Enforcing Strict Mode

The following function looks harmless, yet the compiler rejects it twice.

function add(a, b) { // Error
  return a + b;
}

error TS7006: Parameter 'a' implicitly has an 'any' type.
error TS7006: Parameter 'b' implicitly has an 'any' type.

These errors come from strict mode, a group of type-checking rules controlled by one option called strict. Most of the rules exist to stop a value from quietly having a type that the compiler cannot check, and the sections below follow the rules that you will meet most often.