interface ConcatArray<T> {     length: number;     join(separator?: string): string;     slice(start?: number, 
end?: number): T[];     [
n: 
number]
: T; }   Properties
Readonly length
length: number
Methods
join
- join(separator?: string): string
 Parameters
Optional separator: string
Returns string
slice
- slice(start?: number, end?: number): T[]
 Parameters
Optional start: numberOptional end: number
Returns T[]