TypeScript Type Locking

When chaining type methods, the first type called determines which keywords TypeScript makes available.


// String-first: string keywords available
new SchemaBuilder()
  .string() // Locks to StringSchemaBuilder
  .number() // Adds "number" to the type array, switches to NumberSchemaBuilder
  .minLength(5) // ❌ TypeScript error — string keyword
  .pattern(/^a/) // ❌ TypeScript error — string keyword
  .minimum(0); // ✅ number keyword available
 
// Number-first: number keywords available
new SchemaBuilder()
  .number() // Locks to NumberSchemaBuilder
  .string() // Adds "string" to the type array, switches to StringSchemaBuilder
  .minimum(0) // ❌ TypeScript error — number keyword
  .multipleOf(5) // ❌ TypeScript error — number keyword
  .minLength(5); // ✅ string keyword available

Why this design? It enforces strict typing — only methods of the active type are accessible. The most recently called type decides the available methods, while still letting you build a multi-type schema constrained by one primary type.

Defining constraints for multiple types

Fully define one type’s constraints, then call the next type to branch.


const schema = new SchemaBuilder()
  .string()
  .minLength(5) // String constraint
  .number() // Branch to NumberSchemaBuilder
  .minimum(0) // Number constraint now available
  .build();
 
// Result: { type: ["string", "number"], minLength: 5, minimum: 0 }

Keywords like const, enum, anyOf, oneOf, and allOf are universal — they aren’t affected by type locking. Only typed constraint keywords (minLength, minimum, pattern, …) are gated by the active type.