Node.js · 19/19 5 min

Reading stack traces and logs

Find your frame, decode the classic TypeErrors, and log like you mean it.

Find your frame

TypeError: Cannot read properties of undefined (reading 'price')
    at calcTotal (src/services/cart.js:14:22)      ← your code: start here
    at Array.reduce (<anonymous>)
    at checkout (src/controllers/checkout.js:9:18)
    at Layer.handle (node_modules/express/lib/router/layer.js:95:5)

Skip node_modules and <anonymous> frames. The first frame in your code is where it blew up; the next ones show who called it.

Decode the message

Message	Usually means
`Cannot read properties of undefined (reading 'x')`	the thing before `.x` is undefined: a missing await, wrong key, empty find
`x is not a function`	wrong import shape, or a value shadowed a function
`x is not defined`	typo, or missing import
`Cannot set headers after they are sent`	responded twice: missing `return`
`Converting circular structure to JSON`	sending a request/response or Mongoose doc with cycles
`Assignment to constant variable`	reassigned a `const`
`Maximum call stack size exceeded`	recursion without a base case, or a getter calling itself

Log with intent

console.log({ id, cart });                 // label every value
console.log(JSON.stringify(order, null, 2));
console.log(typeof price, price);          // "string 10" explains a lot

Log at the boundary where expected and actual diverge, not everywhere.
Log types, not just values: "10" and 10 print the same.
Remove the logs once the test is green.

Narrate

"The test expects 201 and gets 500. The stack says cart.js:14, item is undefined, so find returned nothing. I think the id is a string and the stored id is a number." That's the hypothesis-driven loop interviewers want to hear.

Practise it now

Debug labs Bug Hunt

Next in Node.js

The event loop, visually