JavaScriptJavaScript编程

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
• INLINECODE7 — use Number.isNaN(), not INLINECODE9
• INLINECODE10 — 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
• INLINECODE15 loses this — use arrow or INLINECODE17
• 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
• INLINECODE20 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)
• INLINECODE28, pop(), shift(), unshift() mutate — spread [...arr, item] for immutable
• INLINECODE33 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
• INLINECODE37 doesn't await — use for...of for sequential async
• INLINECODE39 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
• INLINECODE44 works now — but parseInt("0x10") is 16, watch prefixes
• INLINECODE46 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
• INLINECODE52 on objects fails — objects aren't iterable, use INLINECODE53
• INLINECODE54 skips non-enumerable — Reflect.ownKeys() gets all including symbols

Implicit Coercion

• - [] + [] is "" — arrays coerce to strings
• INLINECODE58 is "[object Object]" — object toString
• INLINECODE60 is 0 in console — {} parsed as block, not object
• INLINECODE63 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
• INLINECODE67 is undefined in strict functions — not global object
• Duplicate parameters and with forbidden