This feature is no longer recommended. Though some browsers might still support it, it may have already been removed from the relevant web standards, may be in the process of being dropped, or may only be kept for compatibility purposes. Avoid using it, and update existing code if possible; see the compatibility table at the bottom of this page to guide your decision. Be aware that this feature may cease to work at any time.

Note: This method is deprecated in favor of readAsArrayBuffer().

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




The DOM File or Blob to read.

Return value

An DOMString representing the input data.


The following exceptions can be raised by this method:

is raised when the resource represented by the DOM File or Blob cannot be found, e.g. because it has been erased.
is raised when 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).
is raised when the resource cannot be read due to a permission problem, like a concurrent lock.
is raised when the resource is a data URL and exceed the limit length defined by each browser.


Specification Status Comment
File API
The definition of 'readAsBinaryStringSync' in that specification.
Working Draft Initial definition.

Browser compatibility

BCD tables only load in the browser

See also