next.config.mjs:

// @ts-check
"use strict"

/**
 *
 * @param { string } phase
 * @param {{ defaultConfig: import("next").NextConfig }} config
 * @returns { Promise<import("next").NextConfig> }
 */
export default async function createNextConfig(phase, { defaultConfig }) {
    return {
        reactStrictMode: true,
    }
}

参考

• https://github.com/vercel/next.js/blob/e57d621fb38562b1d5ae02fb6f7efd343e1db405/packages/next/server/config-shared.ts#L475-L481