Coalescing is performed via || operator, i.e.

let str:string = name || FirstName || "name is null and FirstName is null";

You can also read this question for more details and explanations.

In Typescript

Typescript introduced null coalescing with version 3.7, so if you're running on 3.7 or higher you can simply write:

const str = name ?? firstName ?? "Name and First Name are both null";
const x = foo?.bar.baz() ?? bizz();

See https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html#nullish-coalescing.

In the Angular Template

Since Angular 12 you can also use ?? in the template.

Maybe what you want achieve is this:

let str =
    typeof (name) !== 'undefined' && name !== null ?
        name : typeof (FirstName ) === 'undefined' || FirstName  === null ?
        "First Name is null" : FirstName