Blackstone-Code-Style-Guide

Eslint and Prettier

Normally all projects use eslint and prettier, install the respective plugins in your favorite IDE.

If your project does not have a prettier configuration, use the basic configuration:

  {
      arrowParens: 'always',
      bracketSameLine: false,
      bracketSpacing: true,
      embeddedLanguageFormatting: 'auto',
      htmlWhitespaceSensitivity: 'css',
      insertPragma: false,
      jsxSingleQuote: false,
      printWidth: 100,
      proseWrap: 'preserve',
      quoteProps: 'as-needed',
      requirePragma: false,
      semi: true,
      singleQuote: true,
      tabWidth: 2,
      trailingComma: 'es5',
      useTabs: false,
      vueIndentScriptAndStyle: false,
  }

References

Use const for all of your references; avoid using var. eslint: prefer-const, no-const-assign

Why? This ensures that you can’t reassign your references, which can lead to bugs and difficult to comprehend code.
```
// bad
var a = 1;
var b = 2;

// good
const a = 1;
const b = 2;
```

If you must reassign references, use let instead of var. eslint: no-var

Why? let is block-scoped rather than function-scoped like var.

// bad
var count = 1;
if (true) {
  count += 1;
}

// good, use the let.
let count = 1;
if (true) {
  count += 1;
}

Objects

Use the literal syntax for object creation. eslint: no-new-object

// bad
const item = new Object();

// good
const item = {};

Use object method shorthand. eslint: object-shorthand

// bad
const atom = {
  value: 1,

  addValue: function (value) {
    return atom.value + value;
  },
};

// good
const atom = {
  value: 1,

  addValue(value) {
    return atom.value + value;
  },
};

Use property value shorthand. eslint: object-shorthand

Why? It is shorter and descriptive.

const lukeSkywalker = 'Luke Skywalker';

// bad
const obj = {
  lukeSkywalker: lukeSkywalker,
};

// good
const obj = {
  lukeSkywalker,
};

Group your shorthand properties at the beginning of your object declaration.

Why? It’s easier to tell which properties are using the shorthand.

const anakinSkywalker = 'Anakin Skywalker';
const lukeSkywalker = 'Luke Skywalker';

// bad
const obj = {
  episodeOne: 1,
  twoJediWalkIntoACantina: 2,
  lukeSkywalker,
  episodeThree: 3,
  mayTheFourth: 4,
  anakinSkywalker,
};

// good
const obj = {
  lukeSkywalker,
  anakinSkywalker,
  episodeOne: 1,
  twoJediWalkIntoACantina: 2,
  episodeThree: 3,
  mayTheFourth: 4,
};

Only quote properties that are invalid identifiers. eslint: quote-props

Why? In general we consider it subjectively easier to read. It improves syntax highlighting, and is also more easily optimized by many JS engines.
```
// bad
const bad = {
  'foo': 3,
  'bar': 4,
  'data-blah': 5,
};

// good
const good = {
  foo: 3,
  bar: 4,
  'data-blah': 5,
};
```

Prefer the object spread syntax over Object.assign to shallow-copy objects. Use the object rest parameter syntax to get a new object with certain properties omitted. eslint: prefer-object-spread

// very bad
const original = { a: 1, b: 2 };
const copy = Object.assign(original, { c: 3 }); // this mutates `original` ಠ_ಠ
delete copy.a; // so does this

// bad
const original = { a: 1, b: 2 };
const copy = Object.assign({}, original, { c: 3 }); // copy => { a: 1, b: 2, c: 3 }

// good
const original = { a: 1, b: 2 };
const copy = { ...original, c: 3 }; // copy => { a: 1, b: 2, c: 3 }

const { a, ...noA } = copy; // noA => { b: 2, c: 3 }

Arrays

Use the literal syntax for array creation. eslint: no-array-constructor

// bad
const items = new Array();

// good
const items = [];

Use Array#push instead of direct assignment to add items to an array.

const someStack = [];

// bad
someStack[someStack.length] = 'abracadabra';

// good
someStack.push('abracadabra');

Use array spreads ... to copy arrays.

