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Map vs Object Key Collision — Object Key Coercion

Diagnose bugs where Object keys are coerced to strings (1 becomes "1") while Map preserves key types, causing lookup mismatches.

node -e "require('./repro')"

Set NODE_OPTIONS=--stack-trace-limit=100 and run with verbose logging

Add console.log / debugger / breakpoint at the line before the throw

Symptom and Error Message

When running the JavaScript code, the runtime raises the following error:

(No error — obj[1] and obj["1"] collide; map.get(1) and map.get("1") do not)

This error indicates that storing and retrieving by numeric keys on an object fails because keys are coerced to strings. The error is reproducible and appears consistently once the triggering condition is met. In production, you may see this error sporadically because the triggering condition depends on runtime state that is not always present.

Root Cause Analysis

The fundamental cause is: plain objects coerce all keys to strings (except symbols), so 1 and “1” map to the same slot; map preserves key identity and type. Two factors converge to produce the visible error. First, object keys are always strings or symbols; numeric keys are silently coerced to strings. Second, code mixes numeric and string keys expecting them to be distinct on a plain object. Either factor alone might not trigger the error, but together they create the conditions for failure.

Diagnosis Walkthrough

Start by reproducing the error in isolation. Strip away everything except the minimum code path that triggers the error. Once you have a minimal reproduction, inspect every variable at the failure point. Look for: undefined where an object is expected, wrong types, stale references, and off-by-one indices. The error message tells you what went wrong; the surrounding state tells you why.

Fix and Code Example

// Before (buggy)
// ... code that triggers the error ...

// After (fixed)
// ... corrected code with proper guard or type check ...

The fix addresses the root cause directly: use map when keys are non-string types (numbers, objects, booleans); use object only for string-keyed records. After applying the fix, the error no longer occurs because the triggering condition is either prevented or handled gracefully.

Verification

Set and get both 1 and “1” on a Map and confirm they are distinct entries. Run the reproduction case again and confirm the error is gone. Then run the full test suite to check for regressions. In production, monitor error logs for 24 hours to confirm the fix is effective under real load.

Common Pitfalls

A frequent mistake is to suppress the error with a try/catch or conditional without addressing the underlying cause. This masks the symptom but the defective logic remains, and the error will resurface in a different form. Always fix the root cause, not just the error message. Another pitfall is fixing the symptom only in the happy path while ignoring edge cases like empty input, null values, or concurrent access.