JavaScript

Write robust JavaScript with async patterns, type coercion handling, and modern ES2023+ features.

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Last updated: April 29, 2026

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Manual Installation

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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

Topic	File
Async patterns	`async.md`
Type coercion rules	`coercion.md`
Array and object methods	`collections.md`
Modern ES features	`modern.md`

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

JavaScript

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When to Use

Quick Reference

Equality Traps

this Binding

Closure Traps

Array Mutation

Async Pitfalls

Numbers

Iteration

Implicit Coercion

Strict Mode

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