fix(runtime-core/scheduler): use undefined instead of null

[Picknight]

1. use undefined instead of null in getId function, because the value of the undeclared job.id is undefined
2. return early in flushPreFlushCbs and flushPostFlushCbs function for better readability

// before
export function flushPostFlushCbs() {
  if (pendingPostFlushCbs.length) {
    // do more
  }
}

// after
export function flushPostFlushCbs() {
  if (!pendingPostFlushCbs.length) return
  // do more
}

[dsonet]

Talking about the readability, I think the current one is better. It's clearer to have the logic in an if branch, focus on what we should deal with. With an extra return, it does reduce the indent but definitely increase the cost of reading the code, you have to keep reminding yourself it's actually introduced an else branch.

[Picknight]

Talking about the readability, I think the current one is better. It's clearer to have the logic in an if branch, focus on what we should deal with. With an extra return, it does reduce the indent but definitely increase the cost of reading the code, you have to keep reminding yourself it's actually introduced an else branch.

Readability is actually a subjective concept, I just think that most people’s default specifications should be followed. The Return Early Pattern :)

[yyx990803]

== null checks for both null and undefined. In this case it doesn't really make a difference. As for the early return - we generally don't encourage readability related PRs since they are subjective but usually ends up in messy diffs that makes the git history harder to follow.

[Picknight]

Thanks for your explanation @yyx990803 , I got it.