this: the call site decides
loose() has no dot, so sloppy mode uses window; strict code and class bodies use undefined. A bound function ignores a later call. The arrow in the object literal took this from the top level of the script (window), not from obj. The arrow class field keeps this even when detached.
Most "what does this print?" interview questions test four rules: how hoisting sets up variables before code runs, how lexical scope decides which variable a name means, how closures keep variables alive, and how a call decides this.
Change the code and press Run (Ctrl+Enter). Predict what will happen first, then break it on purpose and read what changes. Your edits are saved and match the lesson page.
Ready for more? This phase has 6 hands-on exercises with automatic checks.
Practice questions
Write the code in the editor, run it, then open the model answer to compare.
Predict the output, then explain: var a = 1; function f() { console.log(a); var a = 2; console.log(a); } f(); console.log(a);
Predict the output: const o = { n: 1, get() { return this.n; } }; const g = o.get; console.log(o.get(), g.call({ n: 5 }), (o.get)(), (0, o.get)()); (classic sloppy script, and assume window.n is undefined).
Write once(fn) that returns a function which runs fn only the first time and returns the first result on every later call, keeping this and arguments.
Explain it without notes
Explain hoisting for var, let/const and function declarations, including the temporal dead zone.
Why does for (var i = 0; i < 3; i++) setTimeout(() => console.log(i)) print 3, 3, 3, and how do you fix it?
What is a closure? Give a real use.
List the rules that decide this, in order of priority.
Why do arrow functions behave differently with this, and when is that a problem?
Preview
Console
Nothing logged yet.Expected console output
Asha
window
undefined
Ravi
Mei
true Obj
lost this Save