CODEWITHSUNDEEP
node.js
Neatoro
Neatoro

Reputation: 173

NodeJS writing binary data to file without overwriting

I am currently trying to write a binary file using NodeJS.

I have now the issue that I have no clue how to insert data between two bytes without overwriting the following bytes.

Example

Given: 04 42 55 43 48 04

I know want to insert 55 between 43 and 48.

Expected result: 04 42 55 43 55 48 04

Actual result: 04 42 55 43 55 04

Code

I am using the following code to write to my file:

fs.write(fd, 0x55, 4, (err) => {
    if (err) throw err;
    fs.close(fd);
});

Upvotes: 3

Views: 446

Answers (1)

ponury-kostek
ponury-kostek

Reputation: 8060

I couldn't find any solution so I wrote it my self

function insert_data(fd, data, position, cb) {
    // ensure data i Buffer
    if (!Buffer.isBuffer(data)) {
        data = Buffer.from(data);
    }
    // get file size
    fs.fstat(fd, (err, stat) => {
        if (err) {
            return cb(err);
        }
        // calculate size for following bytes in file and allocate
        const buffer_size = stat.size > position ? stat.size - position : 0;
        const buffer = Buffer.alloc(buffer_size);
        // read bytes to by written after inserted data
        fs.read(fd, buffer, 0, buffer_size, position, (err, bytesRead, buffer) => {
            if (err) {
                return cb(err);
            }
            // concatenate new buffer containing data to be inserted and remaining bytes 
            const new_buffer = Buffer.concat([
                data,
                buffer
            ], data.length + buffer_size);
            // write new buffer starting from given position
            fs.write(fd, new_buffer, 0, new_buffer.length, position, (err) => {
                if (err) {
                    return cb(err);
                }
                fs.close(fd, cb);
            });
        });
    });
}

fs.open("test.bin", "r+", (err, fd) => {
    insert_data(fd, [0x55], 4, console.dir);
});

Upvotes: 1

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