DevGradsData Types and data structures
JavaScript is a loosely typed language. This means that you don't have to specify the data type of a variable when you declare it. JavaScript automatically converts the variable to the correct data type. For example, if you assign an integer value to a variable, then later on assign a string value to the same variable, JavaScript will convert the integer to a string.
JavaScript is also a dynamic language with dynamic types. Variables in JavaScript are not directly associated with any particular value type, and any variable can be assigned (and re-assigned) values of all types:
let foo = 42; // foo is now a number
foo = "bar"; // foo is now a string
foo = true; // foo is now a boolean
Primitive data-types
| Type | typeof-return value |
|---|---|
| Null | "object" |
| Undefined | "undefined" |
| Boolean | "boolean" |
| Number | "number" |
| BigInt | "bigint" |
| String | "string" |
| Symbol | "symbol" |
Null type
The Null type is inhabited by exactly one value: null.
Undefined type
The Undefined type is inhabited by exactly one value: undefined.
Boolean type
The Boolean type represents a logical entity and is inhabited by two values: true and false.
Boolean values are usually used for conditional operations, including ternary operators, if...else, while, etc.
Number type
In JavaScript, there's one main numerical data type: the number. Unlike some other programming languages, JavaScript doesn't differentiate between integers (whole numbers) and floating-point numbers (decimals). All numbers are stored as 64-bit floating-point numbers following the IEEE 754 standard.
BigInt
The BigInt type is a numeric primitive in JavaScript that can represent integers with arbitrary magnitude. With BigInts, you can safely store and operate on large integers even beyond the safe integer limit (Number.MAX_SAFE_INTEGER) for Numbers.
strings
The String type represents textual data and is encoded as a sequence of 16-bit unsigned integer values representing UTF-16 code units. Each element in the string occupies a position in the string. The first element is at index 0, the next at index 1, and so on. The length of a string is the number of UTF-16 code units in it, which may not correspond to the actual number of Unicode characters; see the String reference page for more details.
JavaScript strings are immutable. This means that once a string is created, it is not possible to modify it. String methods create new strings based on the content of the current string — for example:
Symbol
A Symbol is a unique and immutable primitive value and may be used as the key of an Object property (see below). In some programming languages, Symbols are called "atoms". The purpose of symbols is to create unique property keys that are guaranteed not to clash with keys from other code.
Non-primitive
In JavaScript, primitive data types represent simple values, while non-primitive data types store collections of data or more complex structures. Non-primitive data types are also sometimes called reference types.
| Type | typeof-return value |
|---|---|
| Object | "{}" |
| Array | "[]" |
| Functions | "(){}" |
Object
An object in JavaScript represents a collection of key-value pairs where the keys are strings and the values are JavaScript values. Objects can be created with figure brackets {}, with the keyword new, and with the class syntax.
An object containing information about a person:
{ name: "Alice", age: 30 }Arrays
Ordered collections of items, all of which can be of different data types.
An array of fruits:
["apple", "banana", "cherry"]Functions
Reusable blocks of code that perform specific tasks. (Technically, functions are a special kind of object in JavaScript)
A function to greet someone:
function sayHi(name) { console.log("Hi, " + name + "!"); }