Distributive conditional — union members resolved independently

What does this output?

// Simulate TypeScript distributive conditional types
// type ToArray<T> = T extends unknown ? T[] : never
// Applied to union: distributes over each member

function toArrayType(values) {
  // Type-level: ToArray<string | number> = string[] | number[]
  // Runtime: demonstrate the distribution
  if (Array.isArray(values)) {
    return values.map(v => [v]); // each element wrapped in array
  }
  return [values];
}

// string | number → distributes → string[] | number[]
// NOT: (string | number)[]

const stringArrays = toArrayType(['a', 'b', 'c']);
const singleNum = toArrayType(42);

console.log(JSON.stringify(stringArrays));
console.log(JSON.stringify(singleNum));

// Prevent distribution with tuple brackets: [T] extends [unknown]
// type ToArrayNonDist<T> = [T] extends [unknown] ? T[] : never
// ToArrayNonDist<string | number> = (string | number)[]
console.log(typeof stringArrays[0][0]);
console.log(typeof singleNum[0]);

Correct Output

[["a"],["b"],["c"]]
[42]
string
number

Why this output?

More Conditional Types Output Questions