(TypeScript/ES6) Exports function flattenArray, which takes a nested array of numbers and flattens it into a non-nested array of numbers

flattenArray.ts

export interface recursiveArray extends Array<number | Array<any>> { }

export function flattenArray(arrayToFlatten: recursiveArray = []): Array<number> {
    let workingArray: Array<number> = []

    for (const value of arrayToFlatten) {
        if (Array.isArray(value)) {
            workingArray = workingArray.concat(flattenArray(value))
        } else {
            workingArray.push(value)
        }
    }

    return workingArray
}