Interview Q&A / Node.js

1. How do you handle errors in asynchronous Node.js code, and what is an unhandled promise rejection?

   Use try/catch around await, .catch() on promise chains, and the error-first argument in callbacks. An unhandled rejection is a promise that fails with no handler attached; since Node 15 it crashes the process by default, so every promise needs a catch somewhere.

   IntermediateIn-depth3 min read
2. What are streams in Node.js and why would you use them for large files?

   A stream processes data in small chunks as it arrives instead of loading everything into memory at once. For a large file, streams keep memory use low and let you start sending data immediately, and pipeline() handles backpressure and errors for you.

   IntermediateIn-depth3 min read
3. What is the difference between process.nextTick, setImmediate and setTimeout in Node.js?

   process.nextTick runs right after the current operation, before promise callbacks and before the event loop moves on. setImmediate runs in the check phase, right after I/O polling. setTimeout(fn, 0) runs in the timers phase after at least about 1 ms.

   IntermediateIn-depth3 min read
4. How can CommonJS code load an ES module?

   Use dynamic import(), which works in CommonJS and returns a promise. Plain require of an ES module was not allowed historically; newer Node versions added limited support, so dynamic import is the safe choice.

   IntermediateQuick answer1 min read
5. How do you read a very large file line by line in Node.js?

   Create a read stream and pass it to readline.createInterface, then loop with for await. Only a small chunk sits in memory at a time, unlike readFile, which loads the whole file.

   IntermediateQuick answer1 min read
6. How do you send a large file to the browser without loading it all in memory?

   Create a read stream of the file and pipe it into the HTTP response. Set the Content-Type and, if known, Content-Length headers first, and handle stream errors before any bytes are sent.

   IntermediateQuick answer1 min read
7. How do you write a JSON file in Node.js without risking a half-written file?

   Write the data to a temporary file in the same folder, then rename it over the target. Rename is atomic on the same filesystem, so readers see either the old file or the complete new one.

   IntermediateQuick answer1 min read
8. Predict the output: setTimeout, a promise, process.nextTick and plain logs together

   Synchronous code runs first, then process.nextTick callbacks, then promise callbacks (microtasks), then timers. So the order is sync, nextTick, promise, setTimeout.

   IntermediateQuick answer1 min read
9. Should you use await inside a for loop or Promise.all for independent tasks?

   For independent tasks use Promise.all with map so they run concurrently. await inside a loop runs them one by one, which is right only when each step depends on the last or order and rate limits matter.

   IntermediateQuick answer1 min read
10. What are the main phases of the Node.js event loop?

    Timers, pending callbacks, idle/prepare, poll, check, and close callbacks. Timers run setTimeout callbacks, poll collects I/O, check runs setImmediate, and microtasks are drained between steps.

    IntermediateQuick answer1 min read
11. What is a circular dependency in Node.js and what happens when it occurs?

    A circular dependency is when module A requires B and B requires A. In CommonJS, the second module receives A's partially filled exports at that moment, so values defined later are undefined. Fix it by restructuring.

    IntermediateQuick answer1 min read
12. What is backpressure in Node.js streams?

    Backpressure is when a writable cannot consume data as fast as a readable produces it. The writable signals this, and the reader should pause, otherwise memory fills up with unwritten data.

    IntermediateQuick answer1 min read
13. What is the difference between stream.pipe and stream.pipeline?

    pipe connects streams but does not forward errors or clean up, so a failure can leave streams open. pipeline connects several streams, destroys all of them on error, and reports the result in one callback or promise.

    IntermediateQuick answer1 min read
14. What is top-level await and where can you use it in Node.js?

    It lets you use await directly at the top of a file without wrapping it in an async function. It works only in ES modules, and it delays anything that imports that module until the promise settles.

    IntermediateQuick answer1 min read
15. Write a Transform stream that converts incoming text to uppercase.

    Extend stream.Transform (or pass a transform function), call this.push with the changed chunk, and call the callback when done. Then pipe a readable into it and into a writable.

    IntermediateQuick answerCoding1 min read