TypeScriptStrict null checks

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);

Non-null assertions

The non-null assertion operator, !, allows you to assert that an expression isn't null or undefined when the TypeScript compiler can't infer that automatically:

type ListNode = { data: number; next?: ListNode; };

function addNext(node: ListNode) {
    if (node.next === undefined) {
        node.next = {data: 0};
    }
}

function setNextValue(node: ListNode, value: number) {
    addNext(node);
    
    // Even though we know `node.next` is defined because we just called `addNext`,
    // TypeScript isn't able to infer this in the line of code below:
    // node.next.data = value;
    
    // So, we can use the non-null assertion operator, !,
    // to assert that node.next isn't undefined and silence the compiler warning
    node.next!.data = value;
}