crockford skill

---
name: crockford-good-parts
description: 'Write JavaScript code in the style of Douglas Crockford, author of "JavaScript: The Good Parts". Emphasizes using only the reliable subset of JavaScript, avoiding problematic features, and writing clear, maintainable code. Use when writing robust, quality JavaScript.'
---

# Douglas Crockford Style Guide

## Overview

Douglas Crockford is the author of "JavaScript: The Good Parts" and creator of JSON. His philosophy centers on using only the reliable, well-designed parts of JavaScript while strictly avoiding the problematic features.

## Core Philosophy

> "JavaScript has some extraordinarily good parts. In JavaScript, there is a beautiful, elegant, highly expressive language that is buried under a steaming pile of good intentions and blunders."

> "It is better to be clear than clever."

Crockford believes that JavaScript, despite its flaws, contains a powerful and beautiful language—if you know which parts to use.

## Design Principles

1. **Use the Good Parts**: Stick to the reliable subset of the language.

2. **Avoid the Bad Parts**: Don't use features that are error-prone or confusing.

3. **Clarity Over Cleverness**: Code should be immediately understandable.

4. **Lint Everything**: Use tools like JSLint to enforce quality.

## When Writing Code

### Always

- Use `===` and `!==` (strict equality)
- Declare variables at the top of their scope
- Use a single `var`/`let`/`const` statement per scope (Crockford's older style)
- Put braces on the same line as control statements
- Use JSLint/ESLint and fix all warnings
- Prefer named functions over anonymous functions

### Never

- Use `==` or `!=` (type coercion equality)
- Use `eval()` or `Function()` constructor
- Use `with` statement
- Use `++` or `--` (prefer `+= 1`)
- Rely on automatic semicolon insertion
- Use bitwise operators for non-bitwise operations
- Use `new` for primitives (`new String`, `new Number`, `new Boolean`)

### Prefer

- `Object.create()` over constructor functions
- Object literals over `new Object()`
- Array literals over `new Array()`
- `Array.isArray()` over `instanceof Array`
- Explicit returns over implicit
- Named functions over arrow functions for methods

## Code Patterns

### Object Creation

```javascript
// BAD: Constructor function with new
function Person(name, age) {
    this.name = name;
    this.age = age;
}
Person.prototype.greet = function () {
    return 'Hello, ' + this.name;
};
var person = new Person('Alice', 30);

// GOOD: Factory function (no new required)
function createPerson(name, age) {
    return {
        name: name,
        age: age,
        greet: function () {
            return 'Hello, ' + name;  // Closure for privacy
        }
    };
}
var person = createPerson('Alice', 30);


// BETTER: Object.create for inheritance
var personPrototype = {
    greet: function () {
        return 'Hello, ' + this.name;
    }
};

function createPerson(name, age) {
    var person = Object.create(personPrototype);
    person.name = name;
    person.age = age;
    return person;
}
```

### Module Pattern

```javascript
// The module pattern for encapsulation
var counter = (function () {
    var count = 0;  // Private variable

    return {
        increment: function () {
            count += 1;
            return count;
        },
        decrement: function () {
            count -= 1;
            return count;
        },
        getCount: function () {
            return count;
        }
    };
}());

counter.increment();  // 1
counter.getCount();   // 1
// count is not accessible directly
```

### Strict Equality

```javascript
// BAD: Type coercion surprises
'' == false      // true (!)
0 == ''          // true (!)
null == undefined  // true (!)

// GOOD: Strict equality, no surprises
'' === false     // false
0 === ''         // false
null === undefined  // false

// Always use strict equality
if (value === null) {
    // handle null
}

if (typeof value === 'undefined') {
    // handle undefined
}
```

### Function Best Practices

```javascript
// BAD: Anonymous function
var numbers = [1, 2, 3];
numbers.map(function (n) {
    return n * 2;
});

// GOOD: Named function (better stack traces, self-documenting)
function double(n) {
    return n * 2;
}
numbers.map(double);


// BAD: Relying on hoisting
greet('Alice');
function greet(name) {
    return 'Hello, ' + name;
}

// GOOD: Define before use
function greet(name) {
    return 'Hello, ' + name;
}
greet('Alice');
```

### Array and Object Literals

```javascript
// BAD: Constructor forms
var arr = new Array();
var obj = new Object();
var str = new String('hello');

// GOOD: Literal forms
var arr = [];
var obj = {};
var str = 'hello';


// BAD: Array constructor ambiguity
var a = new Array(3);     // [undefined, undefined, undefined]
var b = new Array(1, 2);  // [1, 2]

// GOOD: Always predictable
var a = [undefined, undefined, undefined];
var b = [1, 2];
```

### Error Handling

```javascript
// Proper try-catch usage
function parseJSON(text) {
    try {
        return JSON.parse(text);
    } catch (e) {
        console.error('Invalid JSON:', e.message);
        return null;
    }
}

// Throw with Error objects, not strings
// BAD:
throw 'Something went wrong';

// GOOD:
throw new Error('Something went wrong');
```

## The Bad Parts to Avoid

1. **Global Variables**: Pollute the namespace, cause conflicts
2. **`eval()`**: Security risk, performance killer
3. **`with`**: Ambiguous scope, deprecated
4. **`==` and `!=`**: Type coercion causes bugs
5. **`++` and `--`**: Encourage trickery
6. **Bitwise Operators**: Rarely needed, often misused
7. **`void`**: Confusing and unnecessary
8. **Typed Wrappers**: `new String()`, `new Number()`, `new Boolean()`
9. **`arguments`**: Use rest parameters instead
10. **Automatic Semicolon Insertion**: Be explicit

## Mental Model

Crockford approaches JavaScript by asking:

1. **Is this a good part?** If not, avoid it entirely
2. **Would a bug here be obvious?** If not, use a safer pattern
3. **Can this be linted?** If JSLint complains, fix it
4. **Is this clear to readers?** Clarity trumps cleverness

## Signature Crockford Moves

- Factory functions instead of constructors
- IIFE module pattern for encapsulation
- Strict equality everywhere
- Object literals for all object creation
- Named functions for debuggability
- JSLint compliance as non-negotiable