class cfilewriter{    
    constructor(name){
        this.tempname = this.uuidv4() + '.bin';
        this.stream = null;
        this.name = name;
        fs.unlink(this.name);        
        this.stream = fs.createWriteStream(this.tempname, {'flags':'a'});        
    }
    write(dt){        
        this.stream.write(dt, (err, data) => {
            // finish renaming file
            if(err) throw err;
            alert("write ok !");
            this.close();
        });
    }
    uuidv4() {
        return "10000000-1000-4000-8000-100000000000".replace(/[018]/g, c =>
          (+c ^ crypto.getRandomValues(new Uint8Array(1))[0] & 15 >> +c / 4).toString(16)
        );
    }
    close(){                
        this.stream.end(()=>{
            // finish closing file
            console.log("finish close file");
            fs.rename(this.tempname, this.name, () => {
                // finish renaming file
                console.log("complete save");
            });
        });
    }
}

var gfile = null;
function save2file(blob){
    var reader = new FileReader();  
    var name = blob.name;    
    reader.onload = function () { // here we save variable 'file' in closure      
        console.log(this.result);
        var arrayBuffer=this.result;
        const buffer = mbuffer.Buffer.from(arrayBuffer);
        if (gfile==null) gfile = new cfilewriter(name);
        gfile.write(buffer);
        //gfile.close();
        
    }     
    reader.readAsArrayBuffer(blob);
}

this code demo download large file (4gb) to stream .
but i dont know how i copy that stream to download folder.

detail:

1. function save2file , read blob only demo to write stream.
2. class cfilewwrite used to write stream, save to download.
3. this code work , but file dont save in download folder.

everyone help me, i dont have experience in javascript, nodejs. thanks !