TypeScript vs JavaScript

Model answer. TypeScript is JavaScript plus a static type system. You annotate (or let it infer) the shapes of values, and the compiler reports mistakes such as calling a missing method, passing a string where a number is expected, or forgetting a null check, before the code runs.

type Student = { name: string; marks: number[] };
function average(s: Student): number {
  return s.marks.reduce((a, b) => a + b, 0) / s.marks.length;
}
average({ name: 'Asha', marks: '90' });   // compile error: string is not number[]

Key facts:

• Types are erased when compiled: the output is plain JavaScript, and nothing is checked at runtime. Data from an API still needs runtime validation.
• Every valid JavaScript file is (mostly) valid TypeScript, so projects can migrate gradually.
• Benefits: earlier bug detection, better editor autocomplete and refactoring, and types that document intent in large codebases.
• Cost: a build step and some learning; overly clever types can hurt readability.