Primitive Types

TypeScript supports 7 primitive types. A primitive data type refers to a type that is not an object and does not have any methods associated with it. In TypeScript, all primitive types are immutable, meaning their values cannot be changed once they are assigned.

string

The string primitive type stores textual data, and the value is always double or single-quoted.

const x: string = 'x';
const y: string = 'y';

Strings can span multiple lines if surrounded by the backtick (`) character:

let sentence: string = `xxx,
   yyy`;

boolean

The boolean data type in TypeScript stores a binary value, either true or false.

const isReady: boolean = true;

number

A number data type in TypeScript is represented with a 64-bit floating point value. A number type can represent integers and fractions. TypeScript also supports hexadecimal, binary, and octal, for instance:

const decimal: number = 10;
const hexadecimal: number = 0xa00d; // Hexadecimal starts with 0x
const binary: number = 0b1010; // Binary starts with 0b
const octal: number = 0o633; // Octal starts with 0o

bigInt

A bigInt represents numeric values that are very large (253 – 1) and cannot be represented with a number.

A bigInt can be created by calling the built-in function BigInt() or by adding n to the end of any integer numeric literal:

const x: bigint = BigInt(9007199254740991);
const y: bigint = 9007199254740991n;

Notes:

• bigInt values cannot be mixed with number and cannot be used with built-in Math, they must be coerced to the same type.
• bigInt values are available only if target configuration is ES2020 or higher.

Symbol

Symbols are unique identifiers that can be used as property keys in objects to prevent naming conflicts.

type Obj = {
    [sym: symbol]: number;
};

const a = Symbol('a');
const b = Symbol('b');
let obj: Obj = {};
obj[a] = 123;
obj[b] = 456;

console.log(obj[a]); // 123
console.log(obj[b]); // 456

null and undefined

null and undefined types both represent no value or the absence of any value.

The undefined type means the value is not assigned or initialized or indicates an unintentional absence of value.

The null type means that we know that the field does not have a value, so value is unavailable, it indicates an intentional absence of value.

Array

An array is a data type that can store multiple values of the same type or not. It can be defined using the following syntax:

const x: string[] = ['a', 'b'];
const y: Array<string> = ['a', 'b'];
const j: Array<string | number> = ['a', 1, 'b', 2]; // Union

TypeScript supports readonly arrays using the following syntax:

const x: readonly string[] = ['a', 'b']; // Readonly modifier
const y: ReadonlyArray<string> = ['a', 'b'];
const j: ReadonlyArray<string | number> = ['a', 1, 'b', 2];
j.push('x'); // Invalid

TypeScript supports tuple and readonly tuple:

const x: [string, number] = ['a', 1];
const y: readonly [string, number] = ['a', 1];

any

The any data type represents literally “any” value, it is the default value when TypeScript cannot infer the type or is not specified.

When using any TypeScript compiler skips the type checking so there is no type safety when any is being used. Generally do not use any to silence the compiler when an error occurs, instead focus on fixing the error as with using any it is possible to break contracts and we lose the benefits of TypeScript autocomplete.

The any type could be useful during a gradual migration from JavaScript to TypeScript, as it can silence the compiler.

For new projects use TypeScript configuration noImplicitAny which enables TypeScript to issue errors where any is used or inferred.

The anytype is usually a source of errors which can mask real problems with your types. Avoid using it as much as possible.