javascript - 出现错误 "property does not exist"

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我修改了我的问题，使其更加具体。现在我不关心所需的行为，我只需要纠正语法错误

我正在学习this tutorial我在这段代码中遇到一个错误。

严重性:“错误”

消息:“类型‘PagerserviceProvider’上不存在属性‘offset’。”

实际上我对这三个变量有相同的错误。

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

当使用关键字function声明函数时，函数的this并不引用上面的this。因此在函数体中使用 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 的类中声明的事实有关，因此您需要一种方法来引用上面的 this code> 在嵌套函数内部时。

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，您可以在代码位于上部作用域时将上部 this 保存在变量中。该变量通常称为 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:

避免使用 that 的另一个技巧是使用 ES6 箭头函数。在箭头函数内，this 始终指的是上层作用域。

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);
                });
            });                    
        }            
      };
    }