Read a file synchronously in Javascript

You can use the standard FileReaderSync, which is a simpler, synchronous, blocking version of the FileReader API, similar to what you are already using:

let reader = new FileReaderSync();
let result_base64 = reader.readAsDataURL(file); 

console.log(result_base64); // aGV5IHRoZXJl...

Keep in mind though that this is only available in worker threads, for obvious reasons.

---

If you need a solution for the main thread that "reads like" a synchronous API, i.e. sequentially, you can wrap the async FileReader in a promise and use async functions (you might need to transpile):

async function readFileAsDataURL(file) {
    let result_base64 = await new Promise((resolve) => {
        let fileReader = new FileReader();
        fileReader.onload = (e) => resolve(fileReader.result);
        fileReader.readAsDataURL(file);
    });

    console.log(result_base64); // aGV5IHRoZXJl...

    return result_base64;
}

And then you can either await this function in another async context:

async function main() {
    let file = new File(...)
    let dataURL = await readFileAsDataURL(file)
    console.log(dataURL); // aGV5IHRoZXJl...
}

... or just consume it using promise callbacks (doesn't need an async context):

readFileAsDataURL(file).then(dataURL => {
    console.log(dataURL); // aGV5IHRoZXJl...
});

Synchronous tasks (blocking) are generally bad. If there is no real reason to do that synchronously, I strongly recommend you to use the event callback.

Imagine your file is broken and the HTML5 api cant read, it wont give you the result. It would break your code and block the site. Or, someone could select a 10GB file, which would freeze your HTML page until the file is completely loaded. With that asynchronous event handler you are able to catch possible errors.

To work around limitations with callbacks, i use a simple trick:

var ready = false;
var result = '';

var check = function() {
    if (ready === true) {
         // do what you want with the result variable
         return;
    }
    setTimeout(check, 1000);
}

check();

var reader = new FileReader();
reader.onloadend = function(evt) {
    // file is loaded
    result = evt.target.result;
    
    ready = true;
};
reader.readAsDataURL(file);

the check function, checks every second if the ready flag variable is set to true. If so, you can be sure the result is available.

It may not be best practice to do so, but i made a webapp using this technique about 30 times with more than 10 setTimeouts at the same time running, and experienced no problem until now.