ReddRadar
Agent skill
data-structures
Master JavaScript objects and arrays including manipulation methods, prototypal inheritance, and complex data structure patterns.
Install this agent skill to your Project
npx add-skill https://github.com/pluginagentmarketplace/custom-plugin-javascript/tree/main/skills/data-structures
SKILL.md
Data Structures Skill
Quick Reference Card
Object Operations
// Create
const obj = { name: 'Alice', age: 30 };
// Access
obj.name; // Dot notation
obj['name']; // Bracket notation
obj?.address?.city; // Optional chaining
// Modify
obj.email = 'a@b.com'; // Add/update
delete obj.age; // Remove
const { name, ...rest } = obj; // Destructure
Object Methods
Object.keys(obj); // ['name', 'age']
Object.values(obj); // ['Alice', 30]
Object.entries(obj); // [['name','Alice'], ['age',30]]
Object.fromEntries(entries); // Reverse
Object.assign({}, a, b); // Shallow merge
{ ...a, ...b }; // Spread merge (preferred)
Object.freeze(obj); // Immutable
Object.seal(obj); // No add/delete
Array Methods Cheat Sheet
Transform (new array)
arr.map(x => x * 2); // Transform each
arr.filter(x => x > 0); // Keep matches
arr.slice(1, 3); // Extract portion
arr.flat(2); // Flatten nested
arr.flatMap(x => [x, x*2]); // Map + flatten
Search
arr.find(x => x > 5); // First match
arr.findIndex(x => x > 5); // First index
arr.findLast(x => x > 5); // Last match (ES2023)
arr.includes(5); // Boolean check
arr.indexOf(5); // Index or -1
Reduce
arr.reduce((acc, x) => acc + x, 0); // Sum
arr.reduce((acc, x) => ({ ...acc, [x.id]: x }), {}); // Index by
Check
arr.every(x => x > 0); // All pass
arr.some(x => x > 0); // Any pass
Mutate (modify original)
arr.push(x); // Add end
arr.pop(); // Remove end
arr.unshift(x); // Add start
arr.shift(); // Remove start
arr.splice(i, n, ...items); // Remove/insert
arr.sort((a,b) => a - b); // Sort
arr.reverse(); // Reverse
Destructuring
// Object
const { name, age = 0 } = user;
const { name: userName } = user; // Rename
const { address: { city } } = user; // Nested
// Array
const [first, , third] = arr; // Skip
const [head, ...tail] = arr; // Rest
[a, b] = [b, a]; // Swap
Data Transformation Patterns
// Group by
const byRole = users.reduce((acc, u) => {
(acc[u.role] ??= []).push(u);
return acc;
}, {});
// Unique values
const unique = [...new Set(arr)];
// Index by ID
const byId = arr.reduce((acc, x) => ({ ...acc, [x.id]: x }), {});
Troubleshooting
Common Issues
| Problem | Symptom | Fix |
|---|---|---|
| Shallow copy bug | Nested changes shared | Use structuredClone() |
| Mutation side effect | Original changed | Use spread/map |
undefined access |
Property doesn't exist | Use ?. optional chain |
| Sort not working | Wrong order | Provide compare function |
Deep Clone
// Modern (preferred)
const deep = structuredClone(original);
// JSON (limited - no functions, dates)
const deep = JSON.parse(JSON.stringify(original));
Debug Checklist
// 1. Inspect structure
console.log(JSON.stringify(obj, null, 2));
// 2. Check array vs object
console.log(Array.isArray(value));
// 3. Check prototype
console.log(Object.getPrototypeOf(obj));
Production Patterns
Immutable Update
// Object
const updated = { ...user, name: 'New Name' };
// Array
const added = [...arr, newItem];
const removed = arr.filter(x => x.id !== id);
const updated = arr.map(x => x.id === id ? { ...x, ...changes } : x);
Safe Access
const city = user?.address?.city ?? 'Unknown';
const first = arr?.[0] ?? defaultValue;
Related
- Agent 03: Objects & Arrays (detailed learning)
- Skill: fundamentals: Variables and types
- Skill: modern-javascript: ES6+ features
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