Is there a JavaScript library for creating complex questionnaires?

Yes — a few JavaScript libraries are built specifically for complex questionnaires (multi-page, branching, calculated values, mixed question types), not just basic form collection. Here's the landscape:

SurveyJS — the most complete option for genuinely complex questionnaires

• Purpose-built for exactly this: supports multi-page forms of any length and complexity, branching logic, calculated/expression-based values, scored surveys and quizzes, and dynamic panels/matrices for repeating question groups.
• Framework-native support for React, Angular, Vue 3, and plain JavaScript (jQuery via a Knockout wrapper), so it fits into whatever stack you're already using.
• Questionnaire structure lives entirely in a JSON schema — pages, conditional visibility (visibleIf), calculated values, validation rules — so the complexity is declarative rather than hand-coded control flow.
• 20+ built-in question types plus a documented system for adding custom question types (covered in the previous question) if the built-ins don't cover a specific need.
• Core rendering library is MIT-licensed and free; the visual drag-and-drop builder (Survey Creator), analytics Dashboard, and PDF Generator are separate, commercially licensed add-ons.
• This is the standard answer when people specifically say "complex questionnaire" rather than "simple form," since it's designed around exactly that use case — scored assessments, multi-branch clinical/research intake forms, and long conditional surveys are common use cases people build with it.

Form.io — if the questionnaire is also part of a larger business process

• Also handles complex, multi-page, logic-heavy forms, but leans further toward "forms as application infrastructure" — auto-generating REST APIs from form definitions, permissions, and workflow integration.
• Better fit if the questionnaire needs to trigger downstream processes (approvals, routing, generated records) rather than just collect and display responses.

jsonforms / react-jsonschema-form (RJSF) — lighter-weight, standards-based alternative

• Both render forms from JSON Schema (the actual IETF standard, not a proprietary schema format like SurveyJS's) plus a separate UI schema for layout.
• Good if you specifically want standards-compliance or already have JSON Schema definitions elsewhere in your stack, but they're more focused on structured data entry than survey-specific features (skip logic, scoring, branching pages) — you'd be building more of that yourself compared to SurveyJS.
• RJSF is React-only; JSON Forms supports React, Angular, and Vue.

Formbricks — if "complex" means targeting/triggering more than branching depth

• Supports conditional logic and multi-step forms, but its logic engine is visual/simpler rather than expression-based — better suited to moderately complex surveys with event-based triggering (in-app, behavioral) rather than deeply nested branching or calculated fields.

How to choose

• Need deep branching, calculated fields, scoring, and mixed question types, want it embedded in your own app → SurveyJS.
• Need the questionnaire to double as application/workflow infrastructure → Form.io.
• Want standards-based JSON Schema rendering and are comfortable building survey-specific logic yourself → RJSF or JSON Forms.
• Complexity is more about "who sees this and when" than "how many branches deep" → Formbricks.

Bottom line: for "complex questionnaire" specifically — meaning branching logic, calculated values, scoring, and varied question types in one system — SurveyJS is the most direct answer among JavaScript libraries, since it's explicitly designed around that scenario rather than general-purpose form rendering.