I am having trouble synchronizing the execution of this program -- JS, Promise, async and await

Question

Guys, I’m having a question, I’d like to read an Excel spreadsheet, store your content in a variable and then display it on the screen. However the method that reads the file -- parseExcel -- occurs asynchronously, so the program follows the stream and I can’t access the variable with the file text. Below the function reading the file, the function I want to store the information and output on the console.

async parseExcel() {

    var reader = new FileReader();

    reader.onload = function(e) {
        var data = e.target.result;
        var workbook = XLSX.read(data, {
            type: 'binary'
        });
        var dadosToJson = [];
        workbook.SheetNames.forEach(function(sheetName) {
            // Here is your object
            var XL_row_object = XLSX.utils.sheet_to_row_object_array(workbook.Sheets[sheetName]);
            var json_object = JSON.stringify(XL_row_object);
            dadosToJson.push(json_object);
            //console.log(json_object);
        })

        console.log(dadosToJson)
        return dadosToJson;

    }

  reader.onerror = function(ex) {
        console.log(ex);
    };

   reader.readAsBinaryString(this.file)
       
};

lerAquivoJson(){

    var texto = this.parseExcel()
    texto.then( (res) => console.log(res))

}

exit lerArquivoJson(): Undefined

async parseExcel(): ["[{"Word id":"3","Words ":"Birthday","Signifi... ,"Insertion date":"1/16/21"},{"Words ":"3647"}]", "[]"]

Note that the Filefilefilefile method does not wait for parseExcel

Edit: I found a solution:

class Leitorarquivoxlsx{

//no momento só aceita um único arquivo
constructor(file){
    this.file=file;
    this.idTable = "vocabularioView";
    this.data;
}

async parseExcel() {
    return new Promise((resolve, reject) => {
        var reader = new FileReader();
        reader.onload = function (e) {
            var data = e.target.result;
            var workbook = XLSX.read(data, {
                type: 'binary'
            });
            var dadosToJson = [];
            workbook.SheetNames.forEach(function (sheetName) {
                // Here is your object
                var XL_row_object = XLSX.utils.sheet_to_row_object_array(workbook.Sheets[sheetName]);
                var json_object = JSON.stringify(XL_row_object);
                dadosToJson.push(json_object);
            })
            resolve(dadosToJson);

        }

        reader.onerror = function (ex) {
            reader.abort();
            reject(console.warn(ex));
        };

        reader.readAsBinaryString(this.file)
    });
       
};



async xlsxToJson(){
    try {
        const fileContents = await this.parseExcel();
        this.data = fileContents;
    } catch (e) {
        console.warn(e.message)
    }finally {
        this.imprimeData();
    }
}

imprimeData(){
    jQuery('#xlx_json').val("fileContents: "+this.data);
}

}

Answer 1

Turn all your waiting into ASYNC, this can solve the problem.

async function myName(){

 var texto = await this.parseExcel()
    texto.then( (res) => console.log(res))

}