Use via `import './enhanced_array'`

enhanced_array.ts

export { }

declare global {
  interface Array<T> {
    last (): T | undefined
    delete (elem: T): boolean
    pushIfNotExist (elem: T): boolean
  }
}
Array.prototype.last = function () {
  return this[this.length - 1]
}
Array.prototype.delete = function <T> (elem: T): boolean {
  const index = this.indexOf(elem)
  if (index === -1) { return false }
  this.splice(index, 1)
  return true
}

Array.prototype.pushIfNotExist = function <T> (elem: T): boolean {
  if (this.includes(elem)) {
    return false
  } else {
    this.push(elem)
    return true
  }
}