Definite Assignment Assertions

The definite assignment assertion is a feature that allows a ! to be placed after instance property and variable declarations to relay to TypeScript that a variable is indeed assigned for all intents and purposes, even if TypeScript’s analyses cannot detect so.

For example:

let x: number;
initialize();
console.log(x + x);
//          ~   ~
// Error! Variable 'x' is used before being assigned.
function initialize() {
    x = 10;
}

With definite assignment assertions, we can assert that x is really assigned by appending an ! to its declaration:

// Notice the '!'
let x!: number;
initialize();
// No error!
console.log(x + x);
function initialize() {
    x = 10;
}

In a sense, the definite assignment assertion operator is the dual of the non-null assertion operator (in which expressions are post-fixed with a !), which we could also have used in the example.

let x: number;
initialize();
// No error!
console.log(x! + x!);
function initialize() {
    x = 10;

In our example, we knew that all uses of x would be initialized so it makes more sense to use definite assignment assertions than non-null assertions.