Array<Type> VS Type[] in Typescript

235
votes

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<string>, Array<MyType>, etc. (e.g. let prop1: Array<string>)

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?)

EDIT since the question is marked as duplicate, I am aware there is the other question about any[] but still I had a look at it before posting and to me it was more about the type 'any' than the different [] VS <> I asked

1
typescript
Possible duplicate of Typescript Array vs any[] – Nitzan Tomer
They are practically the same constructs, and are effectively the same objects at runtime. Reflect-metadata will also treat them both as having the Array object as their constructor. See the above linked answer. – John Weisz
@NitzanTomer that question is out-of-date -- Array<T> didn't exist back then. – Nathan Shively-Sanders
@NathanShively-Sanders the answer is the same with or without the generics because this question can be reduced to "what's the different between let x: Array; to let x: any[];" – Nitzan Tomer

1 Answers

265
votes

There isn't any semantic difference

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

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

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

Or:

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

And here is a quote from some release notes:

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

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

The following declaration is equivalent to readonly boolean[]:

let okay2: ReadonlyArray<boolean>;