// bad
const len = items.length;
const itemsCopy = [];
let i;

for (i = 0; i < len; i += 1) {
  itemsCopy[i] = items[i];
}

// good
const itemsCopy = [...items];

To convert an iterable object to an array, use spreads ... instead of Array.from.

const foo = document.querySelectorAll('.foo');

// good
const nodes = Array.from(foo);

// best
const nodes = [...foo];

Use Array.from for converting an array-like object to an array.

const arrLike = { 0: 'foo', 1: 'bar', 2: 'baz', length: 3 };

// bad
const arr = Array.prototype.slice.call(arrLike);

// good
const arr = Array.from(arrLike);

Destructuring

Use object destructuring when accessing and using multiple properties of an object. eslint: prefer-destructuring

Why? Destructuring saves you from creating temporary references for those properties, and from repetitive access of the object. Repeating object access creates more repetitive code, requires more reading, and creates more opportunities for mistakes. Destructuring objects also provides a single site of definition of the object structure that is used in the block, rather than requiring reading the entire block to determine what is used.
```
// bad
function getFullName(user) {
  const firstName = user.firstName;
  const lastName = user.lastName;

  return `${firstName} ${lastName}`;
}

// good
function getFullName(user) {
  const { firstName, lastName } = user;
  return `${firstName} ${lastName}`;
}

// best
function getFullName({ firstName, lastName }) {
  return `${firstName} ${lastName}`;
}
```

Use array destructuring. eslint: prefer-destructuring

const arr = [1, 2, 3, 4];

// bad
const first = arr[0];
const second = arr[1];

// good
const [first, second] = arr;

Use object destructuring for multiple return values, not array destructuring.

Why? You can add new properties over time or change the order of things without breaking call sites.

// bad
function processInput(input) {
  // then a miracle occurs
  return [left, right, top, bottom];
}

// the caller needs to think about the order of return data
const [left, __, top] = processInput(input);

// good
function processInput(input) {
  // then a miracle occurs
  return { left, right, top, bottom };
}

// the caller selects only the data they need
const { left, top } = processInput(input);

Strings

Use single quotes '' for strings. eslint: quotes

// bad
const name = "Capt. Janeway";

// bad - template literals should contain interpolation or newlines
const name = `Capt. Janeway`;

// good
const name = 'Capt. Janeway';

When programmatically building up strings, use template strings instead of concatenation. eslint: prefer-template template-curly-spacing

Why? Template strings give you a readable, concise syntax with proper newlines and string interpolation features.

// bad
function sayHi(name) {
  return 'How are you, ' + name + '?';
}

// bad
function sayHi(name) {
  return ['How are you, ', name, '?'].join();
}

// bad
function sayHi(name) {
  return `How are you, ${ name }?`;
}

// good
function sayHi(name) {
  return `How are you, ${name}?`;
}

Do not unnecessarily escape characters in strings. eslint: no-useless-escape

Why? Backslashes harm readability, thus they should only be present when necessary.
```
// bad
const foo = '\'this\' \i\s \"quoted\"';

// good
const foo = '\'this\' is "quoted"';
const foo = `my name is '${name}'`;
```

Functions

Use named function expressions instead of function declarations. eslint: func-style

Why? Function declarations are hoisted, which means that it’s easy - too easy - to reference the function before it is defined in the file. This harms readability and maintainability. If you find that a function’s definition is large or complex enough that it is interfering with understanding the rest of the file, then perhaps it’s time to extract it to its own module! Don’t forget to explicitly name the expression, regardless of whether or not the name is inferred from the containing variable (which is often the case in modern browsers or when using compilers such as Babel). This eliminates any assumptions made about the Error’s call stack. (Discussion)
```
// bad
function foo() {
  // ...
}

// bad
const foo = function () {
  // ...
};

// good
// lexical name distinguished from the variable-referenced invocation(s)
const short = function longUniqueMoreDescriptiveLexicalFoo() {
  // ...
};
```

Never name a parameter arguments. This will take precedence over the arguments object that is given to every function scope.

// bad
function foo(name, options, arguments) {
  // ...
}

// good
function foo(name, options, args) {
  // ...
}

Never use arguments, opt to use rest syntax ... instead. eslint: prefer-rest-params

Why? ... is explicit about which arguments you want pulled. Plus, rest arguments are a real Array, and not merely Array-like like arguments.
```
// bad
function concatenateAll() {
  const args = Array.prototype.slice.call(arguments);
  return args.join('');
}

// good
function concatenateAll(...args) {
  return args.join('');
}
```

Use default parameter syntax rather than mutating function arguments.

