TypeScript 2.0 introduces two new types: null and undefined. You have to set the compiler option strictNullChecks to true to use these types. In this mode, all other types cannot contain undefined or null anymore. If you want to declare a variable that can be undefined or null, you have to annotate it with a union type. For instance, if you want a variable that should contain a string or undefined, you can declare it as let x: string | undefined;.
Before assignments, the type of the variable will be undefined. Assignments and type guards will modify the type locally.
TypeScript has various ways to check whether a variable could be undefined or null. The next code block demonstrates them:
let x: string | null | undefined = ...;
if (x !== null) {
// x: string | undefined
}
if (x !== undefined) {
// x: string | null
}
if (x != null) {
// x: string
}
if (x) {
//...