Strict null checks in action

By default, all types in TypeScript allow null:

function getId(x: Element) {
  return x.id;
}
getId(null);  // TypeScript does not complain, but this is a runtime error.

TypeScript 2.0 adds support for strict null checks. If you set --strictNullChecks when running tsc (or set this flag in your tsconfig.json), then types no longer permit null:

function getId(x: Element) {
  return x.id;
}
getId(null);  // error: Argument of type 'null' is not assignable to parameter of type 'Element'.

You must permit null values explicitly:

function getId(x: Element|null) {
  return x.id;  // error TS2531: Object is possibly 'null'.
}
getId(null);

With a proper guard, the code type checks and runs correctly:

function getId(x: Element|null) {
  if (x) {
    return x.id;  // In this branch, x's type is Element
  } else {
    return null;  // In this branch, x's type is null.
  }
}
getId(null);

Found a mistake? Have a question or improvement idea? Let me know.

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