isNaN (is Not a Number):

const theAnswer = new Number(42);
console.log(theAnswer);  // prints: 42

console.log(Number.isNaN(theAnswer));  // prints: false

5. Boolean

The Boolean object is used to represent and manipulate Boolean (true/false) values.

const wasPublished = new Boolean(true);
console.log(wasPublished);  // prints: "Boolean { true }"

console.log(wasPublished.valueOf());  // prints: true

6. Date

The Date object is used to represent and manipulate dates and times and also provides methods for getting and setting dates, formatting dates, etc:

var currentDate = new Date();

console.log(currentDate.getFullYear()); // prints: 2023
console.log(currentDate.getMonth());    // prints: 0 (January, as months are zero-indexed)
console.log(currentDate.getDate());     // prints: 16
console.log(currentDate.getDay());      // prints: 1 
  // days are also zero-indexed, with Sunday being 0
console.log(currentDate.getHours());    // prints: current hour in 24-hour format
console.log(currentDate.getMinutes());  // prints: current minute
console.log(currentDate.getSeconds());  // prints: current second
console.log(currentDate.getMilliseconds()); // prints: current millisecond
console.log(currentDate.getTime());     // prints: number of milliseconds since January 1, 1970


// parse a string into a Date object
var dateString = "2023-01-16T07:30:00.000Z";
var parsedDate = new Date(Date.parse(dateString));
console.log(parsedDate);  // prints: "Date Mon Jan 16 2023 07:30:00 GMT+0000 (Greenwich Mean Time) ..."

7. Math

The Math object provides mathematical constants and functions, such as trigonometric functions, logarithms, and more:

// get the value of pi
var pi = Math.PI;
console.log(pi);  // prints: 3.141592653589793

// round down to the nearest integer
var roundedDown = Math.floor(3.8);
console.log(roundedDown);  // prints: 3

// round up to the nearest integer
var roundedUp = Math.ceil(3.2);
console.log(roundedUp);  // prints: 4

// round to the nearest integer
var rounded = Math.round(3.5);
console.log(rounded);  // prints: 4

// find the maximum value in a list of numbers
var max = Math.max(3, 5, 7, 2, 8);
console.log(max);  // prints: 8

// find the minimum value in a list of numbers
var min = Math.min(3, 5, 7, 2, 8);
console.log(min);  // prints: 2

// to generate a random number between 0 and 1
var random = Math.random();
console.log(random);  // prints: a random number between 0 and 1

8. RegExp

The RegExp object is used to represent regular expressions, which are used to search for and replace text patterns.

// Create a regular expression to match phone numbers 
// using the format XX-XXXX-XXXXXX
var phoneRegex = /^\d{2}-\d{4}-\d{6}$/;

// test if a string matches the regular expression
var phoneNumber = "44-7700-900123";
var isPhoneNumber = phoneRegex.test(phoneNumber);
console.log(isPhoneNumber);  // prints: true

var invalidPhoneNumber = "123-456-789";
var isInvalidPhoneNumber = phoneRegex.test(invalidPhoneNumber);
console.log(isInvalidPhoneNumber); // prints: false

9. Function

The Function object is used to create functions in JavaScript - the syntax might look a little convoluted and unfamiliar:

const sum = new Function("a", "b", "return a + b;");

console.log(sum(2, 3));  // prints: 5

10. Error

The Error object is used to represent runtime errors in JavaScript and provides information about the error and a stack trace for debugging purposes.

var anError = new Error("Something went wrong");

The message property contains the error message:

var anError = new Error("Something went wrong");

Here is an error generated by a division by zero:

try {
  console.log(42/0);
} catch (error) {
  console.log(error.message);  // prints: "Infinity"
}

Errors can also be created and thrown where appropriate:

function divide(x, y) {
  if (y === 0) {
    throw new Error("Cannot divide by zero");
  }
  return x / y;
}

try {
  console.log(divide(42, 0));
} catch (error) {
  console.log(error.message);  // prints: "Cannot divide by zero"
}

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For more, see JavaScript Built-in Objects (part 2).