收到错误 "property does not exist"

Question

我修改了我的问题以使其更具体。现在我不关心期望的行为，我只需要更正语法错误

我正在研究 this tutorial 我遇到了这段代码中的错误。

严重性：'Error'

消息：'属性 'offset' 在类型 'PagerserviceProvider' 上不存在。'

实际上我对这三个变量有同样的错误。

that.pageSize,that.offset,that.size

public async getPager(tableName:string,pageSize: number = 10) {
let pageSize = pageSize;
let offset = 0;
let limit = pageSize;
let size = await this.getTotal(tableName);
let that = this;
return  {
        initialPage:function(){

            return new Promise((resolve,reject)=>{
                var d = [];
                that.executeSql(tableName,limit,offset).then((data)=>{
                    console.log(JSON.stringify(data));
                    for(var i = 0 ; i < data.rows.length ; i++)
                    {
                        d.push(data.rows.item(i));
                    }
                    resolve(d);
                },(e)=>{
                    reject(e);
                });
            });

        },
        nextPage:function(){
            if(that.offset <= that.size - that.pageSize )
            {  
                that.offset +=  that.pageSize;
            }
            return new Promise((resolve,reject)=>{
                var d = [];
                that.executeSql(tableName,limit,offset).then((data)=>{
                    for(var i = 0 ; i < data.rows.length ; i++)
                    {
                        d.push(data.rows.item(i));
                    }
                    resolve(d);
                },(e)=>{
                    reject(e);
                });
            });                    
        }            
    };}

Answer 1

'that' 只是用于在代码开头存储 'this' 原始值的变量名称，随着 'this' 值的变化。变量名可以是 'dog' 或 'apple'.

如果您打算在那个时间点访问 'this' 的当前值，您可以稍后在您的代码中选择使用 'this'。否则，您可能会引用存储其值的原始变量，例如'that'、'dog' 或 'apple'。

Answer 2

当你使用关键字function声明一个函数时，函数的this不引用上层this。所以在函数体中使用this，指的是函数本身。

您面临的问题与您的函数是在已经定义了 this 的 class 中声明的事实有关，因此您需要一种方法来引用上面的 this 而在嵌套函数中。

class Test {

  hello () { console.log('hello') }

  method () {
    this.hello() // It will work because `this` refers to the class
    function sayHello () {
      return this.hello()
      // it won't work because `this` refers to the function sayHello
    }
    return sayHello()
  }
}

要绕过此限制，您可以在代码处于上层作用域时将上层 this 保存在变量中。此变量通常称为 that 或 self.

class Test {

  hello () { console.log('hello') }

  method () {
    var that = this // that is now refering to the class
    this.hello() // It will work because `this` refers to the class
    function sayHello () {
      return that.hello()
      // that is still refering to the class so it will succeed
    }
    return sayHello()
  }
}

编辑：

另一个避免使用 that 的技巧是使用 ES6 箭头函数。在箭头函数中，this 总是指上层作用域。

class Test {

  hello () { console.log('hello') }

  method () {
    this.hello() // It will work because `this` refers to the class
    // `this` refers to the upper scope by default so it works
    const sayHello = () => this.hello()
    return sayHello()
  }
}

编辑 2：

您的代码应该是：

  public async getPager(tableName: string, pageSize: number = 10) {
    let pageSize = pageSize;
    let offset = 0;
    let limit = pageSize;
    let size = await this.getTotal(tableName);
    let that = this;
    return  {
        initialPage: function () {

            return new Promise((resolve,reject)=>{
                var d = [];
                that.executeSql(tableName, limit, offset).then(data => {
                    console.log(JSON.stringify(data));
                    for(var i = 0 ; i < data.rows.length ; i++) {
                        d.push(data.rows.item(i));
                    }
                    resolve(d);
                }, e => {
                    reject(e);
                });
            });

        },
        nextPage: function () {
            if(offset <= size - pageSize ) {  
                offset += pageSize;
                // no need to use `that` because you used `let`
            }
            return new Promise((resolve, reject) => {
                var d = [];
                that.executeSql(tableName, limit, offset).then(data => {
                    for(var i = 0 ; i < data.rows.length ; i++) {
                        d.push(data.rows.item(i));
                    }
                    resolve(d);
                }, e => {
                    reject(e);
                });
            });                    
        }            
      };
    }

Answer 3

getPager 是一种方法：如果您使用已经丢失的上下文调用它，that 将获得当前的 this 值，该值未指向正确的上下文。

const someInstance = new SomeClass()

someInstance.getPager() // You call getPager directly from the someInstance context

someHTMLButton.onClick = someInstance.getPager // Here the getPager method lost its context

一个解决方案是将 getPager 绑定到 someInstance。这样它的上下文 this 将始终指向 someInstance.

someHTMLButton.onClick = someInstance.getPager.bind(someInstance)

收到错误 "property does not exist"

getting error "property does not exist"

javascript

cordova

ionic2