// really bad
function handleThings(opts) {
  // No! We shouldn’t mutate function arguments.
  // Double bad: if opts is falsy it'll be set to an object which may
  // be what you want but it can introduce subtle bugs.
  opts = opts || {};
  // ...
}

// still bad
function handleThings(opts) {
  if (opts === void 0) {
    opts = {};
  }
  // ...
}

// good
function handleThings(opts = {}) {
  // ...
}

Always put default parameters last. eslint: default-param-last

// bad
function handleThings(opts = {}, name) {
  // ...
}

// good
function handleThings(name, opts = {}) {
  // ...
}

Never reassign parameters. eslint: no-param-reassign

Why? Reassigning parameters can lead to unexpected behavior, especially when accessing the arguments object. It can also cause optimization issues, especially in V8.

// bad
function f1(a) {
  a = 1;
  // ...
}

function f2(a) {
  if (!a) { a = 1; }
  // ...
}

// good
function f3(a) {
  const b = a || 1;
  // ...
}

function f4(a = 1) {
  // ...
}

Prefer the use of the spread syntax ... to call variadic functions. eslint: prefer-spread

Why? It’s cleaner, you don’t need to supply a context, and you can not easily compose new with apply.

// bad
const x = [1, 2, 3, 4, 5];
console.log.apply(console, x);

// good
const x = [1, 2, 3, 4, 5];
console.log(...x);

// bad
new (Function.prototype.bind.apply(Date, [null, 2016, 8, 5]));

// good
new Date(...[2016, 8, 5]);

Functions with multiline signatures, or invocations, should be indented just like every other multiline list in this guide: with each item on a line by itself, with a trailing comma on the last item. eslint: function-paren-newline

// bad
function foo(bar,
             baz,
             quux) {
  // ...
}

// good
function foo(
  bar,
  baz,
  quux,
) {
  // ...
}

// bad
console.log(foo,
  bar,
  baz);

// good
console.log(
  foo,
  bar,
  baz,
);

When developing functions or methods that require the use of more than three arguments, consider grouping these arguments into one object. This practice helps improve code readability and maintainability. The following are the advantages of this strategy.

// bad
function createUser(name, age, email, isAdmin, isActive) {
  // ...
}

// good
function createUser(userDetails) {
  // ...
}

// Example of function call
const user = createUser({
  name: "Pepe",
  age: 30,
  email: "[email protected]",
  isAdmin: false,
  isActive: true,
});

Arrow Functions

When you must use an anonymous function (as when passing an inline callback), use arrow function notation. eslint: prefer-arrow-callback, arrow-spacing

Why? It creates a version of the function that executes in the context of this, which is usually what you want, and is a more concise syntax.

Why not? If you have a fairly complicated function, you might move that logic out into its own named function expression.
```
// bad
[1, 2, 3].map(function (x) {
  const y = x + 1;
  return x * y;
});

// good
[1, 2, 3].map((x) => {
  const y = x + 1;
  return x * y;
});
```
If the function body consists of a single statement returning an expression without side effects, omit the braces and use the implicit return. Otherwise, keep the braces and use a return statement. eslint: arrow-parens, arrow-body-style

Why? Syntactic sugar. It reads well when multiple functions are chained together.
```
// bad
[1, 2, 3].map((number) => {
  const nextNumber = number + 1;
  `A string containing the ${nextNumber}.`;
});

// good
[1, 2, 3].map((number) => `A string containing the ${number + 1}.`);
```

Modules

Always use modules (import/export) over a non-standard module system. You can always transpile to your preferred module system.

Why? Modules are the future, let’s start using the future now.

// bad
const AirbnbStyleGuide = require('./AirbnbStyleGuide');
module.exports = AirbnbStyleGuide.es6;

// ok
import AirbnbStyleGuide from './AirbnbStyleGuide';
export default AirbnbStyleGuide.es6;

// best
import { es6 } from './AirbnbStyleGuide';
export default es6;

Do not use wildcard imports.

Why? This makes sure you have a single default export.

// bad
import * as AirbnbStyleGuide from './AirbnbStyleGuide';

// good
import AirbnbStyleGuide from './AirbnbStyleGuide';

And do not export directly from an import.

Why? Although the one-liner is concise, having one clear way to import and one clear way to export makes things consistent.

// bad
// filename es6.js
export { es6 as default } from './AirbnbStyleGuide';

// good
// filename es6.js
import { es6 } from './AirbnbStyleGuide';
export default es6;

Only import from a path in one place. eslint: no-duplicate-imports

Why? Having multiple lines that import from the same path can make code harder to maintain.

// bad
import foo from 'foo';
// … some other imports … //
import { named1, named2 } from 'foo';

// good
import foo, { named1, named2 } from 'foo';

// good
import foo, {
  named1,
  named2,
} from 'foo';

Put all imports above non-import statements. eslint: import/first

Why? Since imports are hoisted, keeping them all at the top prevents surprising behavior.

