javascript - getting error "property does not exist" -

i modified question more specific. don't care desired behavior , need correct syntax error

i studying this tutorial face error in code.

severity: 'error'

message: 'property 'offset' not exist on type 'pagerserviceprovider'.'

actually have same error 3 variables.

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 = 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 = 0 ; < 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 = 0 ; < data.rows.length ; i++)                     {                         d.push(data.rows.item(i));                     }                     resolve(d);                 },(e)=>{                     reject(e);                 });             });                             }                 };} 

when use keyword function declare function, function's not refer upper this. using this in function body, refers function itself.

the problem facing related fact function declared inside class have this defined, need way reference upper this while being inside nested function.

class test {    hello () { console.log('hello') }    method () {     this.hello() // work because `this` refers class     function sayhello () {       return this.hello()       // won't work because `this` refers function sayhello     }     return sayhello()   } } 

to bypass limitation, can save upper this in variable while code in upper scope. variable called that or self.

class test {    hello () { console.log('hello') }    method () {     var = // refering class     this.hello() // work because `this` refers class     function sayhello () {       return that.hello()       // still refering class succeed     }     return sayhello()   } } 

edit:

another trick avoid using that use es6 arrow function. inside arrow function, this alway refers upper scope.

class test {    hello () { console.log('hello') }    method () {     this.hello() // work because `this` refers class     // `this` refers upper scope default works     const sayhello = () => this.hello()     return sayhello()   } } 

edit 2:

your code should be:

  public async getpager(tablename: string, pagesize: number = 10) {     let pagesize = pagesize;     let offset = 0;     let limit = pagesize;     let size = await this.gettotal(tablename);     let = 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 = 0 ; < 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 use `that` because used `let`             }             return new promise((resolve, reject) => {                 var d = [];                 that.executesql(tablename, limit, offset).then(data => {                     for(var = 0 ; < data.rows.length ; i++) {                         d.push(data.rows.item(i));                     }                     resolve(d);                 }, e => {                     reject(e);                 });             });                             }                   };     }