The Modern JavaScript Variables and Data Types Cheat Sheet

This article provides a succinct and comprehensive cheat sheet about variables and data types in modern JavaScript. You can bookmark this page for a quick and convenient lookup later.

Variables

Variables in modern JavaScript can be declared using the var, let, or const keywords. The var keyword has some scoping issues and is not recommended, whereas let and const are block-scoped.

Keyword	Description
var	Declares a variable with function or global scope. Not recommended.
let	Declares a block-scoped variable that can be reassigned.
const	Declares a block-scoped variable that cannot be reassigned.

Example:

var myVar = 42;
let myLet = "Sling Academy";
const myConst = [1, 2, 3];

Data Types

JavaScript has several built-in data types, including primitive types and complex types.

Primitive Types

Primitive types are immutable and are passed by value.

Type	Description	Examples
string	A sequence of characters	“Hello, world!”, ‘JavaScript’
number	A numerical value, including integers and floats	42, 3.14, NaN, Infinity
boolean	A value that is either true or false	true, false
null	A value that represents the absence of a value	null
undefined	A value that represents a variable that has not been assigned a value	undefined
symbol	A primitive data type that represents a unique identifier	Symbol(“mySymbol”)

Complex Types

Complex types are mutable and are passed by reference.

Type	Description	Examples
object	A collection of key-value pairs also called properties	{name: “Foo”, age: 88}
array	An ordered list of values is enclosed in square brackets	[1, 2, 3], [‘a’, ‘b’, ‘c’]
function	A reusable block of code that performs a specific task	function add(a, b) { return a + b; }
RegExp	An object that describes a pattern of characters	/[a-z]/, /hello/g
Date	An object that represents a date and time	new Date(), new Date(2021, 9, 10)
Map	An object that stores key-value pairs	new Map([[“key1”, “value1”], [“key2”, “value2”]])
Set	An object that stores unique values	new Set([1, 2, 3])
Error	An object that represents an error	new Error(“Something went wrong”)
Promise	An object that represents the eventual completion of an asynchronous operation	new Promise((resolve, reject) => { /* code */ })

Type Coercion

JavaScript also performs type coercion, which is the process of converting a value from one type to another type. This can be implicit or explicit.

Implicit Coercion

Implicit coercion occurs when JavaScript automatically converts a value from one type to another type. This can happen in certain situations, such as when using the + operator with strings and numbers.

console.log("Hello " + 42); // "Hello 42"
console.log(42 + "");      // "42"
console.log("42" - 0);     // 42

Explicit Coercion

Explicit coercion occurs when the developer intentionally converts a value from one type to another type using a built-in method or operator. Some common examples include parseInt(), parseFloat(), toString(), and Number().

console.log(parseInt("42"));       // 42
console.log(parseFloat("3.14"));   // 3.14
console.log((42).toString());      // "42"
console.log(Number("42"));         // 42

Variables Scope

JavaScript has function scope and block scope. A variable declared inside a function is only accessible within that function, whereas a variable declared inside a block is only accessible within that block.

Function Scope

Variables declared inside a function have function scope and are only accessible within that function.

function myFunction() {
  var x = 42;
  console.log(x); // 42
}

console.log(x);   // ReferenceError: x is not defined

Block Scope

Variables declared inside a block have block scope and are only accessible within that block.

if (true) {
  let y = "Hello";
  console.log(y); // "Hello"
}

console.log(y);   // ReferenceError: y is not defined

Final Words

Hope this cheat sheet is helpful to you. If you see anything that needs to be added or corrected, let me know by leaving comments.

If you need more detailed instructions and more concrete examples about JavaScript variables and data types, continue reading the articles in this series. Happy coding, and have a nice day!