Question 1

How do I prepare for JavaScript output prediction questions?

Accepted Answer

Know the event loop (sync > microtask > macrotask), hoisting (var=undefined, let/const=TDZ), closures (capture variable not value), this (5 binding rules), and promise chaining (return values). Practice 20+ questions.

Question 2

What is the output of console.log('1'); setTimeout(() => console.log('2'), 0); Promise.resolve().then(() => console.log('3')); console.log('4')?

Accepted Answer

1, 4, 3, 2. Sync code (1, 4) runs first. Then microtasks (3, Promise.then). Then macrotasks (2, setTimeout). This is the fundamental event loop order.

Question 3

What is the output of for (var i = 0; i < 3; i++) setTimeout(() => console.log(i), 0)?

Accepted Answer

3, 3, 3. var is function-scoped; all callbacks share one i, which is 3 by the time they run. Use let to fix (logs 0, 1, 2).

Question 4

What is the output of async function with await and console.log?

Accepted Answer

1, 3, 2. Code before await runs synchronously (1). await pauses the function and schedules the rest as a microtask. Code after the foo() call runs (3). Then the rest of foo runs (2, microtask).

How do I prepare for JavaScript output prediction questions?

More FAQs in JavaScript Interview: Output Prediction Questions

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