What is the best way to store the result from a JavaScript FileReader into a variable for future reference?

Question

What is the best way to store the result from a JavaScript FileReader into a variable for future reference?

I am currently facing an issue uploading a local .json file to my web application. I have managed to display the file in the dev tools, but I am unable to make it accessible for further use. It seems like the problem lies in how I handle (or fail to handle) the asynchronous behavior of the file reader.

Everything is set up in an Angularjs (1.7) environment, which explains the syntax used in the snippet. The goal is to showcase the retrieved data on an openlayers map.

    this.jsonSelected = function(newFile) {
        let reader = new FileReader();
        let result = 'empty';
        reader.readAsText(newFile);
        reader.onload = function(e) {
            result = e.target.result;
            console.log('in onload', result);
            this.result = e.target.result;
        };
        console.log(this.result);
    };

    this.test = function() {
        console.log(this.file);
    }

I anticipated the code to log the file content twice. Once at "console.log ('in onload', result);" and another time at "console.log (this. result);". The first one functions as expected, however, the second one does not. Additionally, the order of logs in the console appears to be inverted, with "console.log (this. result)" showing up before the inload log, as shown in this console screenshot.

I have attempted various modifications, such as switching the names around and adjusting the references to "this.", yet no success. This leads me to believe that I am mishandling the asynchronous data. The details in the screenshot, particularly the timing of the log lines, hint at a timing issue.

javascript angularjs file gis openlayers

Answer 1

Answer №1

After conducting additional research and experimenting with different approaches, I was able to resolve the issue at hand. The code has been modified as follows:

function readFile(newFile) {
    return $q(function(resolve, reject) {
        let reader = new FileReader();
        reader.onload = function(event) {
            resolve(event.target.result);
        };
        reader.onerror = function(event) {
            reject(event);
        };
        reader.readAsText(newFile);
    });
}

this.jsonSelected = function(newFile) {
    readFile(newFile).then(function(data) {
        console.log(data);
        set$scopeFile(data);
    }, function(errData) {

    });
};

Special thanks to Ben for guiding me in the right direction. I was too focused on tackling the asynchronous aspects that I overlooked the scoping issues. If anyone has any reliable resources or documentation on asynchronous JavaScript, I would greatly appreciate it as I continue to struggle with grasping this topic.

I utilized AngularJS's built-in $q service to encapsulate the entire reading process into a promise, which can then be chained with .then.

Answer 2

After conducting additional research and experimenting with different approaches, I was able to resolve the issue at hand. The code has been modified as follows:

function readFile(newFile) {
    return $q(function(resolve, reject) {
        let reader = new FileReader();
        reader.onload = function(event) {
            resolve(event.target.result);
        };
        reader.onerror = function(event) {
            reject(event);
        };
        reader.readAsText(newFile);
    });
}

this.jsonSelected = function(newFile) {
    readFile(newFile).then(function(data) {
        console.log(data);
        set$scopeFile(data);
    }, function(errData) {

    });
};

Special thanks to Ben for guiding me in the right direction. I was too focused on tackling the asynchronous aspects that I overlooked the scoping issues. If anyone has any reliable resources or documentation on asynchronous JavaScript, I would greatly appreciate it as I continue to struggle with grasping this topic.

I utilized AngularJS's built-in $q service to encapsulate the entire reading process into a promise, which can then be chained with .then.

Answer 3

Answer №2

This is a common issue with asynchronous functions and scoping...

The problem arises when the scope is not set correctly. The this inside the reader.onload function does not refer to the same object as in the console.log(this.result); statement. To fix this, you can either use the bind method or switch to arrow functions.

Additionally, since the onload function is asynchronous, it will execute after the console.log(this.result); call. One way to handle this is by using Promises (such as using $q in AngularJS). Here's an example:

const deferred = $q.defer(); 
deferred.then((data) => { console.log('result', data); });
reader.onload = function(e) {
     result = e.target.result;
     console.log('in onload', result);
     deferred.promise.resolve(e.target.result);
};

Answer 4

This is a common issue with asynchronous functions and scoping...

The problem arises when the scope is not set correctly. The this inside the reader.onload function does not refer to the same object as in the console.log(this.result); statement. To fix this, you can either use the bind method or switch to arrow functions.

Additionally, since the onload function is asynchronous, it will execute after the console.log(this.result); call. One way to handle this is by using Promises (such as using $q in AngularJS). Here's an example:

const deferred = $q.defer(); 
deferred.then((data) => { console.log('result', data); });
reader.onload = function(e) {
     result = e.target.result;
     console.log('in onload', result);
     deferred.promise.resolve(e.target.result);
};

What is the best way to store the result from a JavaScript FileReader into a variable for future reference?

Answer №1

Answer №2

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