// bad
import foo from 'foo';
foo.init();

import bar from 'bar';

// good
import foo from 'foo';
import bar from 'bar';

foo.init();

Do not include JavaScript filename extensions eslint: import/extensions

Why? Including extensions inhibits refactoring, and inappropriately hardcodes implementation details of the module you're importing in every consumer.
```
// bad
import foo from './foo.js';
import bar from './bar.jsx';
import baz from './baz/index.jsx';

// good
import foo from './foo';
import bar from './bar';
import baz from './baz';
```

Iterators and Generators

Don’t use iterators. Prefer JavaScript’s higher-order functions instead of loops like for-in or for-of. eslint: no-iterator no-restricted-syntax

Why? This enforces our immutable rule. Dealing with pure functions that return values is easier to reason about than side effects.

Use map() / every() / filter() / find() / findIndex() / reduce() / some() / ... to iterate over arrays, and Object.keys() / Object.values() / Object.entries() to produce arrays so you can iterate over objects.

const numbers = [1, 2, 3, 4, 5];

// bad
let sum = 0;
for (let num of numbers) {
  sum += num;
}
sum === 15;

// good
let sum = 0;
numbers.forEach((num) => {
  sum += num;
});
sum === 15;

// best (use the functional force)
const sum = numbers.reduce((total, num) => total + num, 0);
sum === 15;

// bad
const increasedByOne = [];
for (let i = 0; i < numbers.length; i++) {
  increasedByOne.push(numbers[i] + 1);
}

// good
const increasedByOne = [];
numbers.forEach((num) => {
  increasedByOne.push(num + 1);
});

// best (keeping it functional)
const increasedByOne = numbers.map((num) => num + 1);

Variables

Always use const or let to declare variables. Not doing so will result in global variables. We want to avoid polluting the global namespace. Captain Planet warned us of that. eslint: no-undef prefer-const
```
// bad
superPower = new SuperPower();

// good
const superPower = new SuperPower();
```

Group all your consts and then group all your lets.

Why? This is helpful when later on you might need to assign a variable depending on one of the previously assigned variables.

// bad
let i, len, dragonball,
    items = getItems(),
    goSportsTeam = true;

// bad
let i;
const items = getItems();
let dragonball;
const goSportsTeam = true;
let len;

// good
const goSportsTeam = true;
const items = getItems();
let dragonball;
let i;
let length;

Avoid using unary increments and decrements (++, --). eslint no-plusplus

Why? Per the eslint documentation, unary increment and decrement statements are subject to automatic semicolon insertion and can cause silent errors with incrementing or decrementing values within an application. It is also more expressive to mutate your values with statements like num += 1 instead of num++ or num ++. Disallowing unary increment and decrement statements also prevents you from pre-incrementing/pre-decrementing values unintentionally which can also cause unexpected behavior in your programs.
```
// bad

const array = [1, 2, 3];
let num = 1;
num++;
--num;

let sum = 0;
let truthyCount = 0;
for (let i = 0; i < array.length; i++) {
  let value = array[i];
  sum += value;
  if (value) {
    truthyCount++;
  }
}

// good

const array = [1, 2, 3];
let num = 1;
num += 1;
num -= 1;

const sum = array.reduce((a, b) => a + b, 0);
const truthyCount = array.filter(Boolean).length;
```

Disallow unused variables. eslint: no-unused-vars

Why? Variables that are declared and not used anywhere in the code are most likely an error due to incomplete refactoring. Such variables take up space in the code and can lead to confusion by readers.

// bad

var some_unused_var = 42;

// Write-only variables are not considered as used.
var y = 10;
y = 5;

// A read for a modification of itself is not considered as used.
var z = 0;
z = z + 1;

// Unused function arguments.
function getX(x, y) {
    return x;
}

// good

function getXPlusY(x, y) {
  return x + y;
}

var x = 1;
var y = a + 2;

alert(getXPlusY(x, y));

// 'type' is ignored even if unused because it has a rest property sibling.
// This is a form of extracting an object that omits the specified keys.
var { type, ...coords } = data;
// 'coords' is now the 'data' object without its 'type' property.

Comparison Operators & Equality

Use === and !== over == and !=. eslint: eqeqeq
Conditional statements such as the if statement evaluate their expression using coercion with the ToBoolean abstract method and always follow these simple rules:
- Objects evaluate to true
- Undefined evaluates to false
- Null evaluates to false
- Booleans evaluate to the value of the boolean
- Numbers evaluate to false if +0, -0, or NaN, otherwise true
- Strings evaluate to false if an empty string '', otherwise true
```
if ([0] && []) {
  // true
  // an array (even an empty one) is an object, objects will evaluate to true
}
```

Use shortcuts for booleans, but explicit comparisons for strings and numbers.

