How to insert text every few lines from a huge file

Question

I have an ASCII file about 1GB, my task here is to write a fixed text every 1000 lines.

e.g.

Expected Output

text line 1
text line 2
text line 3
...
text line 999
text line 1000
// My fixed text
text line 1001
text line 1002

Attemp 1 - JavaScript heap out of memory

Trying to write one byte at a time, and keep track number of newlines.

Write my fixed text every 1000 lines

let src = `/path/to/src`;
let dst = `/path/to/dst`;

let readStream = fs.createReadStream(src);
let writeStream = fs.createWriteStream(dst);
let newline = 0;

readStream.on('data', chunk => {
    for(let i in chunk) {
        let c = chunk[i];

        // write one byte at a time 
        writeStream.write(Buffer.from(chunk.buffer, i, 1))

        if(c == 10) {
            newline++;

            // write fixed text for every 1000th line
            if(newline && newline % 1000 == 0) {
                writeStream.write("My Fixed Text");
            }
        }
    }
}).on('end', () => {
    console.log('done');
});

FATAL ERROR: Ineffective mark-compacts near heap limit Allocation failed - JavaScript heap out of memory

Attemp 2 - Output file scrambled and out of order

Same as attemp 1, but writing one byte at a time from my pre-allocated buffer.

let src = `/path/to/src`;
let dst = `/path/to/dst`;

let readStream = fs.createReadStream(src);
let writeStream = fs.createWriteStream(dst);
let newline = 0;
let buffer = Buffer.alloc(1);

readStream.on('data', chunk => {
    for(let i in chunk) {
        let c = chunk[i];

        buffer[0] = c;

        // write one byte at a time from my pre-allocated buffer
        writeStream.write(buffer)

        if(c == 10) {
            newline++;

            // write fixed text for every 1000th line
            if(newline && newline % 1000 == 0) {
                writeStream.write("My Fixed Text");
            }
        }
    }
}).on('end', () => {
    console.log('done');
});

No error, the script took a while to exit after done is printed

However, the output file looks completely scrambled and out of order

L,a'-etLL'7P62 ,,NL,n''he'L A,r'0et,L ,ei2'vrUb32oa0on' L,c27CirU 7' 2- tN Ng7Bha'0atLV7al'0e'pUbLe2,3 goLA0n'0,v',L9lc'1C,LI0Et0-m',UT5,nD1e''NU,On91arNE40r20ilHLVEn''''''''mwRL (3(0-r''NUB o00at S7Eo'7'clUR32 20vowL 64v80vaL 0,u00hNNUE4n,1'aa L 2 AC1p,,NU e2'Cit, 22y F8etLL56Ola-1C'NLS3n '3mrUS34nn1-CL,g5tw7ma'L)'4t60h'L

This task sounds easy and straightforward, but I'm having trouble to do this.

Any help is appreciated.