First Length (Array Assert)

You can mix checks and assertions. If a value is an array, you can still assert its element type when you know it.

Here the input is unknown. If it is an array of numbers, return the double of the first number. Otherwise return 0.

function firstDouble(a: unknown) {
  if (Array.isArray(a)) {
    const n = (a as number[])[0];
    return typeof n === "number" ? n * 2 : 0;
  }
  return 0;
}

Your Task

Return the length of the first string when the input is a string array.

• Implement firstLength(list: unknown): number.
• If list is an array, assert it as string[] and return the length of its first element.
• If it is not an array or is empty, return 0.
• Do not mutate the input.