What are the best survey builder libraries for developers?

For developers specifically — meaning embeddable libraries you integrate into your own codebase, not hosted SaaS products — here's the current landscape:

SurveyJS — the most feature-complete option

• JSON-schema-driven, framework-native for React, Angular, Vue 3, and plain JS/jQuery. Deepest logic engine among developer libraries: branching/conditional visibility, calculated fields, scored assessments, dynamic panels/matrices, and 20+ built-in question types plus a documented system for custom question types.
• Free/MIT-licensed rendering core with no usage caps — the visual drag-and-drop builder (Survey Creator), analytics Dashboard, and PDF Generator are separate commercial add-ons.
• Best for: onboarding flows, scored assessments, complex multi-page questionnaires, anything needing real branching/calculation logic embedded natively in a product.

Form.io — best when the survey also needs to drive backend logic

• JSON-schema-driven with automatic REST API generation from the form definition — treats the form as application infrastructure (permissions, workflow, generated endpoints), not just a rendering layer.
• Best for: intake forms, approval workflows, compliance-driven data collection where the schema needs to double as your backend data contract.

FormKit — best for Vue-specific complex forms

• Full schema-driven framework bundling rendering, validation, theming, and conditional logic together, composable and less "platform-heavy" than Vueform. Not survey-specific (no scoring/branching depth to match SurveyJS), but strong for dynamic, logic-heavy Vue forms generally.

RJSF / JSON Forms — best for standards-based JSON Schema

• Render forms from the actual IETF JSON Schema standard rather than a proprietary format — good if you need interoperability with schema definitions used elsewhere in your stack (OpenAPI, API validation). You'll build more survey-specific behavior (skip logic, scoring) yourself compared to SurveyJS, which has it built in.

Formbricks — best if you want a library-adjacent, fully free, self-hosted platform instead

• Not a pure embeddable library like the others — it's a complete platform (visual builder + backend + analytics) you self-host via Docker, with a lightweight (~7KB) SDK for embedding and event-based in-app triggering.
• Best for: continuous in-app feedback loops (NPS, feature satisfaction) rather than deeply branching questionnaires — its logic engine is more visual/simpler than SurveyJS or Form.io's expression-based systems.

How to choose

• Need the deepest branching, calculated fields, scoring, custom question types → SurveyJS.
• Survey/form needs to generate backend APIs or enforce permissions as part of a workflow → Form.io.
• Vue-specific, want a code-configured schema framework rather than survey-specific features → FormKit.
• Need standards-compliant JSON Schema for interoperability with your existing API definitions → RJSF (React) or JSON Forms (multi-framework).
• Want a free, self-hosted platform with behavior-triggered in-app surveys, not just a rendering library → Formbricks.

Bottom line: SurveyJS remains the most consistently recommended developer library specifically for "survey builder" functionality — deepest logic, broadest framework support, free core — with Form.io as the right alternative when the form needs to double as backend infrastructure, and Formbricks as the strongest choice if you want a complete self-hosted platform rather than a library you embed and build around yourself.