parallax/jsPDF · error · TypeError

Failed to execute '" + kind + "' on 'FileReader': parameter

Error message

Failed to execute '" + kind + "' on 'FileReader': parameter 1 is not of type 'Blob'.

What it means

Thrown by the Blob.js polyfill's internal _read helper (used by readAsText/readAsArrayBuffer/readAsDataURL/readAsBinaryString) when the first argument is not an instance of Blob. The polyfill type-checks before scheduling the async read to fail fast and match the native DOM TypeError message format.

Source

Thrown at src/libs/Blob.js:332

  function FileReader() {
    if (!(this instanceof FileReader))
      throw new TypeError(
        "Failed to construct 'FileReader': Please use the 'new' operator, this DOM object constructor cannot be called as a function."
      );

    var delegate = document.createDocumentFragment();
    this.addEventListener = delegate.addEventListener;
    this.dispatchEvent = function(evt) {
      var local = this["on" + evt.type];
      if (typeof local === "function") local(evt);
      delegate.dispatchEvent(evt);
    };
    this.removeEventListener = delegate.removeEventListener;
  }

  function _read(fr, blob, kind) {
    if (!(blob instanceof Blob))
      throw new TypeError(
        "Failed to execute '" +
          kind +
          "' on 'FileReader': parameter 1 is not of type 'Blob'."
      );

    fr.result = "";

    setTimeout(function() {
      this.readyState = FileReader.LOADING;
      fr.dispatchEvent(new Event("load"));
      fr.dispatchEvent(new Event("loadend"));
    });
  }

  FileReader.EMPTY = 0;
  FileReader.LOADING = 1;
  FileReader.DONE = 2;
  FileReader.prototype.error = null;

View on GitHub (pinned to a3930ce03a)

Solutions

1. Wrap the data in a Blob first: fr.readAsDataURL(new Blob([data])).
2. If you already have text, skip FileReader and use the string directly.
3. Type-check the argument before calling any read method.

Example fix

// before
fr.readAsText(rawString);
// after
fr.readAsText(new Blob([rawString]));

function isBlob(v){ return v instanceof Blob; }
if (isBlob(data)) fr.readAsDataURL(data); else fr.readAsDataURL(new Blob([data]));

function isBlob(v){ return typeof Blob !== 'undefined' && v instanceof Blob; }

try { fr.readAsText(data); } catch (e) { if (/is not of type 'Blob'/.test(e.message)) fr.readAsText(new Blob([data])); else throw e; }

When it happens

Trigger: Calling fr.readAsDataURL(stringOrObject) where the argument is a plain string, ArrayBuffer, or object instead of a Blob/File. Passing a base64 string directly; passing the wrong variable; calling read on already-extracted raw bytes.

Common situations: Forgetting to wrap bytes in a Blob before reading; confusing FileReader (which needs a Blob) with a raw byte consumer; legacy code that passed strings to a non-conforming shim.

Related errors

• Failed to construct 'FileReader': Please use the 'new' opera
• Line join style of '" + style + "' is not recognized. See or
• Invalid argument passed to jsPDF.setLineMiterLimit
• Invalid argument passed to jsPDF.addField.
• getTextDimensions expects text-parameter to be of type Strin

AI-assisted analysis of parallax/jsPDF@a3930ce03a (2026-08-13). Data as JSON: /api/errors/5ab08b81500060d8. Report an issue: GitHub.