Const

@tsed/schema

Usage

import { Const } from "@tsed/schema";

See /packages/specs/schema/src/decorators/common/const.ts.

Overview

function Const(constValue: JSONSchema6Type | any): (...args: any[]) => any;

• constValue (JSONSchema6Type | any): - The exact value that this property must have

Description

Restricts a property to a single fixed value using JSON Schema's const keyword.

The @Const() decorator ensures that a property must have exactly the specified value. This is useful for literal types, discriminator fields, API version indicators, and other cases where only one specific value is valid.

String Constants

class ApiResponse {
  @Const("success")
  status: "success";
  // Only "success" is valid, any other value fails validation
}

Generated schema:

{
  "type": "object",
  "properties": {
    "status": {
      "type": "string",
      "const": "success"
    }
  }
}

Number Constants

class ConfigModel {
  @Const(1)
  version: 1;
  // Only version 1 is accepted
}

Boolean Constants

class FeatureFlag {
  @Const(true)
  enabled: true;
  // Must always be true
}

Discriminator Pattern

class DogAnimal {
  @Const("dog")
  type: "dog";

  @Property()
  breed: string;
}

class CatAnimal {
  @Const("cat")
  type: "cat";

  @Property()
  indoor: boolean;
}

API Versioning

class V2Request {
  @Const("v2")
  apiVersion: "v2";

  @Property()
  data: any;
}

Use Cases

• Type Discriminators: Distinguish between union type variants
• Literal Types: Enforce TypeScript literal types at runtime
• API Versioning: Validate API version in requests
• Status Fields: Ensure specific status values in responses
• Configuration: Lock configuration values to specific constants

vs Enum

• @Const(): Single fixed value only
• @Enum(): Multiple allowed values from a set