feat: display expected constant for const validation errors (#162)

.changeset/const-error-display.md

@@ -0,0 +1,5 @@
+---
+'better-ajv-errors': minor
+---
+
+Format `const` validation errors with expected constant value and suggestions (fixes #162)

src/__tests__/index.ts

@@ -321,5 +321,32 @@ describe('Main', () => {
         },
       ]);
     });
+
+    it('formats const validation errors including the expected constant value (fixes #162)', () => {
+      const schema = {
+        type: 'object',
+        properties: {
+          node: { const: '>=18.12.0' },
+        },
+      };
+      const data = { node: '>=18.12.1' };
+      const validate = new Ajv().compile(schema);
+      expect(validate(data)).toBeFalsy();
+
+      const cliRes = stripVTControlCharacters(betterAjvErrors(schema, data, validate.errors));
+      expect(cliRes).toContain('CONST must be equal to constant\n(>=18.12.0)');
+      expect(cliRes).toContain('Did you mean >=18.12.0 here?');
+
+      const jsRes = betterAjvErrors(schema, data, validate.errors, { format: 'js' });
+      expect(jsRes).toEqual([
+        {
+          start: { line: 1, column: 9, offset: 8 },
+          end: { line: 1, column: 20, offset: 19 },
+          error: '/node must be equal to constant: >=18.12.0',
+          path: '/node',
+          suggestion: 'Did you mean >=18.12.0?',
+        },
+      ]);
+    });
   });
 });

src/helpers.ts

@@ -1,6 +1,7 @@
 import type {
   AdditionalPropertiesParams,
   AjvError,
+  ConstParams,
   ErrorContext,
   ErrorNode,
   RequiredParams,
@@ -11,6 +12,7 @@ import {
   getInstancePath,
   getSiblings,
   isAnyOfError,
+  isConstError,
   isEnumError,
   isRequiredError,
   concatAll,
@@ -87,7 +89,7 @@ export function filterRedundantErrors(root: ErrorNode, parent?: ErrorNode, key?:
         acc.anyOfErrors.push(error);
       }
 
-      if (isEnumError(error)) {
+      if (isEnumError(error) || isConstError(error)) {
         acc.enumErrors.push(error);
       }
 
@@ -150,8 +152,16 @@ export function createErrorInstances(
   options: ErrorContext
 ): BaseValidationError[] {
   const errors = getErrors(root);
-  if (errors.length && errors.every(isEnumError)) {
-    const uniqueValues = new Set(concatAll<unknown>([])(errors.map(e => e.params.allowedValues)));
+  if (errors.length && errors.every(e => isEnumError(e) || isConstError(e))) {
+    const uniqueValues = new Set(
+      concatAll<unknown>([])(
+        errors.map(e =>
+          isEnumError(e)
+            ? e.params.allowedValues
+            : [(e as AjvError<ConstParams>).params.allowedValue]
+        )
+      )
+    );
     const allowedValues = [...uniqueValues];
     const error = errors[0];
     return [

src/types.ts

@@ -14,6 +14,7 @@ export interface AjvError<Params = Record<string, unknown>> {
 }
 
 export type EnumParams = { allowedValues: unknown[] };
+export type ConstParams = { allowedValue: unknown };
 export type RequiredParams = { missingProperty: string };
 export type AdditionalPropertiesParams = { additionalProperty: string };
 // ajv-errors replaces the errors it customizes with one `errorMessage` error.

src/utils.ts

@@ -1,4 +1,11 @@
-import type { AjvError, EnumParams, ErrorMessageParams, ErrorNode, RequiredParams } from './types';
+import type {
+  AjvError,
+  ConstParams,
+  EnumParams,
+  ErrorMessageParams,
+  ErrorNode,
+  RequiredParams,
+} from './types';
 
 // Basic
 const eq =
@@ -23,6 +30,7 @@ export const isRequiredError: (error: AjvError) => error is AjvError<RequiredPar
   isXError('required');
 export const isAnyOfError: (error: AjvError) => boolean = isXError('anyOf');
 export const isEnumError: (error: AjvError) => error is AjvError<EnumParams> = isXError('enum');
+export const isConstError: (error: AjvError) => error is AjvError<ConstParams> = isXError('const');
 export const getErrors = (node: ErrorNode | undefined): AjvError[] =>
   node && node.errors
     ? node.errors.map(e =>

src/validation-errors/enum.ts

@@ -12,9 +12,11 @@ const toText = (value: unknown): string => String(value);
 export default class EnumValidationError extends BaseValidationError<EnumParams> {
   print(): string[] {
     const {
+      keyword,
       message,
       params: { allowedValues },
     } = this.options;
+    const isConst = keyword === 'const';
     const bestMatch = this.findBestMatch();
 
     // Needed to handle nullable enums, as joining on null just prints ", "
@@ -24,16 +26,22 @@ export default class EnumValidationError extends BaseValidationError<EnumParams>
       toText(firstValue || '')
     );
 
+    const header = isConst ? 'CONST' : 'ENUM';
     const output = [
-      styleText('red', styleText('bold', 'ENUM') + ' ' + message),
+      styleText('red', styleText('bold', header) + ' ' + message),
       styleText('red', `(${allowedValuesMessage})\n`),
     ];
 
+    const fallbackMessage = isConst
+      ? '👈🏽  Unexpected value, should be equal to constant ' +
+        styleText('magentaBright', toText(firstValue))
+      : '👈🏽  Unexpected value, should be equal to one of the allowed values';
+
     return output.concat(
       this.getCodeFrame(
         bestMatch !== null
           ? '👈🏽  Did you mean ' + styleText('magentaBright', toText(bestMatch)) + ' here?'
-          : '👈🏽  Unexpected value, should be equal to one of the allowed values'
+          : fallbackMessage
       )
     );
   }