// bad
if (isValid === true) {
  // ...
}

// good
if (isValid) {
  // ...
}

// bad
if (name !== '') {
  // ...
}

// good
if (name) {
  // ...
}

// bad
if (collection.length > 0) {
  // ...
}

// good
if (collection.length) {
  // ...
}

Ternaries should not be nested and generally be single line expressions. eslint: no-nested-ternary

// bad
const foo = maybe1 > maybe2
  ? "bar"
  : value1 > value2 ? "baz" : null;

// split into 2 separated ternary expressions
const maybeNull = value1 > value2 ? 'baz' : null;

// better
const foo = maybe1 > maybe2
  ? 'bar'
  : maybeNull;

// best
const foo = maybe1 > maybe2 ? 'bar' : maybeNull;

Avoid unneeded ternary statements. eslint: no-unneeded-ternary

// bad
const foo = a ? a : b;
const bar = c ? true : false;
const baz = c ? false : true;

// good
const foo = a || b;
const bar = !!c;
const baz = !c;

When mixing operators, enclose them in parentheses. The only exception is the standard arithmetic operators: +, -, and ** since their precedence is broadly understood. We recommend enclosing / and * in parentheses because their precedence can be ambiguous when they are mixed. eslint: no-mixed-operators

Why? This improves readability and clarifies the developer’s intention.

// bad
const foo = a && b < 0 || c > 0 || d + 1 === 0;

// bad
const bar = a ** b - 5 % d;

// bad
// one may be confused into thinking (a || b) && c
if (a || b && c) {
  return d;
}

// bad
const bar = a + b / c * d;

// good
const foo = (a && b < 0) || c > 0 || (d + 1 === 0);

// good
const bar = a ** b - (5 % d);

// good
if (a || (b && c)) {
  return d;
}

// good
const bar = a + (b / c) * d;

Blocks

Use braces with all multiline blocks. eslint: nonblock-statement-body-position

// bad
if (test)
  return false;

// good
if (test) return false;

// good
if (test) {
  return false;
}

// bad
function foo() { return false; }

// good
function bar() {
  return false;
}

If you’re using multiline blocks with if and else, put else on the same line as your if block’s closing brace. eslint: brace-style

// bad
if (test) {
  thing1();
  thing2();
}
else {
  thing3();
}

// good
if (test) {
  thing1();
  thing2();
} else {
  thing3();
}

If an if block always executes a return statement, the subsequent else block is unnecessary. A return in an else if block following an if block that contains a return can be separated into multiple if blocks. eslint: no-else-return

// bad
function foo() {
  if (x) {
    return x;
  } else {
    return y;
  }
}

// bad
function cats() {
  if (x) {
    return x;
  } else if (y) {
    return y;
  }
}

// bad
function dogs() {
  if (x) {
    return x;
  } else {
    if (y) {
      return y;
    }
  }
}

// good
function foo() {
  if (x) {
    return x;
  }

  return y;
}

// good
function cats() {
  if (x) {
    return x;
  }

  if (y) {
    return y;
  }
}

// good
function dogs(x) {
  if (x) {
    if (z) {
      return y;
    }
  } else {
    return z;
  }
}

Control Statements

In case your control statement (if, while etc.) gets too long or exceeds the maximum line length, each (grouped) condition could be put into a new line. The logical operator should begin the line.

Why? Requiring operators at the beginning of the line keeps the operators aligned and follows a pattern similar to method chaining. This also improves readability by making it easier to visually follow complex logic.

// bad
if ((foo === 123 || bar === 'abc') && doesItLookGoodWhenItBecomesThatLong() && isThisReallyHappening()) {
  thing1();
}

// bad
if (foo === 123 &&
  bar === 'abc') {
  thing1();
}

// bad
if (foo === 123
  && bar === 'abc') {
  thing1();
}

// bad
if (
  foo === 123 &&
  bar === 'abc'
) {
  thing1();
}

// good
if (
  foo === 123
  && bar === 'abc'
) {
  thing1();
}

// good
if (
  (foo === 123 || bar === 'abc')
  && doesItLookGoodWhenItBecomesThatLong()
  && isThisReallyHappening()
) {
  thing1();
}

// good
if (foo === 123 && bar === 'abc') {
  thing1();
}

Don't use selection operators in place of control statements.

