Is there a way in typescript define fixed size array. say, for example, in a function definition, I need to able to say
coord: (c:any) => number[]; //how to say it is an array of size 4
can I define an interface like we define a hash map
//this doesn't work
interface IArray{
[number]
}
and also restrict max length to be 4.
You could return a tuple instead of an array:
type array_of_4 = [number, number, number, number];
var myFixedLengthArray :array_of_4 = [1,2,3,4];
// the tuple can be used as an array:
console.log(myFixedLengthArray.join(','));
It's sort of awkward, but you can do it like this:
interface SizeFour<T> {
0: T;
1: T;
2: T;
3: T;
}
function fn(): SizeFour<string> {
// Need to cast
return <any>['', '', '', ''];
}
var x = fn();
var a = x[0]; // a: string
var b = x[4]; // b: any; error if --noImplicitAny
var c = x[10]. so no safety on the bounds. How this is possible as 10 (index/method) is not defined in the interface.Preventing someone from using an index into an array outside the bounds is not something that even C# can do. e.g. what is preventing you from doing :
var index = 123;
sizeFour[index];
Or even requesting the index from the server?
So short answer "there isn't a declarative way of preventing this if you insist on using []"
You could always do
var bar = {
one : 'something',
two: 'somethingelse'
// etc.
}
and then only use . i.e. bar.one
There is a (hacky) way of doing that. It's basically adding a new optional element, which is the impossible type never, to the inner array interface. However, that doesn't work if the third element is undefined.
The code below is with the latest Typescript v4.3.2:
interface ArraySizeOfTwo {
0: number;
1: number;
2?: never;
}
interface ArrayList {
[index: number]: ArraySizeOfTwo;
}
const myArray: ArrayList = [
[1, 2],
[15, 20],
];
// Error: Type 'number' is not assignable to type 'undefined'.
const myArray2: ArrayList = [
[1, 2, 3],
[15, 20],
];
// Error: Property '1' is missing in type '[number]' but required in type 'ArraySizeOfTwo'.
const myArray3: ArrayList = [
[1, 2],
[15],
];
// Error: Type 'undefined' is not assignable to type 'number'.
const myArray4: ArrayList = [
[1, 2],
[15, undefined],
];
// no errors :(
const myArray5: ArrayList = [
[1, 2],
[15, 20, undefined],
];
let a: number[2] = [ 0, 1 ]