Async APIs for accessing files from different sources

chrome-file-system.js

// Chrome extensions/apps API
// Uses Callback #3: (data => { let err = chrome.runtime.lastError; ... }) for Chrome API
// Uses Callback #2: (data => { ... }, err => { ... }) for FileSystem API (also Chrome-specific!)
// Uses Events onload and onerror for File API (HTML5)
// All implemented within one vendor for different APIs (sic!)

chrome.fileSystem.chooseEntry({ type: 'openFile' }, fileEntry => {
    if (chrome.runtime.lastError) {
        console.error('Error in file open dialog:', chrome.runtime.lastError);
    } else {
        fileEntry.file(file => {
            let reader = new FileReader();
            reader.onload = () => console.log(reader.result);
            reader.onerror = () => console.error('Error during file reading:', reader.error);
            reader.readAsText(file);
        }, error => console.error('Error while retrieving a File instance:', error));
    }
});

html5-file-api.js

// HTML5 File API
// Events: onload + onerror

document.getElementById('fileInput').addEventListener('change', event => {
    let file = event.target.files[0];
    let reader = new FileReader();
    reader.onload = () => console.log(reader.result);
    reader.onerror = () => console.error(reader.error);
    reader.readAsText(file);
});

node.js

// Node.js
// Callbacks #1: ((err, data) => { ... })

const { readFile } = require('fs');

fs.readFile('file.txt', 'utf-8', (err, contents) => {
    if (err) {
        console.error(err);
    } else {
        console.log(contents);
    }
});

request-file-system.js

// HTML5 requestFileSystem proposal (turned out to be Chrome-only, but widely used)
// Callback #2: (data => { ... }, err => { ... }) + Events onload and onerror for File API

requestFileSystem(PERSISTENT, 1024, fileSystem => {
    fileSystem.root.getFile('file.txt', fileEntry => {
        fileEntry.file(file => {
            let reader = new FileReader();
            reader.onload = () => console.log(reader.result);
            reader.onerror = () => console.error(reader.error);
            reader.readAsText(file);
        }, error => console.error(error));
    }, error => console.error(error));
}, error => console.error(error));