OrchardCMS/OrchardCore · error · TypeError

Cannot call a class as a function

Error message

Cannot call a class as a function

What it means

Popper's babel-compiled class guard throws this TypeError when a constructor (here Popper itself, via classCallCheck) is invoked without `new`, so `instance instanceof Constructor` is false. The ES5 transpilation of ES6 classes cannot distinguish a plain function call from construction without this runtime check.

Solutions

1. Invoke the constructor with `new`: new Popper(referenceElement, popperElement, options).
2. If you only need positioning for Bootstrap tooltips/dropdowns, use bootstrap's bundled Popper and remove manual Popper calls.
3. Verify the imported symbol is the Popper constructor (default export of popper.js), not a helper or utils build.
4. Consider upgrading to Popper 2 (@popperjs/core), which uses native ES classes and reports `Class constructor cannot be invoked without 'new'` more clearly.

Example fix

// before
var popper = Popper(referenceEl, tooltipEl, { placement: 'top' });

// after
var popper = new Popper(referenceEl, tooltipEl, { placement: 'top' });

function createPopper(ref, popperEl, options) {
  if (typeof Popper !== 'function') throw new Error('Popper not loaded');
  return new Popper(ref, popperEl, options); // always construct with new
}

function isPopperInstance(x) { return typeof Popper === 'function' && x instanceof Popper; }

let instance;
try {
  instance = new Popper(refEl, popEl, { placement: 'bottom' });
} catch (e) {
  if (e instanceof TypeError && /class as a function/.test(e.message)) {
    console.error('Popper must be called with new; check import/build');
  } else throw e;
}

When it happens

Trigger: Calling Popper(reference, popper, options) without `new`; passing a bundled/re-exported Popper around in a way that loses construction context; calling an internal compiled class as a function after minification/concatenation mistakes.

Common situations: Old tutorial snippets or scripts using Popper 0.x/1.x API without `new`; mixing Popper builds (popper.js vs popper-utils, UMD vs ES) so the wrong export is invoked as a function; Bootstrap wiring where the Popper reference was overwritten.

Related errors

• is missing the required "orchardBaseUrl" field.
• could not find global
• 'EventSource' is not supported in your environment.
• Failed to load
• HttpConnection.stopConnection

AI-assisted analysis of OrchardCMS/OrchardCore@4306c0717f (2026-09-13). Data as JSON: /api/errors/750c991368116e96. Report an issue: GitHub.

Appendix: source

Thrown at src/OrchardCore.Modules/OrchardCore.Resources/wwwroot/Vendor/popper-1.16.1/popper.js:375

function getSize(axis, body, html, computedStyle) {
  return Math.max(body['offset' + axis], body['scroll' + axis], html['client' + axis], html['offset' + axis], html['scroll' + axis], isIE(10) ? parseInt(html['offset' + axis]) + parseInt(computedStyle['margin' + (axis === 'Height' ? 'Top' : 'Left')]) + parseInt(computedStyle['margin' + (axis === 'Height' ? 'Bottom' : 'Right')]) : 0);
}

function getWindowSizes(document) {
  var body = document.body;
  var html = document.documentElement;
  var computedStyle = isIE(10) && getComputedStyle(html);

  return {
    height: getSize('Height', body, html, computedStyle),
    width: getSize('Width', body, html, computedStyle)
  };
}

var classCallCheck = function (instance, Constructor) {
  if (!(instance instanceof Constructor)) {
    throw new TypeError("Cannot call a class as a function");
  }
};

var createClass = function () {
  function defineProperties(target, props) {
    for (var i = 0; i < props.length; i++) {
      var descriptor = props[i];
      descriptor.enumerable = descriptor.enumerable || false;
      descriptor.configurable = true;
      if ("value" in descriptor) descriptor.writable = true;
      Object.defineProperty(target, descriptor.key, descriptor);
    }
  }

  return function (Constructor, protoProps, staticProps) {
    if (protoProps) defineProperties(Constructor.prototype, protoProps);
    if (staticProps) defineProperties(Constructor, staticProps);
    return Constructor;

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