// bad
!isRunning && startRunning();

// good
if (!isRunning) {
  startRunning();
}

The first function is a bad practice because it uses an else if string that makes the code more difficult to read and can complicate the addition or modification of conditions in the future. The second approach is a better practice because it improves clarity and maintainability: by eliminating the else, each condition is handled independently, making the code more readable and facilitating future modifications.

let isFirstStepWorking  = true;
let isSecondStepWorking = true;
let isThirdStepWorking  = true;
let isFourthStepWorking = false;

  // bad
  const workingSteps = () => {
    if (isFirstStepWorking === true) {
      if (isSecondStepWorking === true) {
        if (isThirdStepWorking === true) {
          if (isFourthStepWorking === true) {
            return "Working properly!";
          } else {
            return "Fourth step broken.";
          }
        } else {
          return "Third step broken.";
        }
      } else {
        return "Second step broken.";
      }
    } else {
      return "First step broken.";
    }
  }

  // good
  const workingSteps = () => {
    if (!isFirstStepWorking) return "First step broken.";

    if (!isSecondStepWorking) return "Second step broken.";

    if (!isThirdStepWorking) return "Third step broken.";

    if (!isFourthStepWorking) return "Fourth step broken.";

    return "Working properly!";
  }

  // bad
   const getPayAmount = ({ isDead = false, isSeparated = true, isRetired = false }) => {
    let result;
    if ( isDead ) {
        result = 1500;
    } else {
        if ( isSeparated ) {
            result = 2500;
        } else {
            if ( isRetired ) {
                result = 3000;
            } else {
                result = 4000; 
            }
        }
    }
    
    return result;
  } 

   // good
   const getPayAmount = ({ isDead = false, isSeparated = true, isRetired = false }) => {
   if ( isDead ) return 1500;

   if ( isSeparated ) return 2500;

   if ( isRetired ) return 3000;

   return 4000;
  }

   // best
   const getPayAmount = ({ isDead = false, isSeparated = true, isRetired = false }) => {
   if ( isDead ) return 1500;

   if ( isSeparated ) return 2500;

   return isRetired ? 3000 : 4000;
  }

Each of the three versions of the getFruitsByColor function aims to do the same thing: return a list of fruits based on the given color or throw an error if the color is not among the expected ones. However, they differ in their clarity, brevity, and maintainability.

Version ('bad', if-else)

Pros: This version is straightforward and easy to understand. Cons: However, using multiple if-else statements can make the code harder to read, especially if more colors and conditions are added in the future. As the list grows, it becomes more cumbersome to follow the logic and maintain the code.

Version ('good', switch-case)

Pros: Using a switch slightly improves readability compared to multiple if-else statements, and it's a bit easier to maintain as it has a clearer structure for handling multiple conditions based on a single variable. Cons: Still, adding or removing colors requires changing the switch structure, which could be seen as more work than simply adding or removing properties from an object.

Version ('best', lookup object)

Pros: This version is more concise and easier to maintain. The logic of mapping colors to fruits is managed through a literal object, which makes adding or removing colors as easy as modifying the fruitsByColor object. There are no complex control flow structures (like if or switch), and the line that throws the error is very short and clear. Efficiency: Accessing an object's property is generally faster than a switch statement or multiple if-else statements, especially if the list of cases is long. Cons: One might argue that for someone not accustomed to working with operators like ?? or the error handling style using throw in expressions, this version could be slightly less readable. However, this is a common pattern in modern TypeScript and JavaScript.
```
 // bad
  const getFruitsByColor = ( color ) => {

    if ( color === 'red' ) {
        return ['manzana','fresa'];
    } else if ( color === 'yellow') {
        return ['piña','banana'];
    } else if ( color === 'purple') {
        return ['moras','uvas']
    } else {
        throw Error('the color must be: red, yellow, purple');
    }
}


// good 
const getFruitsByColor = ( color ) => {

    switch( color ) {
         case 'red':
             return ['manzana','fresa'];
         case 'yellow':
             return ['piña','banana'];
         case 'purple':
             return ['moras','uvas'];
         default:
             throw Error('the color must be: red, yellow, purple');
     }
}


// best
const getFruitsByColor = ( color ) => {

    const fruitsByColor = {
        red:    ['manzana','fresa'],
        yellow: ['piña','banana'],
        purple: ['moras','uvas'],
    };

     return fruitsByColor[color] ?? throw new Error('the color must be: red, yellow, purple');
}
```

Version ('bad')

The first block of code (bad) uses multiple logical operators || to compare the variable color with a list of values. This makes the code:

Verbose: There's a lot of repeated text, which makes the code longer and less readable. Hard to maintain: If you need to add or remove a color, you have to modify the condition and make sure you do not make mistakes while manipulating the logical operators.

