dianping/cat · error · TypeError

reduceRight of empty array with no initial value

Error message

reduceRight of empty array with no initial value

What it means

ES5-shim Array.prototype.reduceRight throws 'reduceRight of empty array with no initial value' when length is 0 and no initialValue argument was given — the right-to-left twin of reduce's rule that an accumulator must come from somewhere.

Source

Thrown at cat-home/src/main/webapp/assets/js/editor/worker-xquery.js:49147

                result = fun.call(void 0, result, self[i], i, object);
            }
        }

        return result;
    };
}
if (!Array.prototype.reduceRight) {
    Array.prototype.reduceRight = function reduceRight(fun /*, initial*/) {
        var object = toObject(this),
            self = splitString && _toString(this) == "[object String]" ?
                this.split("") :
                object,
            length = self.length >>> 0;
        if (_toString(fun) != "[object Function]") {
            throw new TypeError(fun + " is not a function");
        }
        if (!length && arguments.length == 1) {
            throw new TypeError("reduceRight of empty array with no initial value");
        }

        var result, i = length - 1;
        if (arguments.length >= 2) {
            result = arguments[1];
        } else {
            do {
                if (i in self) {
                    result = self[i--];
                    break;
                }
                if (--i < 0) {
                    throw new TypeError("reduceRight of empty array with no initial value");
                }
            } while (true);
        }

        do {

View on GitHub (pinned to e815e74d4c)

Solutions

1. Always supply the initial value: arr.reduceRight(fn, init).
2. Short-circuit empty inputs with a default result before calling reduceRight.
3. Initialize the accumulator consistently so first-iteration behavior is deterministic.

Example fix

// before
var composed = fns.reduceRight(compose2);

// after
var composed = fns.reduceRight(compose2, function(x) { return x; });

if (arr.length === 0 && arguments.length < 2) {
  return identity; // or default — avoids the empty-array TypeError
}
return arr.reduceRight(fn, initialValue);

try { return arr.reduceRight(fn); }
catch (e) {
  if (e instanceof TypeError && /no initial value/.test(e.message)) {
    return fallbackValue;
  }
  throw e;
}

When it happens

Trigger: [].reduceRight(fn); — empty array after a filter/map chain, a queue drained to zero, or data not yet arrived at reduce time.

Common situations: Reducing an empty filtered set; race where reduce runs before data loads; empty tail recursion helper arrays.

Related errors

• reduce of empty array with no initial value
• {fun} is not a function
• Object.getOwnPropertyDescriptor called on a non-object: {obj
• typeof prototype[{typeof prototype}] != 'object'
• Object.defineProperty called on non-object: {object}

AI-assisted analysis of dianping/cat@e815e74d4c (2026-08-14). Data as JSON: /api/errors/82caf107a2678976. Report an issue: GitHub.