karatelabs/karate · error · TypeError

Set-like object's 'keys' is not callable

Error message

Set-like object's 'keys' is not callable

What it means

This TypeError is thrown when code passes a Set-like object (to Set APIs such as union/intersection/difference) whose 'keys' property is not a callable function. The engine validates the Set-like protocol: a Set-like must expose a numeric 'size' and callable 'has' and 'keys' methods. If 'keys' is missing or non-callable, the object cannot be iterated as a Set record.

Solutions

1. Add a callable keys() method to your Set-like object that returns an iterator over its elements
2. Check the object's shape before passing it: typeof obj.keys === 'function'
3. Use a real Set instance instead of a custom Set-like if iteration support is not needed

Example fix

// before
const setLike = { size: 2, has: v => true };
new Set(setLike); // TypeError
// after
const setLike = { size: 2, has: v => true, keys: function* () { yield 1; yield 2; } };
new Set(setLike);

function isSetLike(o) { return o !== null && typeof o === 'object' && typeof o.has === 'function' && typeof o.keys === 'function' && !isNaN(Number(o.size)); }

const isSetLike = (o) => o instanceof Set || (typeof o === 'object' && o !== null && typeof o.keys === 'function' && typeof o.has === 'function');

try { return new Set(candidate); } catch (e) { if (String(e).includes('keys')) throw new TypeError('Set-like missing keys(): ' + candidate); throw e; }

When it happens

Trigger: Passing an object with 'size' and 'has' but no 'keys', or with keys set to a non-function value, into a Set method that accepts Set-like arguments (e.g. new Set(setLike), set.union(obj), set.symmetricDifference(obj)) via getSetRecord.

Common situations: Hand-rolled Set-like objects missing one method; objects ported from another environment where 'keys' was removed or renamed; typos like 'key' instead of 'keys'; objects built dynamically where the method failed to attach.

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

• Set.prototype method called with non-object
• toBytes() argument must be a list of numbers, got
• toBytes() list must contain only numbers, got
• xmlPath() first argument must be XML node or string, but was
• NoSuchElementException

AI-assisted analysis of karatelabs/karate@a22eb90246 (2026-09-12). Data as JSON: /api/errors/74bb3604f22d9860. Report an issue: GitHub.

Appendix: source

Thrown at karate-js/src/main/java/io/karatelabs/js/JsSetPrototype.java:185

            throw JsErrorException.typeError("Set-like object's size is not a number");
        }
        // ToIntegerOrInfinity, then RangeError on negative — spec step 7.
        long intSize;
        if (Double.isInfinite(d)) {
            intSize = d > 0 ? Long.MAX_VALUE : Long.MIN_VALUE;
        } else {
            intSize = (long) d;
        }
        if (intSize < 0) {
            throw JsErrorException.rangeError("Set-like object's size is negative");
        }
        Object hasFn = obj.getMember("has", obj, cc);
        if (!(hasFn instanceof JsCallable has)) {
            throw JsErrorException.typeError("Set-like object's 'has' is not callable");
        }
        Object keysFn = obj.getMember("keys", obj, cc);
        if (!(keysFn instanceof JsCallable keys)) {
            throw JsErrorException.typeError("Set-like object's 'keys' is not callable");
        }
        return new SetRecord(obj, intSize, has, keys);
    }

    private static JsIterator keysIter(SetRecord rec, Context ctx) {
        return IterUtils.iteratorFromCallable(rec.keys, rec.setObj, ctx);
    }

    /** Spec normalize: -0 → +0 on values pulled from a foreign keys() iteration. */
    private static Object normalize(Object v) {
        return JsMap.normalizeKey(v);
    }

    /** Direct-populate a JsSet, bypassing Set.prototype.add per spec. */
    private static void rawAdd(JsSet s, Object value) {
        Object normalized = normalize(value);
        // Linear-scan match for cross-Java-numeric-type SameValueZero, mirroring
        // JsSet.has's findStoredValue logic.

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