What are the different ways to execute external scripts?
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
There are three different ways to execute external scripts,
- async: The script is downloaded in parallel to parsing the page, and executed as soon as it is available even before parsing completes. The parsing of the page is going to be interuppted once the script is downloaded completely and then the script is executed. Thereafter, the parsing of the remaining page will continue.
The syntax for async usage is as shown below,
<script src="demo.js" async></script>
- defer: The script is downloaded in parallel to parsing the page, and executed after the page has finished parsing.
The syntax for defer usage is as shown below,
<script src="demo.js" defer></script>
- Neither async or defer: The script is downloaded and executed immediately by blocking parsing of the page until the script execution is completed.
Note: You should only use either async or defer attribute if the src attribute is present.
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.