TypeScript supports a lot of fixits, which are automated helpers to refactor your code. For example if you select the text in line 7, and click on the light bulb which pops up, you'll get offered some fixits.

function addOne(x: number) {
  return x + 1;
}

// This feature is available as of TypeScript version 3.7, which will also include nightly builds. They might not be something you need to use inside the playground when you're making code samples, or learning. However, having fixits available means we can document them in the playground and that's really valuable: example:big-number-literals example:const-to-let example:infer-from-usage-changes