Hello,
You can use fs.existsSync():
const fs = require("fs"); // Or `import fs from "fs";` with ESM
if (fs.existsSync(path)) {
    // Do something
}
You've specifically asked for a synchronous check, but if you can use an asynchronous check instead (usually best with I/O), use fs.promises.access if you're using async functions or fs.access if not:
In an async function:
try {
    await fs.promises.access("somefile");
    // The check succeeded
} catch (error) {
    // The check failed
}
Or with a callback:
fs.access("somefile", error => {
    if (!error) {
        // The check succeeded
    } else {
        // The check failed
    }
});