Console.log Output is Sometimes Incorrect in Chrome

1 minute read

TIL that the output of console.log in chrome can be misrepresentative of the actual state of an object at the point of logging.

During a particularly grueling debugging session that involved a misconception of mine about how the spread operator copied properties, I ran in to some strange observed behavior by chrome’s console.log method that made me think I was going crazy.

The Observed Behavior

I was mutating an object, running some validation on it, then mutating it back to its original state. I was trying to log the intermediate state of this object to be sure it was being mutated properly. However, the intermediate logs were not what I was expecting…

  const wallet = {value: "123"};
  console.log(wallet); // {value: "123"}

  wallet.value = +wallet.value; //converts to integer
  console.log(wallet); // {value: "123"} <-- WHAT THE HECK???

  /* validation omitted */

  wallet.value = wallet.value.toString();
  console.log(wallet); // {value: "123"}

I called a coworker over to my desk at this point, so he could see what was happening and confirm that I wasn’t going crazy.

The second log statement should have logged the object with the value property as a number, not a string, since I had just converted it the line prior.

Next, I opened the debugger in chrome so I could step through the code… I accessed the value directly after it was mutated to a number, and it did indeed return as a number.

Logging the value in the following line, however, returned the object with string output.

My validation logic was operating correctly, so I know the value was actually being passed to it as an integer, but it still didn’t explain why the logging was mismatched to reality.

The Resolution?

My coworker and I stumbled across this stack overflow post, where another person was having similar problems. It doesn’t really explain why the issue is happening, but it does point out that console.log is somewhat lazy in its evaluation, and will log the last state of an object reference.

Another weird caveat… if the same code I posted above is executed in Firefox, the console.log statements will log the correct intermediate mutated value! I guess there must be some difference between webkit and gecko implementations of the console object, or at least the resolution of logs.

Not a very satisfying answer as to why this occurs, but still a TIL none the less!