no-implicit-config-flags

Disallow implicit truthy or falsy checks for Vite config callback flags such as isPreview and isSsrBuild.

Rule catalog ID: R013

Targeted pattern scope

• vite.config.*
• conditional config factory functions that receive isPreview or isSsrBuild
• if, ternary, loop, and similar conditional tests inside the config callback

What this rule reports

This rule reports config callback conditions such as:

• if (isPreview) { ... }
• !isSsrBuild ? clientConfig : serverConfig
• if (process.env.CI === "true" || isPreview) { ... }

Why this rule exists

Vite's config callback receives isPreview and isSsrBuild as optional flags. The Vite docs note that some tools loading a config can pass undefined for these values instead of a strict boolean.

That makes generic truthy or falsy checks easy to misread:

• if (isPreview) treats undefined the same as false
• if (!isPreview) treats undefined the same as false

An explicit comparison communicates intent better and prevents undefined from silently joining one branch.

❌ Incorrect

import { defineConfig } from "vite";

export default defineConfig(({ isPreview, isSsrBuild }) => {
    if (isPreview) {
        return {
            preview: {
                open: true,
            },
        };
    }

    return !isSsrBuild
        ? {
              build: {
                  sourcemap: true,
              },
          }
        : {
              build: {
                  ssr: "src/entry-server.ts",
              },
          };
});

✅ Correct

import { defineConfig } from "vite";

export default defineConfig(({ isPreview, isSsrBuild }) => {
    if (isPreview === true) {
        return {
            preview: {
                open: true,
            },
        };
    }

    return isSsrBuild === false
        ? {
              build: {
                  sourcemap: true,
              },
          }
        : {
              build: {
                  ssr: "src/entry-server.ts",
              },
          };
});

Behavior and migration notes

• compare isPreview with === true when a branch should run only during preview
• compare isSsrBuild with === true or === false when SSR and client branches must stay distinct
• this rule is intentionally scoped to Vite config callback flags, not arbitrary boolean values in the file

ESLint flat config example

import vite from "@typpi/eslint-plugin-vite";

export default [vite.configs.configs, vite.configs.strict];

When not to use it

Disable this rule only if your team intentionally treats undefined the same as one branch and prefers that looser conditional style.

Package documentation

• Vite conditional config

Further reading

• configs preset
• strict preset