Get variable from a asynchronous function

I’m using the following javascript solution to get the number of pages of a file :

const reader = new FileReader()
reader.readAsBinaryString(file)
reader.onloadend = function () {
 const count = reader.result.match(//Type[s]*/Page[^s]/g).length
 console.log('Number of Pages:', count)
}

The number of pages is correct in the console but I don’t know how to extract that number from the scope of the reader so I can use it elsewhere. I’ve read How to return the response from an asynchronous call but I don’t understand how to implement it for my case

Answer

Wrap it in a promise and resolve the value you want:

function getPageNumber() {
  return new Promise((resolve, reject) => {
    const reader = new FileReader()
    reader.readAsBinaryString(file)
    reader.onloadend = function () {
       const count = reader.result.match(//Type[s]*/Page[^s]/g).length
       console.log('Number of Pages:', count);
      resolve(count);
    }
  }
}

getPageNumber().then(count => {
  // here, now you have count
});