By default null and undefined are assignable to all types in TypeScript e.g.
let foo: number = 123;
foo = null; // Okay
foo = undefined; // Okay
This is modelled after how a lot of people write JavaScript. However, like all things, TypeScript allows you to be explicit about what can and cannot be assigned a null or undefined.
In strict null checking mode, null and undefined are different:
let foo = undefined;
foo = null; // NOT Okay
Let's say we have a Member interface:
interface Member {
name: string,
age?: number
Not every Member will provide their age, so age is an optional property, meaning the value of age may or may not be undefined.
undefined is the root of all evil. It often leads to runtime errors. It is easy to write code that will throw Error at runtime:
.then(member: Member => {
const stringifyAge = member.age.toString() // Cannot read property 'toString' of undefined
But in strict null checking mode, this error will be caught at compile time:
.then(member: Member => {
const stringifyAge = member.age.toString() // Object is possibly 'undefined'

Non-Null Assertion Operator

A new ! post-fix expression operator may be used to assert that its operand is non-null and non-undefined in contexts where the type checker is unable to conclude that fact. For example:
// Compiled with --strictNullChecks
function validateEntity(e?: Entity) {
// Throw exception if e is null or invalid entity
function processEntity(e?: Entity) {
let a =; // TS ERROR: e may be null.
let b = e!.name; // OKAY. We are asserting that e is non-null.
Note that it is just an assertion, and just like type assertions you are responsible for making sure the value is not null. A non-null assertion is essentially you telling the compiler "I know it's not null so let me use it as though it's not null".

Definite Assignment Assertion Operator

TypeScript will also complain about properties in classes not being initialized e.g.:
class C {
foo: number; // OKAY as assigned in constructor
bar: string = "hello"; // OKAY as has property initializer
baz: boolean; // TS ERROR: Property 'baz' has no initializer and is not assigned directly in the constructor.
constructor() { = 42;
You can use the definite assignment assertion postfixed to the property name to tell TypeScript that you are initializing it somewhere other than the constructor e.g.
class C {
foo!: number;
// ^
// Notice this exclamation point!
// This is the "definite assignment assertion" modifier.
constructor() {
initialize() { = 0;
You can also use this assertion with simple variable declarations e.g.:
let a: number[]; // No assertion
let b!: number[]; // Assert
a.push(4); // TS ERROR: variable used before assignment
b.push(4); // OKAY: because of the assertion
function initialize() {
a = [0, 1, 2, 3];
b = [0, 1, 2, 3];
Like all assertions, you are telling the compiler to trust you. The compiler will not complain even if the code doesn't actually always assign the property.