How to capture FileReader base64 as variable?

Question

This prints base64 out to console:

function getBase64(file) {
    var reader = new FileReader();
    reader.readAsDataURL(file);
    reader.onload = function() {
        console.log(reader.result);
    };
    reader.onerror = function(error) {
        console.log('Error: ', error);
    };
}

var file = document.querySelector('#files > input[type="file"]').files[0];
getBase64(file); // prints the base64 string

Source: https://stackoverflow.com/a/36281449/1063287

jsFiddle: jsFiddle demo of the above working code

I want to be able to assign the base64 to a variable, so I tried the following, based on this answer:

function getBase64(file, onLoadCallback) {
    var reader = new FileReader();
    reader.readAsDataURL(file);
    reader.onload = onLoadCallback;
    reader.onerror = function(error) {
        console.log('Error when converting PDF file to base64: ', error);
    };
}

var my_pdf_file = document.querySelector("#my_pdf_file").files[0];
var my_pdf_file_as_base64 = "";
getBase64(my_pdf_file, function(e) {
    my_pdf_file_as_base64 = e.target.result
});

// print out the base64 to show i have captured the value correctly
console.log(my_pdf_file_as_base64);

It is currently printing nothing out to the console.

Question:

How can I save the base64 value as a variable?

Edit:

As requested, for context:

I am submitting a form in a Google Apps Script environment.

I have done this previously and passed a form object (which included a file) through to the Google Apps Script function.

However, one of the constraints of this approach is that if passing a form object as a parameter, it is the only parameter allowed.

A form element within the page is also legal as a parameter, but it must be the function’s only parameter

Source

In this instance, I am passing multiple parameters through, and one of the parameters will be a pdf file, converted to base64.

In response to @Aasmund's great answer, I would like the variable assignment to block further code execution:

var my_pdf_file = [ converted file here ];

// don't do this stuff until the above variable is assigned

Otherwise, I will have to refactor the remaining code to take place in the then block (suggested by @Aasmund), and that might be messy / impossible due to the amount of validation / variable preparation / conditional handling that is taking place before the form is submitted.

Answer 1

FileReader.readAsDataURL() is asynchronous - the download happens in the background while the rest of the code keeps executing. So the reason console.log(my_pdf_file_as_base64); prints an empty string is that the line my_pdf_file_as_base64 = e.target.result hasn't been executed yet: the call to getBase64() finishes almost immediately, and the subsequent statement is executed; only later (when the download is complete) will the callback be executed.

The way to handle this is to place the code that uses the downloaded file inside the callback:

getBase64(my_pdf_file, function(e) {
    my_pdf_file_as_base64 = e.target.result;
    console.log(my_pdf_file_as_base64);
});

Alternatively, you can repeatedly (e.g. inside a setTimeout callback or inside some DOM event handler) check if reader.readyState === FileReader.DONE - whenever this becomes true, reader.result will contain the file.

A more flexible approach is to use a Promise, which is an object that encapsulates an asynchronous computation:

function getBase64(file, onLoadCallback) {
    return new Promise(function(resolve, reject) {
        var reader = new FileReader();
        reader.onload = function() { resolve(reader.result); };
        reader.onerror = reject;
        reader.readAsDataURL(file);
    });
}

var promise = getBase64(my_pdf_file);
promise.then(function(result) {
    console.log(result);
});

So far, this looks pretty similar to the first solution, but the advantage is that promise is an object that you can pass around to other functions, so that you can start a computation in one place and decide in another place what should happen when it's finished.

As you have probably noticed, neither of these approaches allow you to block further code execution until the file content has been assigned to the global variable my_pdf_file_as_base64. This is by design; however, if you really really need to block on the download because you don't have time to refactor old code, see the discussion in https://stackoverflow.com/a/39914235/626853. If your user's browsers are sufficiently modern, you can use async/await:

$(document).on("click", ".clicker", async function() {
    var promise = getBase64(my_pdf_file);
    var my_pdf_file_as_base64 = await promise;
}

(Note that await only works inside async functions, so your click handler must be async. I also experimented with adding a busy waiting loop, but that caused my browser to hang.)