karatelabs/karate · error · JsErrorException

toISOString is not a function

Error message

toISOString is not a function

What it means

Date.prototype.toJSON finishes by invoking the object's `toISOString` property. If the result of ToPrimitive is an object whose `toISOString` is missing or not callable, Karate throws this TypeError per spec step 4.b/c. It means the receiver does not actually behave like a Date.

Solutions

1. Ensure the object exposes a callable toISOString, or use a real Date instance.
2. Add toISOString to your mock/stub if you deliberately use Date-like objects in tests.
3. Check typeof obj.toISOString === 'function' before serializing.

Example fix

// before
const fake = Object.create(Date.prototype);
JSON.stringify({ d: fake }); // TypeError: toISOString is not a function
// after
const fake = Object.create(Date.prototype);
fake.toISOString = () => new Date().toISOString();

if (typeof (obj && obj.toISOString) !== 'function') throw new Error('receiver has no callable toISOString');

function hasToISOString(v) { return v !== null && typeof v === 'object' && typeof v.toISOString === 'function'; }

try { return JSON.stringify(obj); } catch (e) { if (e instanceof TypeError && /toISOString is not a function/.test(e.message)) return JSON.stringify(fallback(obj)); throw e; }

When it happens

Trigger: Date.prototype.toJSON.call({ toISOString: 42 }), .call({}) (after numeric coercion yields an object), or a plain object inheriting Date.prototype.toJSON without defining a callable toISOString.

Common situations: Objects created via Object.create(Date.prototype) or mixin patterns, mocked Date-like objects in tests lacking toISOString, and serialization code that assumes any date-ish object has the method.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• this is not a Date object
• invalid hint to Symbol.toPrimitive:
• Date.prototype[@@toPrimitive] called on non-object
• Cannot convert null or undefined to object
• toBytes() argument must be a list of numbers, got

AI-assisted analysis of karatelabs/karate@a22eb90246 (2026-09-12). Data as JSON: /api/errors/14b70ca3c633b32b. Report an issue: GitHub.

Appendix: source

Thrown at karate-js/src/main/java/io/karatelabs/js/JsDatePrototype.java:245

        // Invoke(O, "toISOString") path doing a redundant getMember dispatch.
        if (o instanceof JsDate d) {
            return formatIso(d);
        }
        // Invoke O.toISOString()
        if (o instanceof ObjectLike ol) {
            Object iso = ol.getMember("toISOString");
            if (iso instanceof JsCallable jc) {
                CoreContext sub = (cc == null) ? null : new CoreContext(cc, null, null);
                if (sub != null) sub.thisObject = o;
                Object r = jc.call(sub == null ? context : sub, new Object[0]);
                if (sub != null && sub.isError() && cc != null) {
                    cc.updateFrom(sub);
                    return null;
                }
                return r;
            }
        }
        throw JsErrorException.typeError("toISOString is not a function");
    }

    private Object getFullYear(Context context, Object[] args) {
        JsDate d = requireDate(context);
        if (d.isInvalid()) return Double.NaN;
        return localZdt(d).getYear();
    }

    private Object getYear(Context context, Object[] args) {
        // Annex B: getYear returns getFullYear() - 1900
        JsDate d = requireDate(context);
        if (d.isInvalid()) return Double.NaN;
        return localZdt(d).getYear() - 1900;
    }

    private Object getMonth(Context context, Object[] args) {
        JsDate d = requireDate(context);
        if (d.isInvalid()) return Double.NaN;

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