打字稿，对象可能在空检查中未定义

Question

我想知道，这段代码如何给出object is possibly undefined error

  if (newFocus) {
    if (viewCache[viewId] !== undefined) {
      dispatch(ViewActions.focusOn(viewCache[viewId].focus));
    } else {
      dispatch(ViewActions.focusOn(newFocus));
    }
  }

第 3 行给了我错误，viewCache[viewId] is possibly undefined 即使包裹在 if (viewCache[viewId] !== undefined) 中

Answer 1

错误似乎指出viewCache 可以是undefined。您也可以添加对其存在的检查

if (newFocus) {
    if (viewCache && viewCache[viewId] !== undefined) {
      dispatch(ViewActions.focusOn(viewCache[viewId].focus));
    } else {
      dispatch(ViewActions.focusOn(newFocus));
    }
  }