vercel/packages/cli/src/util/dev/validate.ts

import Ajv from 'ajv';
import {
  routesSchema,
  cleanUrlsSchema,
  headersSchema,
  redirectsSchema,
  rewritesSchema,
  trailingSlashSchema,
} from '@vercel/routing-utils';
import { VercelConfig } from './types';
import {
  functionsSchema,
  buildsSchema,
  NowBuildError,
  getPrettyError,
} from '@vercel/build-utils';
import { fileNameSymbol } from '@vercel/client';

const vercelConfigSchema = {
  type: 'object',
  // These are not all possibilities because `vc dev`
  // doesn't need to know about `regions`, `public`, etc.
  additionalProperties: true,
  properties: {
    builds: buildsSchema,
    routes: routesSchema,
    cleanUrls: cleanUrlsSchema,
    headers: headersSchema,
    redirects: redirectsSchema,
    rewrites: rewritesSchema,
    trailingSlash: trailingSlashSchema,
    functions: functionsSchema,
  },
};

const ajv = new Ajv();
const validate = ajv.compile(vercelConfigSchema);

export function validateConfig(config: VercelConfig): NowBuildError | null {
  if (!validate(config)) {
    if (validate.errors && validate.errors[0]) {
      const error = validate.errors[0];
      const fileName = config[fileNameSymbol] || 'vercel.json';
      const niceError = getPrettyError(error);
      niceError.message = `Invalid ${fileName} - ${niceError.message}`;
      return niceError;
    }
  }

  if (config.functions && config.builds) {
    return new NowBuildError({
      code: 'FUNCTIONS_AND_BUILDS',
      message:
        'The `functions` property cannot be used in conjunction with the `builds` property. Please remove one of them.',
      link: 'https://vercel.link/functions-and-builds',
    });
  }

  return null;
}