get-video-mime/lib/detect-mime.js

A simple CLI tool to get a video's MIME string including relevant codes for use with MediaSource Extension.

/**
 * Define a simple Transform stream to process each chunk of the video
 * and try to detect the MIME
 */
const { Transform } = require('stream');
const MP4Box = require('mp4box');

class DetectMIME extends Transform {

    constructor(setMime) {
        super();

        this.mime = '';
        this.mp4boxfile = MP4Box.createFile();
        this.chuckStart = 0;

        // If MP4Box emits an error pass it down the stream
        this.mp4boxfile.onError = (error) => { this.emit(error); };

        // When MP4Box has enough to parse the file, grab the MIME
        this.mp4boxfile.onReady = (info) => {

            if (! info || ! info.mime) {

                this.emit(new Error('Non MIME returned from MP4Box.js'));
                return;
            }

            this.mime = info.mime;

            // MP4Box.js reports opus audio as Opus in the codec string and Firefox doesn't like that
            // and it says the mime type is not supported. We'll special case this and lower case opus.
            this.mime = this.mime.replace('Opus', 'opus');

            setMime(this.mime);
        };
    }

    _transform(chunk, encoding, next) {

        // No need to process if we already have the MIME
        if (this.mime) {

            next();
            return;
        }

        // Convert the Buffer into an ArrayBuffer
        // https://stackoverflow.com/a/31394257/931860
        const arrayBuffer = chunk.buffer.slice(chunk.byteOffset, chunk.byteOffset + chunk.byteLength);

        // This is required for MP4Box.js
        arrayBuffer.fileStart = this.chuckStart;

        // Add this chunk to MP4Box.js
        this.mp4boxfile.appendBuffer(arrayBuffer);

        // Increment the chunkStart
        this.chuckStart += chunk.byteLength;

        next();
    }

    _flush(next) {

        // When we're done, output the detected MIME
        this.push(this.mime);

        // Signal that we are done processing
        next();
    }
}

module.exports = DetectMIME;