principletypescriptCritical

Array<Type> VS Type[] in Typescript

Submitted by: @import:stackoverflow-api·Mar 10, 2026·

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arraytypetypescript

Problem

As far as I know a property's type can be defined in two ways when it's an array:

property_name: type

where type can be either

Array
Array
// etc. (e.g. let prop1: Array)

and

string[]
MyType[]
// etc. (e.g. let prop1: string[])

What is the difference between the two cases? Or am I misunderstanding something (perhaps something about <> used in casting)?

Solution

There isn't any semantic difference

There is no difference at all. Type[] is the shorthand syntax for an array of Type. Array is the generic syntax. They are completely equivalent.

The handbook provides an example here. It is equivalent to write:

function loggingIdentity(arg: T[]): T[] {
    console.log(arg.length);
    return arg;
}

Or:

function loggingIdentity(arg: Array): Array {
    console.log(arg.length);
    return arg;
}

And here is a quote from some release notes:

Specifically, number[] is a shorthand version of Array, just as Date[] is a shorthand for Array.

About the readonly type modifier

TypeScript 3.4, introduces the readonly type modifier. With a precision:

the readonly type modifier can only be used for syntax on array types and tuple types

let err2: readonly Array; // error!    
let okay: readonly boolean[]; // works fine

The following declaration is equivalent to readonly boolean[]:

let okay2: ReadonlyArray;

Code Snippets

function loggingIdentity<T>(arg: T[]): T[] {
    console.log(arg.length);
    return arg;
}

function loggingIdentity<T>(arg: Array<T>): Array<T> {
    console.log(arg.length);
    return arg;
}

let err2: readonly Array<boolean>; // error!    
let okay: readonly boolean[]; // works fine

let okay2: ReadonlyArray<boolean>;

Context

Stack Overflow Q#36842158, score: 474

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