Function placeChild

code/new-context-api/public/app.js:8337–8359

(newFiber, lastPlacedIndex, newIndex)

  }

  function placeChild(newFiber, lastPlacedIndex, newIndex) {
    newFiber.index = newIndex;
    if (!shouldTrackSideEffects) {
      // Noop.
      return lastPlacedIndex;
    }
    var current = newFiber.alternate;
    if (current !== null) {
      var oldIndex = current.index;
      if (oldIndex < lastPlacedIndex) {
        // This is a move.
        newFiber.effectTag = Placement;
        return lastPlacedIndex;
      } else {
        // This item can stay in place.
        return oldIndex;
      }
    } else {
      // This is an insertion.
      newFiber.effectTag = Placement;
      return lastPlacedIndex;
    }
  }

  function placeSingleChild(newFiber) {
    // This is simpler for the single child case. We only need to do a