karatelabs/karate · error · TypeError

Invalid value used in weak set

Error message

Invalid value used in weak set

What it means

WeakSet can only store values eligible for weak references — objects (and registered symbols). Karate's JsWeakSet.addValue checks JsWeakMap.canBeHeldWeakly(value) and throws a TypeError when add() receives a primitive, matching V8's 'Invalid value used in weak set'.

Solutions

1. Add only objects/functions: weakSet.add(obj).
2. For primitive values use a regular Set instead of WeakSet.
3. Guard before adding: if (value && typeof value === 'object') weakSet.add(value).
4. Wrap primitives in holder objects if weak semantics are genuinely needed: weakSet.add({ v: 42 }).

Example fix

// before
const seen = new WeakSet();
seen.add(id); // id is a string

// after
const seen = new Set();
seen.add(id);

function isWeakValue(v) { return (typeof v === 'object' && v !== null) || typeof v === 'function'; }
if (!isWeakValue(value)) throw new Error('WeakSet values must be objects');
ws.add(value);

function isWeakValue(v) { return (typeof v === 'object' && v !== null) || typeof v === 'function'; }

When it happens

Trigger: weakSet.add(42), weakSet.add('str'), weakSet.add(null), weakSet.add(undefined), or weakSet.add(Symbol('local')) — any non-object, non-function value passed to add().

Common situations: Deduplicating IDs that are strings/numbers rather than objects; adding array elements from JSON-parsed data (all primitives); variables that are unexpectedly undefined at call time.

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

• Constructor WeakSet requires 'new'
• WeakSet.prototype.add is not callable
• Method WeakSet.prototype called on incompatible receiver
• Cannot set property ' ' which has only a getter
• assignment to constant

AI-assisted analysis of karatelabs/karate@a22eb90246 (2026-09-12). Data as JSON: /api/errors/587d5c7874e210e4. Report an issue: GitHub.

Appendix: source

Thrown at karate-js/src/main/java/io/karatelabs/js/JsWeakSet.java:49

 * identity-keyed.
 */
class JsWeakSet extends JsObject {

    private static final Object PRESENT = new Object();

    private final IdentityHashMap<Object, Object> elements = new IdentityHashMap<>();

    JsWeakSet() {
        super(null, JsWeakSetPrototype.INSTANCE);
    }

    boolean has(Object value) {
        return JsWeakMap.canBeHeldWeakly(value) && elements.containsKey(value);
    }

    void addValue(Object value) {
        if (!JsWeakMap.canBeHeldWeakly(value)) {
            throw JsErrorException.typeError("Invalid value used in weak set");
        }
        elements.put(value, PRESENT);
    }

    boolean deleteValue(Object value) {
        if (!has(value)) {
            return false;
        }
        elements.remove(value);
        return true;
    }

}

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