Invariant in TypeScript

Invariants.ts

function notNull<T>(val: T | null): T {
  if (val === null) {
    throw new Error(`Invariant: value should not be null`);
  }
  return val;
}

function notNil<T>(val: T | null | undefined): T {
  if (val === null || val === undefined) {
    throw new Error(`Invariant: value should not be null | undefined`);
  }
  return val;
}

function notUndefined<T>(val: T | undefined): T {
  if (val === undefined) {
    throw new Error(`Invariant: value should not be undefined`);
  }
  return val;
}