How do you add a key value pair in javascript?

Peer-reviewed by HireXTech Technical Panel • Updated for 2025/2026 hiring • Editorial standards

Interviewer Expectations for this Question

Core Competency

Assesses fundamental understanding of JavaScript conventions, runtime behavior, and memory/performance considerations.

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 two possible solutions to add new properties to an object.

Let's take a simple object to explain these solutions.

      var object = {
        key1: value1,
        key2: value2,
      };

Using dot notation: This solution is useful when you know the name of the property

      object.key3 = "value3";

Using square bracket notation: This solution is useful when the name of the property is dynamically determined or the key's name is non-JS like "user-name"

      obj["key3"] = "value3";

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.

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