Wait for function to finish reading excel file-CodePudding

I want to read an excel file and return the data in json format. When I call 'readExcelSheet' function on button click, it returns 'undefined' as function hasn't been finished reading the excel data. On subsequent click, data does return properly. I want to wait for this function until it read complete data.

constructor(private httpClient: HttpClient) {
    }

readExcelSheet() {
    let dataJson;
        this.httpClient.get(this.filePath, { responseType: 'blob' })
            .subscribe((data: any) => {
                const reader: FileReader = new FileReader();

                reader.onload = (e: any) => {
                    //   reader.readAsBinaryString(e.target.files[0]);
                    const bstr: string = e.target.result;
                    const wb: XLSX.WorkBook = XLSX.read(bstr, { type: 'binary' });

                    /* grab first sheet */
                    const wsname1: string = wb.SheetNames[0];
                    const ws1: XLSX.WorkSheet = wb.Sheets[wsname1];

                    dataJson = XLSX.utils.sheet_to_json(ws1);
                };
                reader.readAsBinaryString(data);
            });

        return dataJson;
    }

CodePudding user response：

When you call subscribe, the code executes this line asynchronously (ie: skips over this line and go to the return immediately). Use lastValueFrom in an async-await function instead.

Try doing this:

async readExcelSheet(){
 let dataJson;
 const data = await lastValueFrom(this.httpClient.get(this.filePath, { responseType: 'blob' }))
 const reader: FileReader = new FileReader();

 reader.onload = (e: any) => {
  const bstr: string = e.target.result;
  const wb: XLSX.WorkBook = XLSX.read(bstr, { type: 'binary' });

  /* grab first sheet */
  const wsname1: string = wb.SheetNames[0];
  const ws1: XLSX.WorkSheet = wb.Sheets[wsname1];

  dataJson = XLSX.utils.sheet_to_json(ws1);
 };
 reader.readAsBinaryString(data);
 return dataJson;
}

CodePudding user response：

Try using ASYNC & AWAIT in your functions

async readExcelSheet()

CodePudding user response：

Try to use toPromise instead of subscribe and then you can return the function. You can then try to use Promise.resolve() to resolve the dataJson.

constructor(private httpClient: HttpClient) {
}

readExcelSheet() {
  return this.httpClient.get(this.filePath, { responseType: 'blob' })
        .toPromise().then((data: any) => {
            const reader: FileReader = new FileReader();

            reader.onload = (e: any) => {
                //   reader.readAsBinaryString(e.target.files[0]);
                const bstr: string = e.target.result;
                const wb: XLSX.WorkBook = XLSX.read(bstr, { type: 'binary' });

                /* grab first sheet */
                const wsname1: string = wb.SheetNames[0];
                const ws1: XLSX.WorkSheet = wb.Sheets[wsname1];

                Promise.resolve(XLSX.utils.sheet_to_json(ws1))
            };
            reader.readAsBinaryString(data);
        });

}