How to detect if a function is called as constructor?
Interviewer Expectations for this Question
01
Core Competency
Assesses fundamental understanding of JavaScript conventions, runtime behavior, and memory/performance considerations.
02
Evaluation Criteria
Hiring managers look for precision, avoidance of ambiguous jargon, and ability to explain trade-offs under real production conditions.
Comprehensive Model Answer
Verified Solution
You can use new.target pseudo-property to detect whether a function was called as a constructor(using the new operator) or as a regular function call.
- If a constructor or function invoked using the new operator, new.target returns a reference to the constructor or function.
- For function calls, new.target is undefined.
function Myfunc() {
if (new.target) {
console.log("called with new");
} else {
console.log("not called with new");
}
}
new Myfunc(); // called with new
Myfunc(); // not called with new
Myfunc.call({}); // not called with new
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.