I used to "learn" the same JavaScript concepts every few months. I'd read about the event loop, nod along, and a month later get a predict-the-output question wrong in an interview.

What finally worked was seeing the rule every day. I turned each concept into a phone wallpaper with a short why at the bottom, so every time I unlock my phone, I revise one thing.

Here are the five that helped most. Try to predict each output before reading the answer.

1. The event loop: why promises beat setTimeout 

setTimeout(() => console.log('C')); Promise.resolve().then(() => console.log('B')); console.log('A');

Output: A, B, C

The rule that made it stick:

  1. Run all synchronous code on the call stack.
  2. When the stack is empty, drain every microtask (promise callbacks, code after await, queueMicrotask).
  3. Then take one macrotask (setTimeout, I/O, clicks) and repeat.

So a promise callback always runs before setTimeout(fn, 0), even though both look "async". The 0 doesn't mean "now"; it means "next macrotask".

2. Closures: functions remember where they were born 

function counter() { let count = 0; return () => ++count; } const next = counter(); next(); next(); console.log(next());

Output: 3

The returned function keeps a live reference to count, not a copy. That's a closure: a function plus the variables that were in scope when it was created.

The classic interview trap is the same idea:

for (var i = 0; i < 3; i++) { setTimeout(() => console.log(i)); } // 3, 3, 3

All three callbacks close over the same var i, which is 3 by the time they run. Change var to let and you get 0, 1, 2, because let creates a new binding for each loop iteration.

3. this: ask how the function was called 

const user = { name: 'Asha', greet() { return `Hi, ${this.name}`; }, }; const greet = user.greet; console.log(greet());

Output: in an ES module or strict mode, TypeError: Cannot read properties of undefined (reading 'name'). In an old-style non-strict script, this falls back to the global object, so you get a wrong name instead of an error. Either way, it's not Hi, Asha.

this isn't decided where a function is written; it's decided by how it's called. Check these in order:

  • Arrow function: never gets its own this; uses the one from the surrounding code.
  • new Fn(): this is the brand-new object.
  • fn.call(obj) / apply / bind: this is obj.
  • obj.fn(): this is whatever is left of the dot.
  • Plain fn(): undefined in strict mode.

greet() has no dot at call time, so it lost user.

4. Type coercion: + and - play by different rules

Cover the answers and predict:

[] + {} // ? "5" - 2 // ? "5" + 2 // ? null == 0 // ? null >= 0 // ?

Answers: "[object Object]", 3, "52", false, true

  • + switches to string concatenation if either side ends up a string. [] becomes "" and {} becomes "[object Object]".
  • - has no string version, so it always converts to numbers.
  • null >= 0 converts null to 0, so 0 >= 0 is true. But == has a special rule: null only equals undefined. That's how null can be >= 0 without being == 0.

5. Hoisting: let is hoisted too, just locked 

console.log(a); // ? var a = 1; console.log(b); // ? let b = 2;

Output: undefined, then ReferenceError: Cannot access 'b' before initialization

Both declarations are hoisted. The difference:

  • var is hoisted and initialized to undefined.
  • let, const and class are hoisted but sit in the temporal dead zone until their line runs.
  • Function declarations are hoisted completely, so you can call them before they appear.

The error is a feature: it catches bugs that var would silently hide as undefined.

Want them on your lock screen?

I run TechCookies, a small platform built around this "predict first, then learn the why" style. These wallpapers came out of it, and there are 26 in total: JavaScript, TypeScript, SQL, DSA, system design, Java and Python.

👉 Get the full wallpaper pack

Tip for iPhone: put them in one album and use Photo Shuffle on the lock screen, so you get a new concept every day.

Which JavaScript concept took you the longest to really understand? I'm making more wallpapers and would like to cover the ones people actually struggle with.