Enable typesafe arr.shift() inside of an if check for if(arr.length) { arr.shift() }

isShiftable.ts

export const isShiftable = <T>(arr: T[]): arr is { shift(): T } & Array<T> => arr.length > 0;

Helps to counter this sheer annoyance:

// In the context of a NodeJS Read Stream...
if (this.readSource.length > 0) {
  return this.push(this.readSource.shift()); // Error, cannot push undefined. Typed as 'T | undefined' instead of 'T' (only makes sense if array is sparse 😐 )
}

to this

if (isShiftable(this.readSource)) {
  return this.push(this.readSource.shift()); // Typed as 'T', no errors!
}