What are the logical assignment operators?

Q: What are the logical assignment operators?

**Target Audience:** Engineers preparing for JavaScript fundamentals interviews. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term: . Every assignment and short-circuit claim below was actually run in Node. ## 1. Why This Even Matters — A Story First Imagine a thermostat that only adjusts the temperature if the room is currently "empty" (nullish) — it should never override a temperature someone deliberately set, even a genuinely cold one like 0°C, which might be exactly what someone wanted. `??=` behaves exactly like that careful thermostat. A DIFFERENT, less careful thermostat that adjusts whenever the current reading is anything it considers "not good enough" (falsy) — including that legitimate 0°C — would be `||=`, and it would genuinely overwrite a setting the first thermostat correctly left alone. ## 2. The Core Idea 📌 **Interview term:** `x ??= v` assigns only if `x` is nullish. `x

The problem

Question presented to candidate: "If you write config.retries ??= 3 and config.retries is already 0, does it get overwritten? What if you'd used ||= instead — same answer?"

What a strong answer should cover:

📌 Interview term: ??=, ||=, &&= — the logical assignment operators, each conditionally assigning the right-hand side to a variable/property based on the CURRENT value, combining a logical check with assignment in one step.
📌 Interview term: the real, direct answer to the prompt — verified directly: x ??= value genuinely only assigns when x is currently null/undefined — a real, legitimate 0 genuinely survives ??= untouched. x ||= value genuinely assigns for ANY falsy value — the identical real 0 genuinely gets overwritten by ||=, a real, sharp, different answer to the prompt's second question.
📌 Interview term: &&= — the mirror case: x &&= value genuinely only assigns when x is currently truthy — verified directly, a null value genuinely stays null, since &&= never assigns to something already falsy.
📌 Interview term: genuine short-circuiting — verified directly with a real call counter: when the condition for assignment is not met, the right-hand side expression is genuinely never evaluated at all — not just its result discarded, the exact same short-circuiting guarantee this bank's own dedicated short-circuit-evaluation question covers for plain &&/||.
A precise answer names the real, practical use case directly matching the prompt: config.retries ??= 3 is the correct, idiomatic way to supply a default ONLY for a genuinely missing config value, without accidentally clobbering a deliberately-set 0, false, or "".

Clarifying questions expected:

None — this is a definitional/technical question; directly answering both halves of the prompt's own scenario (??= vs. ||= on a real 0) is the strong signal.

Code / implementation expected: Yes — the direct side-by-side ??=-vs-||=-on-a-real-0 comparison is the clearest, most convincing demonstration.

logical-assignment

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

Stuck? Get a hint.

A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Engineers preparing for JavaScript fundamentals interviews. Difficulty: Easy

How to read this doc: Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every assignment and short-circuit claim below was actually run in Node.

1. Why This Even Matters — A Story First

Imagine a thermostat that only adjusts the temperature if the room is currently "empty" (nullish) — it should never override a temperature someone deliberately set, even a genuinely cold one like 0°C, which might be exactly what someone wanted. ??= behaves exactly like that careful thermostat. A DIFFERENT, less careful thermostat that adjusts whenever the current reading is anything it considers "not good enough" (falsy) — including that legitimate 0°C — would be ||=, and it would genuinely overwrite a setting the first thermostat correctly left alone.

The solution is hidden so you can try first

6 min read · 8 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

JavaScriptReal proof: ??= preserves a legitimate 0 while ||= overwrites it, and all three logical assignment operators genuinely short-circuit