I need to read a text file from a javascript

javascript, xmlhttprequest

Solution

Use an asynchronous request (or `fetch`, see below, which is also asynchronous):

function doGET(path, callback) {
    var xhr = new XMLHttpRequest();
    xhr.onreadystatechange = function() {
        if (xhr.readyState == 4) {
            // The request is done; did it work?
            if (xhr.status == 200) {
                // ***Yes, use `xhr.responseText` here***
                callback(xhr.responseText);
            } else {
                // ***No, tell the callback the call failed***
                callback(null);
            }
        }
    };
    xhr.open("GET", path);
    xhr.send();
}

function handleFileData(fileData) {
    if (!fileData) {
        // Show error
        return;
    }
    // Use the file data
}

// Do the request
doGET("/path/to/file", handleFileData);

Or using promises, which are the more modern way to handle callbacks (but keep reading):

function doGET(path, callback) {
    return new Promise(function(resolve, reject) {
        var xhr = new XMLHttpRequest();
        xhr.onreadystatechange = function() {
            if (xhr.readyState == 4) {
                // The request is done; did it work?
                if (xhr.status == 200) {
                    // Yes, use `xhr.responseText` to resolve the promise
                    resolve(xhr.responseText);
                } else {
                    // No, reject the promise
                    reject(xhr);
                }
             }
        };
        xhr.open("GET", path);
        xhr.send();
    });
}

// Do the request
doGET("/path/to/file")
    .then(function(fileData) {
        // Use the file data
    })
    .catch(function(xhr) {
        // The call failed, look at `xhr` for details
    });

Here in 2019, there's no reason to use XHR wrapped in a promise like that, just use `fetch`:

function doGET(url) {
    return fetch(url).then(response => {
        if (!response.ok) {
            throw new Error("HTTP error " + response.status); // Rejects the promise
        }
    });
}

Problem

I'm writing webpage with a javascript to read data files in text format from the server per user request. Once the text file has been loaded, I need to manipulate the data somewhat. I have been using XMLHttpRequest for the loading, however, now I see that synchronous requests are "deprecated". I can't start manipulating the data before it's loaded, so what can I do in this case?

Original source