What does the following type-level Fibonacci implementation use? type BuildArr = Arr[‘length’] extends N ? Arr : BuildArr; type Add = […BuildArr, …BuildArr][‘length’];

Question

TypeScript Professional — Hard

What does the following type-level Fibonacci implementation use? type BuildArr = Arr[‘length’] extends N ? Arr : BuildArr; type Add = […BuildArr, …BuildArr][‘length’];

Accepted Answer

The type-level Fibonacci implementation uses tuple lengths to represent numbers and the spread operator to add them together. This allows for type-level arithmetic to be performed within TypeScript.