Values, variables, and what a reference really is
By the end of today you can predict whether changing one variable will change another, explain the difference between a name and the value it points at, and say why two lists can look identical yet behave differently.
YesterdayOn Day 1 you learned that values live in memory while a process runs. Today you find out what a variable name actually is.
TomorrowTomorrow you use these values in conditions and loops, where the shape of your logic starts to matter.
Why this matters
The single most common beginner bug is expecting a copy and getting a reference. This one idea explains it, and it comes back on Day 5 with collections, on Day 21 with objects, and on Day 44 with database rows.
- Variable binding
- References and aliasing
- Mutability
- Primitive types
Learn it
70 minCopy this into Claude or ChatGPT. It quizzes you before it explains anything, which is deliberate. The resources under it are how you check what it told you.
Today's Master Prompt
Free · sign inA prompt written for this day alone: your level, the exact scope, what to leave out, and an instruction to quiz you before it explains anything. Paste it into Claude or ChatGPT and it teaches you today's material.
Check it against something that is not a model
An assistant can be fluent and wrong, and on a topic you met today you will not catch it. These cover the same ground and were made by people who do this for a living, so they are what you hold the explanation up against. They are other people's work and we only link to them, so judge them for yourself.
3 hand-picked resources
Free · sign inVideos, official docs and articles covering the same ground, each opened and annotated by hand. They are what you check the assistant against on a day you cannot yet catch it being wrong.
Build it
35 minWrite a script with two variables bound to the same list. Mutate one and print both. Then do the same with two integers and observe the difference. Finally write a function that takes a list, appends to it, and returns nothing, and prove to yourself that the caller's list changed.
Recall it
20 minAnswer out loud, reveal, then mark honestly whether you had it. That score is the only thing on this page you do not get to choose.
4 recall questions
Free · sign inQuestions you answer from memory, then grade yourself against the real answer. The score is carried into the mastery rating below it, so an honest miss cannot quietly become a tick.
Rate it
Completion and mastery are tracked separately. Be honest, because an inflated rating only means the concept resurfaces sooner.
Mastery tracking
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Recap
- 01A variable is a name bound to an object, not a container holding one
- 02Two names can point at the same object, so mutation is visible through both
- 03`is` compares identity, `==` compares value
- 04Immutable types produce new objects instead of changing in place
Your progress
Free · sign inMark days complete, pick up where you left off across devices, and watch completion and mastery diverge. Free, and the account exists only so ninety days of work cannot vanish with a cleared browser.