alvarotrigo/fullPage.js · error · TypeError

predicate must be a function

Error message

predicate must be a function

What it means

Thrown by the Array.prototype.find polyfill (src/js/polyfills/array.find.js:17) at spec step 3, IsCallable(predicate). After the receiver is validated and length coerced, the polyfill checks typeof predicate !== 'function' and throws this message if the callback is not callable. The native method throws the same TypeError, so this polyfill path is only reached when Array.prototype.find is absent.

Source

Thrown at src/js/polyfills/array.find.js:17

// https://tc39.github.io/ecma262/#sec-array.prototype.find
if (!Array.prototype.find) {
    Object.defineProperty(Array.prototype, 'find', {
        value: function(predicate) {
            // 1. Let O be ? ToObject(this value).
            if (this == null) {
                throw new TypeError('"this" is null or not defined');
            }

            var o = Object(this);

            // 2. Let len be ? ToLength(? Get(O, "length")).
            var len = o.length >>> 0;

            // 3. If IsCallable(predicate) is false, throw a TypeError exception.
            if (typeof predicate !== 'function') {
                throw new TypeError('predicate must be a function');
            }

            // 4. If thisArg was supplied, let T be thisArg; else let T be undefined.
            var thisArg = arguments[1];

            // 5. Let k be 0.
            var k = 0;

            // 6. Repeat, while k < len
            while (k < len) {
                // a. Let Pk be ! ToString(k).
                // b. Let kValue be ? Get(O, Pk).
                // c. Let testResult be ToBoolean(? Call(predicate, T, « kValue, k, O »)).
                // d. If testResult is true, return kValue.
                var kValue = o[k];
                if (predicate.call(thisArg, kValue, k, o)) {
                    return kValue;
                }

View on GitHub (pinned to 49f15effa7)

Solutions

1. Pass a real function predicate: arr.find(item => item.active).
2. If the predicate comes from config or an optional argument, default it or guard before calling: arr.find(predicate || (() => false)).
3. Trace predicate back to its source and fix the assignment/import that produced a non-function.
4. Verify the polyfill is active only on target browsers; on modern browsers the native find throws the equivalent error, so the fix is the same.

Example fix

// before
const match = items.find('id');
// predicate must be a function

// after
const match = items.find(item => item.id === 'id');

function findWith(items, predicate) {
  if (!Array.isArray(items)) {
    throw new TypeError('expected an array');
  }
  if (typeof predicate !== 'function') {
    throw new TypeError('predicate must be a function, got ' + typeof predicate);
  }
  return items.find(predicate);
}

const isFunction = (v) => typeof v === 'function';
// usage: if (isFunction(predicate)) items.find(predicate);

When it happens

Trigger: Calling arr.find() with no arguments; passing a non-function such as arr.find(null), arr.find(undefined), arr.find('name'), arr.find({}), or arr.find(someVar) where someVar resolved to a non-function value.

Common situations: Typo in a method name passed as the predicate; conditional code that omits the callback; reading a callback from config that is undefined when a feature flag is off; passing a property name string instead of a comparison function.

Related errors

• Array.from: when provided, the second argument must be a fun
• "this" is null or not defined
• Array.from requires an array-like object - not null or undef
• Cannot convert undefined or null to object

AI-assisted analysis of alvarotrigo/fullPage.js@49f15effa7 (2026-08-13). Data as JSON: /api/errors/06b9cb85c248f0d8. Report an issue: GitHub.