Or is it null?

What is Blogvent? (tl;dr I am going to write one post a day for all of December as a way to practice writing.)

Specific behavior

C++ has this concept of Undefined Behavior. It’s often touted as the wild west, the land of the unknown, here be dragons, we will uninstall your hard drive, etc, etc.

What it is in actuality isn’t that strange. Undefined Behavior is when the standard doesn’t specify what should happen in certain cases, so it’s up to the compiler writers to do whatever they think is best, either logically or from a performance point of view.

Other defines

Javascript has something called undefined as well. It’s not even remotely related to C++’s undefined but it’s fun to think about. JS programmers usually compare undefined with null.

So what’s the difference?


Both signify the absence of value. “It’s nothing”, but I feel this is the key difference.

  • null is intentional indication of no value
  • undefined is indication of no value (intentional or not)

Usually, when you get a null, someone put that there. Someone returned null or assigned a null to a variable.

undefined can happen in more ways, either deliberate by assigning undefined to a variable or by not returning a value from a function and assigning that value to a variable.

function f(){}

const value = f();
console.log(value); // undefined

Fun fact, undefined is actually a variable while null is a value.
This means you can assign values to undefined

() => {
  const undefined = null;
  console.log(undefined); // null

But I don’t recommend doing this… obviously.


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