JavaScript code to read unicode text from a file on the browser side

read-text-from-file.js

function readTextFromFile(file, maxMegabytes = 1) {
    const reader = new FileReader();
    return new Promise(resolve => {
    
        // check file size
        if (!file || !file.size) {
            resolve({ error: `The file cannot be empty` });
            return;
        }
        if (file.size > maxMegabytes * 0x100000) {
            resolve({ error: `The file size should not exceed ${maxMegabytes} MB` });
            return;
        }
        const filename = file.name;
        
        reader.onerror = e => {
             resolve({ error: 'Cannot read a file from a specified location' });
        };

        reader.onload = e => {
            try {
                const data = new Uint8Array(e.target.result);
                
                // https://en.wikipedia.org/wiki/Byte_order_mark
                let encoding = 'utf-8';
                let checkMojibakes = false;
                if (data[0] == 0xFE && data[1] == 0xFF) {
                    encoding = 'utf-16be';
                } else if (data[0] == 0xFF && data[1] == 0xFE) {
                    encoding = 'utf-16';
                } else {
                    checkMojibakes = data[0] != 0xEF || data[1] != 0xBB || data[2] != 0xBF;
                }
                
                // parse and remove null terminators
                let text = new TextDecoder(encoding).decode(data);
                text = text.replace(/\0+$/, '');
                
                // check invalid text
                if (text.indexOf('\0') >= 0 ||
                    checkMojibakes && text.indexOf('\uFFFD') >= 0) {
                    resolve({ error: 'The file must use UTF-8 or UTF-16 encoding' });
                } else {
                    resolve({ text, filename });
                }
            } catch(e) {
                resolve({ error: e.toString() });
            }
        };
        try {
            reader.readAsArrayBuffer(file);
        } catch(e) {
            resolve({ error: e.toString() });
        }
    });
}