karatelabs/karate · error · TypeError

Reflect.apply: target is not callable

Error message

Reflect.apply: target is not callable

What it means

Reflect.apply(target, thisArgument, argumentsList) requires the first argument to be a callable function (spec §28.1.1). Karate's JsReflect throws this TypeError when args is empty or args[0] is not a JsCallable — e.g. a number, string, object, or undefined was passed as the target.

Solutions

1. Check the first argument is a function before calling: `typeof fn === 'function'`
2. Fix typos/undefined variables in the target position
3. Use Function.prototype.call/bind or `fn(...args)` only on confirmed callables

Example fix

// before
Reflect.apply(handlers[name], this, args); // handlers[name] may be undefined
// after
if (typeof handlers[name] !== 'function') throw new Error('no handler: ' + name);
Reflect.apply(handlers[name], this, args);

if (typeof target !== 'function') throw new Error('Reflect.apply requires a callable target, got: ' + target);

const isCallable = v => typeof v === 'function';
if (!isCallable(target)) throw new Error('not callable: ' + target);

try {
  return Reflect.apply(target, thisArg, args);
} catch (e) {
  if (String(e.message).includes('not callable')) {
    throw new Error('Reflect.apply target is not a function: ' + String(target));
  }
  throw e;
}

When it happens

Trigger: Reflect.apply() with no arguments; Reflect.apply(fnName, ...) where fnName is undefined due to a typo or failed import; passing a non-callable like an object, string, or number as the first argument.

Common situations: Dynamic function lookup that returns undefined at runtime; refactoring that renamed a function but missed a Reflect.apply call site; passing `this` or a value from a lookup table that holds non-function members.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• argumentsList must be an iterable object
• Cannot set property ' ' which has only a getter
• assignment to constant
• assignment to constant
• Generator is already running

AI-assisted analysis of karatelabs/karate@a22eb90246 (2026-09-12). Data as JSON: /api/errors/963a641e10bfa9b6. Report an issue: GitHub.

Appendix: source

Thrown at karate-js/src/main/java/io/karatelabs/js/JsReflect.java:101

        if (args.length >= 3) {
            if (!(args[2] instanceof JsCallable nt)) {
                throw JsErrorException.typeError("Reflect.construct: newTarget is not a constructor");
            }
            check = nt;
        }
        if (!check.isConstructable()) {
            throw JsErrorException.typeError("Reflect.construct: target is not a constructor");
        }
        if (!(context instanceof CoreContext cc)) {
            throw JsErrorException.typeError("Reflect.construct: bad context");
        }
        return Interpreter.constructFromHost(target, cArgs, cc);
    }

    // Reflect.apply(target, thisArgument, argumentsList) — spec §28.1.1.
    private Object apply(Context context, Object[] args) {
        if (args.length < 1 || !(args[0] instanceof JsCallable target)) {
            throw JsErrorException.typeError("Reflect.apply: target is not callable");
        }
        Object thisArg = args.length >= 2 ? args[1] : Terms.UNDEFINED;
        Object[] cArgs = listToArray(args.length >= 3 ? args[2] : null);
        if (context instanceof CoreContext cc) {
            cc.thisObject = thisArg;
        }
        return target.call(context, cArgs);
    }

    private static Object[] listToArray(Object value) {
        if (value == null || value == Terms.UNDEFINED) {
            return new Object[0];
        }
        if (value instanceof List<?> list) {
            return list.toArray();
        }
        if (value instanceof Object[] arr) {
            return arr;

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