JavaScript

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 Gotchas

• 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 — good riddance