Comparison with constant not working in find array function in javascript, while direct comparison with the string works

Question

The following code does not work:

try {
  const _id = req.params.id
  await req.user.populate("tasks").execPopulate();
  console.log("_id: ",_id)
  console.log("_id type", typeof(_id))
  //console.log(req.user.tasks);
  const task = await req.user.tasks.find(o =>
    o._id.toString() === _id)
  updates.forEach((update) => task[update] = req.body[update])
  await task.save();

Here is the console output:

_id:  5f1b5164acfd1a264d1589d5

src/routers/task.js:68
_id type string
src/routers/task.js:69
task: undefined

While if I change this line:

  const task = await req.user.tasks.find(o =>
    o._id.toString() === _id)

into this:

  const task = await req.user.tasks.find(o =>
    o._id.toString() === "5f1b5164acfd1a264d1589d5")

it works perfectly. Anyone know why?

---

Additional information: As asked in the comments, I looked at the exact characters in each string:

console.log(_id.split('').map(x => x.charCodeAt(0)))
/*
[
  53, 102, 49, 98, 53, 49, 54, 52, 97, 99,
  102, 100, 49, 97, 50, 54, 52, 100, 49, 53,
  56, 57, 100, 53, 10
]
*/

console.log("5f1b5164acfd1a264d1589d5".split('').map(x => x.charCodeAt(0)))
/*
[
  53, 102, 49, 98, 53, 49, 54, 52, 97, 99,
  102, 100, 49, 97, 50, 54, 52, 100, 49, 53,
  56, 57, 100, 53
]
*/

Answer 1

There is a newline character (character code 10, or \n in JavaScript) at the end of your _id value.

Actually this can even be seen in your debug output - there is an empty line after you print the ID. Instead of using console.log, I'd recommend using a debugger and stepping through the code and looking at variables this way. It would probably have displayed the string as "5f1b5164acfd1a264d1589d5\n" instead.

You can get rid of it using .trim, which removes all leading and trailing whitespace from a string:

const _id = req.params.id.trim()

This way, the code will work, despite the stray newline character passed into your URL.

---

However, this should make you wonder how that can even happen in the first place. I initially ruled out such a problem when I saw this question, because the _id seems to come from an URL path parameter req.params.id, and a newline is not a valid character in a URL, unless you explicitly pass it in as %0A which sounded unlikely to me at first. You should double-check where this URL comes from, because the actual bug probably originates from there!