JavaScript

When to Use

User needs JavaScript expertise — from core language features to modern patterns. Agent handles async/await, closures, module systems, and ES2023+ features.

Quick Reference

Equality Traps

• == coerces: "0" == false is true — use === always

• NaN !== NaN — use Number.isNaN(), not === NaN

• typeof null === "object" — check === null explicitly

• Objects compare by reference — {} === {} is false

this Binding

• Regular functions: this depends on call site — lost in callbacks

• Arrow functions: this from lexical scope — use for callbacks

• setTimeout(obj.method) loses this — use arrow or .bind()

• Event handlers: this is element in regular function, undefined in arrow (if no outer this)

Closure Traps

• Loop variable captured by reference — let in loop or IIFE to capture value

• var hoisted to function scope — creates single binding shared across iterations

• Returning function from loop: all share same variable — use let per iteration

Array Mutation

• sort(), reverse(), splice() mutate original — use toSorted(), toReversed(), toSpliced() (ES2023)

• push(), pop(), shift(), unshift() mutate — spread [...arr, item] for immutable

• delete arr[i] leaves hole — use splice(i, 1) to remove and reindex

• Spread and Object.assign are shallow — nested objects still reference original

Async Pitfalls

• Forgetting await returns Promise, not value — easy to miss without TypeScript

• forEach doesn't await — use for...of for sequential async

• Promise.all fails fast — one rejection rejects all, use Promise.allSettled if need all results

• Unhandled rejection crashes in Node — always .catch() or try/catch with await

Numbers

• 0.1 + 0.2 !== 0.3 — floating point, use integer cents or toFixed() for display

• parseInt("08") works now — but parseInt("0x10") is 16, watch prefixes

• Number("") is 0, Number(null) is 0 — but Number(undefined) is NaN

• Large integers lose precision over 2^53 — use BigInt for big numbers

Iteration

• for...in iterates keys (including inherited) — use for...of for values

• for...of on objects fails — objects aren't iterable, use Object.entries()

• Object.keys() skips non-enumerable — Reflect.ownKeys() gets all including symbols

Implicit Coercion

• [] + [] is "" — arrays coerce to strings

• [] + {} is "[object Object]" — object toString

• {} + [] is 0 in console — {} parsed as block, not object

• "5" - 1 is 4, "5" + 1 is "51" — minus coerces, plus concatenates

Strict Mode

• "use strict" at top of file or function — catches silent errors

• Implicit globals throw in strict — x = 5 without declaration fails

• this is undefined in strict functions — not global object

• Duplicate parameters and with forbidden