The child_process documentation for execFileSync, execSync and spawnSync notes that options.stdio is an array.
But it seems it can also be a string, similar to the stdio options given to child_process.spawn.
Actually execFileSync, execSync and spawnSync all ends up here, where the options are accepted as string or array. See https://github.com/nodejs/node/blob/master/lib/internal/child_process.js#L758