Whats the difference between `watch()` and `watchEffect()`?
Assesses fundamental understanding of Vue.js conventions, runtime behavior, and memory/performance considerations.
Hiring managers look for precision, avoidance of ambiguous jargon, and ability to explain trade-offs under real production conditions.
Both watch and watchEffect are Composition API functions used for reactivity, but they serve different purposes and
behave differently.
watch: It is used to watch specific reactive sources (like ref or computed) and respond to changes. This
function is lazy. i.e, The callback is not called immediately but only on changes. It is useful when you need to
access both old and new values, and performs side-effects conditionally.
watch(count, (newVal, oldVal) => {
console.log('Count changed:', newVal);
});
watchEffect: This function is automatic and eager. It runs a function immediately and tracks its dependencies
automatically. It cannot access old values directly.
watchEffect(() => {
console.log(`Count is ${count.value}`);
});
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Candidate Response Strategy & Interview Tips
- Start with a concise one-sentence summary: Deliver a direct, confident answer first before expanding into nuances.
- Demonstrate real-world trade-offs: Discuss where this approach excels and when you would avoid it in production systems.
- Discuss complexity & edge cases: Proactively explain time/space complexity or boundary conditions (null values, scale limits).
- Prepare for interviewer follow-ups: Technical hiring panels frequently probe deeper into concurrency, backward compatibility, or alternative libraries.