how to make sure callback of one function call, gets executed only after callback of the second func-CodePudding

I am trying to parse a pdf containing data in the form of a table using javascript. My pdf file is a university time table in the form of a grid which has data in each grid. I am making use of pdfreader npm package(I am quite new to javascript so I am not sure if a better package exists for the usecase)

My plan is first parse through the pdf and with the help of few words I know are present in time table(eg name of days) and fetch the approximate x and y cordinates of the grid in the pdf. Secondly Parse throgh the pdf once again and now from the text I get populate some 2d array with the help of cordinates I fetched in first pass.

In the sample for pdf parser there is a function, parseFileItems which calls itemHandler(error, item) on each item parsed from the pdf file. So for the first pass I call the function like

  new PdfReader().parseFileItems("./timetable.pdf", (err, item) => {
    if (err) console.error("error:", err);
    else if (!item) console.warn("end of file");
    else if (item.text) {
        
        cordinatesofGrid=//my logic to fetch the cordinates of the grid
    }
  });

Now what I need to do is to parse through the pdf once again and make use of the "cordinatesofGrid" variable I have fetched in the first pass. so I again intend to call parseFileItems second time. But this should happen only once the first pass and its callback code is done executing.

I tried to call the first parseFileItems with await and then call parse function second time but this is not working as expected. (I assume this is because the wait does not apply to the callback)

  await new PdfReader().parseFileItems("./timetable.pdf", (err, item) => {
    //first call 
  });
  
  new PdfReader().parseFileItems("./timetable.pdf", (err, item) => {
    //second  call 
  });

Could anybody suggest how to make sure callback of one fucntion call, gets executed only after callback of the second function call

CodePudding user response：

You could try using .then() function for promise handling.

Here's a LINK to some documentation on how it works/how to use it. I believe this would only work if your functions use promises (which I assumed since you tried awaiting).

CodePudding user response：

You could create a Promise wrapper around the new PdfReader()

Quick FYI don't know if the pdf library is promise based! this is from the information you provided!

//This code will always resolve and never throw a reject
const PdfReaderAsync = new Promise((res, rej) => {
    new PdfReader().parseFileItems("./timetable.pdf", (err, item) => {
            if (err || !item) rej(err)
            res(item)
    })
})

//Make the function you are calling this code async

//So you can use it like this
try {
    let item = await PdfReaderAsync()
    // Run through your code
} catch (err) {
    //handleError
}
// Code 2
try {
    let item = await PdfReaderAsync()
    // Run through your code
} catch (err) {
    //handleError
}