Why Ascertain

Ascertain is a compiled schema-and-constraint validator for JavaScript-native runtime values.

That matters because it keeps the schema model close to ordinary JavaScript while still giving you compiled-validator performance. You write schemas with constructors, literals, regexes, arrays, object shapes, unions, and constraints, then call compile() once and reuse the generated validator.

What You Get

• JavaScript-native schemas instead of a separate authoring format
• Compiled validators instead of interpreting the schema on every call
• Fast first-error mode by default
• Fast all-errors mode when you need full issue lists
• Zero dependencies
• Pathful issues for invalid data

When It Fits

Ascertain is strongest when validation happens at runtime boundaries and throughput matters:

• API request and response validation
• message queue consumers
• config and environment parsing
• hot loops that validate many similarly shaped values
• systems that need detailed issues without giving up too much speed

What It Is Not

Ascertain is not trying to be a JSON Schema authoring tool, a form-first validation DSL, or a data transformation framework. The center of the library is compiled validation over JavaScript-native runtime values.

The Core Workflow

import { compile, optional, or } from 'ascertain';

const validateUser = compile({
  id: Number,
  role: or('admin', 'user', 'guest'),
  score: optional(Number),
});

if (!validateUser(userData)) {
  console.error(validateUser.issues);
}

Use ascertain(schema, data) when you want the convenience wrapper that validates once and throws on failure.