getVideoDuration.js

const fs = require("fs").promises;

const buff = Buffer.alloc(100);
const header = Buffer.from("mvhd");

async function length() {
    const file = await fs.open("video.mp4", "r");
    const { buffer } = await file.read(buff, 0, 100, 0);

    await file.close();

    const start = buffer.indexOf(header) + 17;
    const timeScale = buffer.readUInt32BE(start);
    const duration = buffer.readUInt32BE(start + 4);

    const audioLength = Math.floor((duration / timeScale) * 1000) / 1000;

    console.log(buffer, header, start, timeScale, duration, audioLength);
}

length();