What's in the list?
An array of prices should only ever hold numbers. Tell TypeScript, and it guards every push, every map, every loop.
Array types and tuples
const prices: number[] = [180, 40, 90];
prices.push("free");
const names: Array<string> = ["Asha", "Ram"];
const upper = names.map((n) => n.toUpperCase());
const point: [number, number] = [27.7, 85.3];
const entry: [string, number] = ["momo", 180];
const [item, cost] = entry;
const days: readonly string[] = ["Sun", "Mon"];
days.push("Tue");