undefined

JavaScript has two primitive values used to signal absent or uninitialized value: null (absent) and undefined (uninitialized).

TypeScript has two corresponding types by the same names. How these types behave depends on whether you have the strictNullChecks option on.

With strictNullChecks off, values that might be null or undefined can still be accessed normally, and the values null and undefined can be assigned to a property of any type. This is similar to how languages without null checks (e.g. C#, Java) behave. The lack of checking for these values tends to be a major source of bugs; TypeScript always recommend people turn strictNullChecks on if it’s practical to do so in the codebase.

With strictNullChecks on, when a value is null or undefined, you will need to test for those values before using methods or properties on that value. Just like checking for undefined before using an optional property, we can use narrowing to check for values that might be null:

function doSomething(x: string | null) {
  if (x === null) {
    // do nothing
  } else {
    console.log('Hello, ' + x.toUpperCase());
  }
}