未捕获的类型错误:无法读取属性'querySelector'的未定义

Uncaught TypeError: Cannot read property 'querySelector' of undefined

本文关键字：属性未定义 querySelector 读取类型错误更新时间：2024-02-16

你能帮我吗？当我运行此程序时，我会在'bindTaskEvents'函数中得到"Uncaught TypeError:无法读取未定义的属性'querySelector'"，该函数处理"completedTasksHolder"列表。在控制台中尝试：

console.log(completedTasksHolder.children[0])

只回李刚刚好。

var taskInput = document.getElementById("new-task"); // new-task
var addButton = document.getElementsByTagName("button")[0]; // first button
var incompleteTasksHolder = document.getElementById("incomplete-tasks"); // ul#incomplete-tasks
var completedTasksHolder = document.getElementById("completed-tasks"); // ul#completed-tasks
// Add a new task
var addTask = function(){
    console.log("Add task...");
}
// Edit an existing task
var editTask = function() {
    console.log("Edit task...");
}
var deleteTask = function() {
    console.log("Delete task...");
// Delete an existing task
    // When the Delete button is pressed
        // Remove the parent li from the ul
}
var taskCompleted = function() {
    console.log("Complete task...");
// Mark a task as complete
    // When the checkbox is checked
        // Append the task li to the #completed-tasks
}
// Mark a task as incomplete
var taskIncomplete = function() {
    console.log("Incomplete task...");
    // When the checkbox is unchecked
        // Append the task li to the #incomplete-tasks
}
var bindTaskEvents = function(taskListItem, checkBoxEventHandler) {
    console.log("Bind li events");
    // select li's children
    var checkbox = taskListItem.querySelector("input[type=checkbox]");
    var editButton = taskListItem.querySelector("button.edit");
    var deleteButton = taskListItem.querySelector("button.delete");
    // bind editTask to editButton
    editButton.onclick = editTask;
    //  bind deleteTask to deleteButton;
    deleteButton.onclick = deleteTask;
    // bind checkBoxEventHandler to checkbox
    checkbox.onchange = checkBoxEventHandler;
}
// Set the click handler to the addTask function
addButton.onclick = addTask;
// Cycle over incompleteTasksHiolder ul list items
for (var i = 0; incompleteTasksHolder.children.length; i++) {
        // bind events to li' children (taskCompleted)
    bindTaskEvents(incompleteTasksHolder.children[i], taskCompleted);
}     
// Cycle over completedTasksHiolder ul list items
for (var i = 0; completedTasksHolder.children.length; i++) {
        // bind events to li' children (taskCompleted)
    bindTaskEvents(completedTasksHolder.children[i], taskIncomplete);
}

这里可能出了什么问题？谢谢

for循环存在问题

更换

for (var i = 0; incompleteTasksHolder.children.length; i++) {

带有

for (var i = 0; i < incompleteTasksHolder.children.length; i++) {

类似地用于completedTasksHolder

基本上，您已经创建了一个没有结束条件的循环，并且在某个点上incompleteTasksHolder.children[i]是undefined。

var db = null;
var db2 = null;
var db3 = null;
var dbUser = null;
var dbName = "estudos.db";
var app = {
    // Application Constructor
    initialize: function() {
        document.addEventListener('deviceready', this.onDeviceReady.bind(this), false);
    },
    // deviceready Event Handler
    //
    // Bind any cordova events here. Common events are:
    // 'pause', 'resume', etc.
    onDeviceReady: function() {
        this.receivedEvent('deviceready');
    },
    // Update DOM on a Received Event
    receivedEvent: function(id) {
        var parentElement = document.getElementById(id);
        var listeningElement = parentElement.querySelector('.listening');
        var receivedElement = parentElement.querySelector('.received');
        listeningElement.setAttribute('style', 'display:none;');
        receivedElement.setAttribute('style', 'display:block;');
        // OPERACOES BD - inicio
        //banco de dados local - aceite de termos e outras coisas
        dbUser = window.sqlitePlugin.openDatabase({name: 'user.db', location: 'default'});
        dbUser.transaction(function(tx) {
            tx.executeSql('CREATE TABLE IF NOT EXISTS Users (flg_aceite, flg_valid_bd)');
        }, function(error) {
            alert('Transaction ERROR: ' + error.message);
        }, function() {
            console.log('Database OK');
        });
        //copia do banco de dados de estudos
        window.plugins.sqlDB.copy(dbName, 0, copysuccess, copyerror);
        // OPERACOES BD - fim
    }
};
app.initialize();
//---------------------------------------------------------------
function copysuccess()
{
    //primeira versão deste banco de dados. o comando anterior.
    //provavelmente realizou a cópia, abro o BD.
    db = window.sqlitePlugin.openDatabase({name: dbName});
    //preciso verificar se existem versões anteriores deste BD. Deleto por precaucao
    dropTable();
    fts_table();
}
function copyerror(e)
{
    //esta versao do banco de dados ja existe.
    //abro o BD
    db = window.sqlitePlugin.openDatabase({name: dbName});
    //db3 = window.sqlitePlugin.openDatabase({name: "vtestudos"});
    //alert("copyerror" + JSON.stringify(e));
}
//---------------------------------------------------------------
function fts_table(){
    db.transaction(function(tx) {
    tx.executeSql('CREATE VIRTUAL TABLE vtestudos USING FTS3(titulo, texto, id_titulo)', [], function(tx,res){
          //alert("nao deu erro");
          //db = window.sqlitePlugin.openDatabase({name: "vtestudos"});
          //alert("uai. deu pra abrir");
          db.transaction(function(tx) {
          tx.executeSql('INSERT INTO vtestudos(titulo, texto, id_titulo) SELECT titulo, texto, id_titulo FROM estudos', [], function(tx,res){
              //db3 = window.sqlitePlugin.openDatabase({name: "vtestudos"});
               console.log('insert ok');
          });
          }, function(err){
              alert(err.message);
          });
    });
    }, function(err){
        alert(err.message);
    });
}
//---------------------------------------------------------------
function dropTable()
{
    window.plugins.sqlDB.remove("estudosprev1", 0, rmsuccess,rmerror); 
    window.plugins.sqlDB.remove("estudosprev2", 0, rmsuccess,rmerror);  
}
function rmsuccess()
{
    //existe versão anterior
    //alert("removesuccess");
    console.log('existe versão anterior');
}
function rmerror(e)
{
    //não existe versão anterior. ignoro.
    //alert("removeerror" + JSON.stringify(e));
    console.log('n existe versão anterior. ignoro.');
}