Array

An array in TypeScript is an ordered, iterable data structure that enforces strict type checking on its elements. It ensures that all items within the collection conform to a predefined type signature, preventing runtime type errors inherent to dynamically typed JavaScript.

TypeScript provides two syntactically equivalent approaches to declare an array type: the square bracket notation and the generic array type.

// Square bracket notation (Standard convention)
let numericArray: number[] = [1, 2, 3];

// Generic Array type
let stringArray: Array<string> = ["a", "b", "c"];

Complex Type Arrays

Arrays can enforce types for complex structures, including interfaces, type aliases, and union types. When using union types, parentheses are required to dictate the order of operations.

// Union types
let mixedArray: (string | number)[] = ["hello", 42];

// Interface typing
interface User {
    id: number;
    name: string;
}
let users: User[] = [{ id: 1, name: "Alice" }];

Multi-dimensional Arrays

Multi-dimensional arrays are declared by chaining the square bracket notation or nesting the generic syntax. This enforces the type signature across nested iterable levels.

// 2D Array using bracket notation
let matrix: number[][] = [
    [1, 2],
    [3, 4]
];

// 2D Array using generic notation
let genericMatrix: Array<Array<number>> = [
    [1, 2],
    [3, 4]
];

Readonly Arrays

TypeScript introduces immutability at the type level via the readonly modifier or the ReadonlyArray<T> generic. These types strip away mutating methods (e.g., push, pop, splice) and prevent index assignment during compilation.

// Using the readonly modifier
let immutableNumbers: readonly number[] = [1, 2, 3];

// Using the ReadonlyArray generic
let immutableStrings: ReadonlyArray<string> = ["a", "b"];

// Compilation Errors:
// immutableNumbers[0] = 99; 
// immutableNumbers.push(4); 

Type Inference

If an array is initialized without an explicit type annotation, the TypeScript compiler infers the type signature based on the union of the types of the initial elements.

// Inferred as (number | string | boolean)[]
let inferredArray = [1, "two", true]; 

Tuples (Fixed-Length Arrays)

A tuple is a specialized array subtype in TypeScript. While standard arrays represent an unbounded collection of uniform (or union) types, a tuple enforces both a fixed length and a specific type at each positional index.

// Tuple declaration
let record: [string, number, boolean] = ["active", 100, true];

// Optional tuple elements
let optionalRecord: [string, number?] = ["pending"];

// Rest elements in tuples
let spreadTuple: [string, ...number[]] = ["scores", 98, 85, 100];

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