Ascertain vs AJV

AJV and Ascertain are much closer in purpose than Ascertain and Zod. Both are built around compiled validation and both care about runtime throughput.

The Main Difference

AJV centers on JSON Schema. Ascertain centers on JavaScript-native schema values.

That creates the practical split:

• AJV is the better fit when JSON Schema is already your interchange format
• Ascertain is the better fit when you want compiled validation without switching authoring models

Why Use Ascertain Instead Of AJV

• schemas are written as ordinary JavaScript values
• unions and discriminated unions are expressed directly in code
• you get compiled validators without carrying JSON Schema as the source format
• the current benchmark suite shows Ascertain ahead of AJV across the published workloads

Example

AJV:

const schema = {
  type: 'object',
  properties: {
    id: { type: 'number' },
    role: { type: 'string', enum: ['admin', 'user', 'guest'] },
  },
  required: ['id', 'role'],
  additionalProperties: false,
};

Ascertain:

import { compile, or, $strict } from 'ascertain';

const validate = compile({
  id: Number,
  role: or('admin', 'user', 'guest'),
  [$strict]: true,
});

How To Think About The Tradeoff

Choose AJV when:

• you need JSON Schema compatibility
• schemas come from external systems or contracts
• the schema itself must be serialized, exchanged, or standardized

Choose Ascertain when:

• the source of truth is JavaScript or TypeScript code
• you want compiled validation over JS-native runtime values
• you care about invalid-path and all-errors throughput