Question

var obj = {a : 2};
console.log(obj.b); //prints undefined
console.log(b);      //ReferenceError

这里没有定义obj.b和b。任何人都可以解释不同产出背后的原因吗？

Answer 1

第一个是缺失属性，第二个是缺失变量。

另见这里的区别：

＆＃13;

import UIKit

class ViewController: UIViewController, UITableViewDelegate, UITableViewDataSource {

    @IBOutlet var tableView: UITableView!
    @IBOutlet var tableViewHeightConstraint: NSLayoutConstraint!

    @IBOutlet var parentView: UIView!

    @IBOutlet var parentViewHeightConstraint: NSLayoutConstraint!

    let data = ["this is a short line.",
                "Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Excepteur sint. deserunt mollit anim id est laborum.",
                "another short line.",
                "Sed ut perspiciatis unde omnis iste natus error sit voluptatem accusantium doloremque laudantium, totam  explicabo. Nemo enim ipsam voluptatem quia voluptas sit aspernatur aut odit aut fugit, sed quia."
                ]

    override func viewDidLoad() {
        super.viewDidLoad()

        tableView.delegate = self
        tableView.dataSource = self

        tableView.rowHeight = UITableViewAutomaticDimension
        tableView.estimatedRowHeight = 50

    } // viewDiDload
    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)

        // If I add this new frame for the table's parent view,
        // the table view height gets chopped off. See 3 lines below
        var newFrameForParent = self.parentView.frame
        newFrameForParent.size.height = self.tableView.contentSize.height
        self.parentView.frame = newFrameForParent
        // But if I comment out the above 3 lines, the table view
        // height is fine (table height fits actual dynamic content size)
        // but the table's parent view is shorter than the table view

        var newFrame = self.tableView.frame
        newFrame.size.height = self.tableView.contentSize.height
        self.tableView.frame = newFrame

        print("viewDidAppear: tableView contentSize w: \(tableView.contentSize.width) h: \(tableView.contentSize.height)")

    } // viewDidAppear

    override func viewWillLayoutSubviews() {
        super.updateViewConstraints()
        self.tableViewHeightConstraint.constant = self.tableView.contentSize.height
        //self.parentViewHeightConstraint.constant = self.tableView.contentSize.height + 20
        print("viewWillLayoutSubviews: tableView contentSize w: \(tableView.contentSize.width) h: \(tableView.contentSize.height)")
    } // viewWillLayoutSubviews
    override func viewWillAppear(_ animated: Bool) {
        //tableView.sizeToFit()
    }

    func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return data.count
    }

    func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        let cell = tableView.dequeueReusableCell(withIdentifier: "CellID", for: indexPath) as! CustomCell
        cell.cellLabel?.text = data[indexPath.row]

        print("cell \(indexPath.row) height is: \(cell.frame.height)")

        return cell
    }

}

＆＃13;

Answer 2

obj.b undefined因为他不存在于您的对象中，var b也不存在。

Answer 3

您可以通过此

使第一部分工作

var obj = {a : 2};
console.log(obj.a); //Your property `b` was not defined or you typed `b` instead of `a`

..和第二部分由

var b = 3;
console.log(b);// `b` was not defined at all

评论中的解释

未声明的对象变量显示未定义而不是ReferenceError

3 个答案: