How to set bitmap with DataURL

```javascript
const setdpi = () => ({

setdpi (dpi, callback) {

if ( typeof dpi !== 'number' ) return throwError.call(this, '( dpi ) must be a number', callback);

this.getBase64Async(this._originalMime).then( b64 => {
const [ none, b64_dpi ] = changeDPI.changeDpiDataUrl(b64, dpi).split(',')
const b64_buffer = Buffer.from( b64_dpi, 'base64' )
this.bitmap.data = b64_buffer
})

return this

}

})
```
the code above does not work, so what is the problem