Non-null assertion operator: !

• You tells the TS compiler that the value of a variable is not null | undefined
• Use it when you are in possession of knowledge that the TS compiler lacks.

Here is a trivial example of what it does:

let nullable1: null | number;
let nullable2: undefined | string;

let foo  = nullable1! // type foo: number
let fooz = nullable2! // type fooz: string

It basically removes null | undefined from the type

When do I use this?

Typescript is already pretty good at inferring types for example using typeguards:

let nullable: null | number | undefined;

if (nullable) {
    const foo = nullable; // ts can infer that foo: number, since if statements checks this
}

However sometimes we are in a scenario which looks like the following:

type Nullable = null | number | undefined;

let nullable: Nullable;

validate(nullable);

// Here we say to ts compiler:
// I, the programmer have checked this and foo is not null or undefined
const foo = nullable!;  // foo: number

function validate(arg: Nullable) {
    // normally usually more complex validation logic
    // but now for an example
    if (!arg) {
        throw Error('validation failed')
    }
}

My personal advice is to try to avoid this operator whenever possible. Let the compiler do the job of statically checking your code. However there are scenarios especially with vendor code where using this operator is unavoidable.

If you run compilation with --strictNullChecks, a compilation error

error TS2531: Object is possibly 'null'.

should be raised. I guess the author intended to state that non-null assertions lead to runtime exception if you continue to use the object that supposedly is non-null.

For example calling a function on taxValue in your example above with the non-null assertion would not warn you that the value might be null but if it is null and you call the function during runtime it will raise an exception.

The non-null assertion can be used for you to remove falsy warning that a knonw-non-null variable whose type could be null can be used as if it were not nullable.