What causes "Invalid WriteConcern cannot parse: ${JSON.stringify(value)}" in mongodb/node-mongodb-native?

Pass an object: `writeConcern: { w: 'majority', j: true }`.

mongodb/node-mongodb-native · error · MongoParseError

Invalid WriteConcern cannot parse

Error message

Invalid WriteConcern cannot parse: ${JSON.stringify(value)}

What it means

The `writeConcern` option transform at src/connection_string.ts:1185 accepts four shapes: an object/WriteConcern instance (record check), the literal string `'majority'`, or a number (used as `w`). Any other value falls through to MongoParseError at src/connection_string.ts:1202.

Solutions

Pass an object: `writeConcern: { w: 'majority', j: true }`.
Or a numeric w: `writeConcern: 1` (number, not string).
Or the literal string: `writeConcern: 'majority'`.

Example fix

// before
new MongoClient(uri, { writeConcern: 'all' });

// after
new MongoClient(uri, { writeConcern: { w: 'majority' } });

Defensive patterns

Strategy: type-guard

Validate before calling

import { WriteConcern } from 'mongodb';
function isWriteConcernInput(v: unknown): boolean {
  return v instanceof WriteConcern || (typeof v === 'object' && v !== null) || v === 'majority' || typeof v === 'number';
}
if (opts.writeConcern != null && !isWriteConcernInput(opts.writeConcern)) {
  throw new Error('writeConcern must be an object, WriteConcern, "majority", or a number');
}

Type guard

function isWriteConcernInput(v: unknown): v is object | 'majority' | number { return (typeof v === 'object' && v !== null) || v === 'majority' || typeof v === 'number'; }

Try / catch

try { client = new MongoClient(uri, opts); } catch (e) { if (e instanceof MongoParseError && /WriteConcern cannot parse/.test(e.message)) { opts.writeConcern = { w: 'majority' }; client = new MongoClient(uri, opts); } else throw e; }

Prevention

Always pass writeConcern as an object `{ w, j, wtimeoutMS }`.
Use numeric `w` values as numbers, not strings.
Type-check values decoded from JSON before forwarding.

When it happens

Trigger: Passing `writeConcern: 'all'`, `writeConcern: true`, `writeConcern: ['majority']`, `writeConcern: '1'` (string number, not number), or `writeConcern: null`.

Common situations: Confusing with `w: 'majority'` syntax; passing a JSON-decoded value that became a string; treating writeConcern as a boolean 'enable safe writes' flag.

Related errors

AI-assisted analysis of mongodb/node-mongodb-native@dce7939f86 (2026-08-11). Data as JSON: /api/errors/6f113727cc425381. Report an issue: GitHub.

Appendix: source

Thrown at src/connection_string.ts:1202

    target: 'writeConcern',
    transform({ values: [value], options }) {
      if (isRecord(value) || value instanceof WriteConcern) {
        return WriteConcern.fromOptions({
          writeConcern: {
            ...options.writeConcern,
            ...value
          }
        });
      } else if (value === 'majority' || typeof value === 'number') {
        return WriteConcern.fromOptions({
          writeConcern: {
            ...options.writeConcern,
            w: value
          }
        });
      }

      throw new MongoParseError(`Invalid WriteConcern cannot parse: ${JSON.stringify(value)}`);
    }
  },
  wtimeout: {
    deprecated: 'Please use wtimeoutMS instead',
    target: 'writeConcern',
    transform({ values: [value], options }) {
      const wc = WriteConcern.fromOptions({
        writeConcern: {
          ...options.writeConcern,
          wtimeout: getUIntFromOptions('wtimeout', value)
        }
      });
      if (wc) return wc;
      throw new MongoParseError(`Cannot make WriteConcern from wtimeout`);
    }
  } as OptionDescriptor,
  wtimeoutMS: {
    target: 'writeConcern',

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