Version ('good')

The second block of code (good) improves upon the former by utilizing the Array.includes() method. This makes the code:

More readable: By placing all the colors into an array and using includes(), the code's purpose becomes clearer. Easier to maintain: Adding or removing colors from the array is straightforward and less error-prone.

Version ('best' first version)

The third block of code (best) further improves by reorganizing the logic:

Less complex: It inverts the logic of the condition, returning immediately if the color is not included, which reduces a level of nesting and enhances clarity. Cleaner code: Eliminates the unnecessary else because if the condition is met, the function ends with a return.

Version ('best')

The last version uses a ternary expression, which is a concise way to write an if-else statement.

 // bad
 const colorForRainbow = (color) => {
     if (
       color === "red" ||
       color === "orange" ||
       color === "yellow" ||
       color === "green" ||
       color === "blue" ||
       color === "indigo" ||
       color === "violet"
     ) {
       return "yes, it is a valid color";
     } else {
       return "no, it is not a valid color";
     }
   };

 // good
 const colorForRainbow = (color) => {
   if (
     ["red", "orange", "yellow", "green", "blue", "indigo", "violet"].includes(
       color
     )
   ) {
     return "yes, it is a valid color";
   } else {
     return "no, it is not a valid color";
   }
 };

 // best
 const colorForRainbow = (color) => {
   if (
     !["red", "orange", "yellow", "green", "blue", "indigo", "violet"].includes(
       color
     )
   ) {
     return "no, it is not a valid color";
   }
   return "yes, it is a valid color";
 };

 // best
 const colorForRainbow = (color) =>
   ["red", "orange", "yellow", "green", "blue", "indigo", "violet"].includes(
     color
   )
     ? "yes, it is a valid color"
     : "no, it is not a valid color";

Comments

Use /** ... */ for multiline comments.

// bad
// make() returns a new element
// based on the passed in tag name
//
// @param {String} tag
// @return {Element} element
function make(tag) {

  // ...

  return element;
}

// good
/**
 * make() returns a new element
 * based on the passed-in tag name
 */
function make(tag) {

  // ...

  return element;
}

Use // for single line comments. Place single line comments on a newline above the subject of the comment. Put an empty line before the comment unless it’s on the first line of a block.

// bad
const active = true;  // is current tab

// good
// is current tab
const active = true;

// bad
function getType() {
  console.log('fetching type...');
  // set the default type to 'no type'
  const type = this.type || 'no type';

  return type;
}

// good
function getType() {
  console.log('fetching type...');

  // set the default type to 'no type'
  const type = this.type || 'no type';

  return type;
}

// also good
function getType() {
  // set the default type to 'no type'
  const type = this.type || 'no type';

  return type;
}

Start all comments with a space to make it easier to read. eslint: spaced-comment

// bad
//is current tab
const active = true;

// good
// is current tab
const active = true;

// bad
/**
 *make() returns a new element
 *based on the passed-in tag name
 */
function make(tag) {

  // ...

  return element;
}

// good
/**
 * make() returns a new element
 * based on the passed-in tag name
 */
function make(tag) {

  // ...

  return element;
}

Prefixing your comments with FIXME or TODO helps other developers quickly understand if you’re pointing out a problem that needs to be revisited, or if you’re suggesting a solution to the problem that needs to be implemented. These are different than regular comments because they are actionable. The actions are FIXME: -- need to figure this out or TODO: -- need to implement.

Use // FIXME: to annotate problems.

class Calculator extends Abacus {
  constructor() {
    super();

    // FIXME: shouldn’t use a global here
    total = 0;
  }
}

Use // TODO: to annotate solutions to problems.

class Calculator extends Abacus {
  constructor() {
    super();

    // TODO: total should be configurable by an options param
    this.total = 0;
  }
}

Naming Conventions

Avoid single letter names. Be descriptive with your naming. eslint: id-length

// bad
function q() {
  // ...
}

// good
function query() {
  // ...
}

Use camelCase when naming objects, functions, and instances. eslint: camelcase

// bad
const OBJEcttsssss = {};
const this_is_my_object = {};
function c() {}

// good
const thisIsMyObject = {};
function thisIsMyFunction() {}

Use PascalCase only when naming constructors or classes. eslint: new-cap

// bad
function user(options) {
  this.name = options.name;
}

const bad = new user({
  name: 'nope',
});

