Exceljs await not working in promise in async function

Question

Im currently working on opening and writing on an excel file using exceljs. However, the await is making an error

SyntaxError: await is only valid in async functions and the top level bodies of modules

Despite the function being in async. How can I resolve this problem? The following is the code that I am using

async function Process(filename){
  const workbook = new ExcelJS.Workbook();

  let myPromise = new Promise(function(myResolve, myReject) {
    // "Producing Code" (May take some time)
    try{
      await workbook.xlsx.readFile(filename)
      myResolve(); // when successful
    }catch(err){
      myReject();  // when error
    }

  });

  // "Consuming Code" (Must wait for a fulfilled Promise)
  myPromise.then(
    function() {
        /* code if successful */ 
    },
    function() {return false;}
  );
}

[What is the explicit promise construction antipattern and how do I avoid it?](https://stackoverflow.com/q/23803743) | [Is it an anti-pattern to use async/await inside of a new Promise() constructor?](https://stackoverflow.com/q/43036229) — VLAZ, Jan 06 '22 at 10:16
The anonymous function you pass to `new Promise` is not `async` (but shouldn't be, see VLAZ's comment). — Quentin, Jan 06 '22 at 10:18

score 0 · Accepted Answer · answered Jan 06 '22 at 14:01

Agreeing with the comments, you should avoid the Promise constructor antipattern and never use await in the executor of a new Promise!

You should write either

function Process(filename){
  const workbook = new ExcelJS.Workbook();
  
  // "Producing Code" (May take some time)
  let myPromise =  workbook.xlsx.readFile(filename);

  // "Consuming Code"
  return myPromise.then(function() {
    /* code if successful, waiting for a fulfilled Promise */
    return …;
  }, function() {
    /* code when error, waiting for a rejected Promise */
    return false;
  });
}

or

async function Process(filename){
  const workbook = new ExcelJS.Workbook();

  try {
    // "Producing Code" (May take some time)
    await workbook.xlsx.readFile(filename);

    // "Consuming Code" (wait for the promise to be fulfilled)
    return …;
  } catch(err) {
    // Code when error in producing or consuming code
    return false;
  }
}

Exceljs await not working in promise in async function

1 Answers1