FileReaderSync: readAsText() method

Note: This feature is only available in Web Workers, except for Service Workers.

The readAsText() method of the FileReaderSync interface allows to read File or Blob objects in a synchronous way into a string. This interface is only available in workers as it enables synchronous I/O that could potentially block.


readAsText(blob, encoding)



The File or Blob to read.

encoding Optional

The optional parameter specifies encoding to be used (e.g., iso-8859-1 or UTF-8). If not present, the method will apply a detection algorithm for it.

Return value

A string representing the input data.


The following exceptions can be raised by this method:

NotFoundError DOMException

Thrown if the resource represented by the DOM File or Blob cannot be found, e.g. because it has been erased.

SecurityError DOMException

Thrown if one of the following problematic situation is detected:

  • the resource has been modified by a third party;
  • too many read are performed simultaneously;
  • the file pointed by the resource is unsafe for a use from the Web (like it is a system file).
NotReadableError DOMException

Thrown if the resource cannot be read due to a permission problem, like a concurrent lock.

EncodingError DOMException

Thrown if the resource is a data URL and exceed the limit length defined by each browser.


File API
# readAsTextSync

Browser compatibility

BCD tables only load in the browser

See also