// good
class User {
  constructor(options) {
    this.name = options.name;
  }
}

const good = new User({
  name: 'yup',
});

Do not use trailing or leading underscores. eslint: no-underscore-dangle

Why? JavaScript does not have the concept of privacy in terms of properties or methods. Although a leading underscore is a common convention to mean “private”, in fact, these properties are fully public, and as such, are part of your public API contract. This convention might lead developers to wrongly think that a change won’t count as breaking, or that tests aren’t needed. tl;dr: if you want something to be “private”, it must not be observably present.
```
// bad
this.__firstName__ = 'Panda';
this.firstName_ = 'Panda';
this._firstName = 'Panda';

// good
this.firstName = 'Panda';

// good, in environments where WeakMaps are available
// see https://kangax.github.io/compat-table/es6/#test-WeakMap
const firstNames = new WeakMap();
firstNames.set(this, 'Panda');
```

A base filename should exactly match the name of its default export.

// file 1 contents
class CheckBox {
  // ...
}
export default CheckBox;

// file 2 contents
export default function fortyTwo() { return 42; }

// file 3 contents
export default function insideDirectory() {}

// in some other file
// bad
import CheckBox from './checkBox'; // PascalCase import/export, camelCase filename
import FortyTwo from './FortyTwo'; // PascalCase import/filename, camelCase export
import InsideDirectory from './InsideDirectory'; // PascalCase import/filename, camelCase export

// bad
import CheckBox from './check_box'; // PascalCase import/export, snake_case filename
import forty_two from './forty_two'; // snake_case import/filename, camelCase export
import inside_directory from './inside_directory'; // snake_case import, camelCase export
import index from './inside_directory/index'; // requiring the index file explicitly
import insideDirectory from './insideDirectory/index'; // requiring the index file explicitly

// good
import CheckBox from './CheckBox'; // PascalCase export/import/filename
import fortyTwo from './fortyTwo'; // camelCase export/import/filename
import insideDirectory from './insideDirectory'; // camelCase export/import/directory name/implicit "index"
// ^ supports both insideDirectory.js and insideDirectory/index.js

Use camelCase when you export-default a function. Your filename should be identical to your function’s name.
```
function makeStyleGuide() {
  // ...
}

export default makeStyleGuide;
```
Use PascalCase when you export a constructor / class / singleton / function library / bare object.
```
const AirbnbStyleGuide = {
  es6: {
  },
};

export default AirbnbStyleGuide;
```

Acronyms and initialisms should always be all uppercased, or all lowercased.

Why? Names are for readability, not to appease a computer algorithm.

// bad
import SmsContainer from './containers/SmsContainer';

// bad
const HttpRequests = [
  // ...
];

// good
import SMSContainer from './containers/SMSContainer';

// good
const HTTPRequests = [
  // ...
];

// also good
const httpRequests = [
  // ...
];

// best
import TextMessageContainer from './containers/TextMessageContainer';

// best
const requests = [
  // ...
];

You may optionally uppercase a constant only if it (1) is exported, (2) is a const (it can not be reassigned), and (3) the programmer can trust it (and its nested properties) to never change.

Why? This is an additional tool to assist in situations where the programmer would be unsure if a variable might ever change. UPPERCASE_VARIABLES are letting the programmer know that they can trust the variable (and its properties) not to change.
- What about all const variables? - This is unnecessary, so uppercasing should not be used for constants within a file. It should be used for exported constants however.
- What about exported objects? - Uppercase at the top level of export (e.g. EXPORTED_OBJECT.key) and maintain that all nested properties do not change.
```
// bad
const PRIVATE_VARIABLE = 'should not be unnecessarily uppercased within a file';

// bad
export const THING_TO_BE_CHANGED = 'should obviously not be uppercased';

// bad
export let REASSIGNABLE_VARIABLE = 'do not use let with uppercase variables';

// ---

// allowed but does not supply semantic value
export const apiKey = 'SOMEKEY';

// better in most cases
export const API_KEY = 'SOMEKEY';

// ---

// bad - unnecessarily uppercases key while adding no semantic value
export const MAPPING = {
  KEY: 'value'
};

// good
export const MAPPING = {
  key: 'value'
};
```

Name		Name	Last commit message	Last commit date
Latest commit History 82 Commits
Documentation		Documentation
git		git
nestjs		nestjs
react		react
README.md		README.md

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