JavaScript

console.log(3 / 2); // 1.5 1

console.log(0.1 + 0.2); // 0.30000000000000004

console.log(0b111110111); // 503
console.log(0o767); // 503
console.log(0x1f7); // 503
console.log(5.03e2); // 503

console.log(-3n / 2n); // -1n

Math.sin(3.5);
const circumference = 2 * Math.PI * r;

console.log("Hello, world");
console.log(""); //  Unicode 

console.log("Hello"[1] === "e"); // true

const age = 25;
console.log("I am " + age + " years old."); // 
console.log(`I am ${age} years old.`); // 

Boolean(""); // false
Boolean(234); // true

let a;
let name = "Simon";

// myLetVariable **

for (let myLetVariable = 0; myLetVariable < 5; myLetVariable++) {
  // myLetVariable 
}

// myLetVariable **

const Pi = 3.14; //  Pi
console.log(Pi); // 3.14

const Pi = 3.14;
Pi = 1; // 

const obj = {};
obj.a = 1; // 
console.log(obj); // { a: 1 }

function foo(x, condition) {
  if (condition) {
    console.log(x);
    const x = 2;
    console.log(x);
  }
}

foo(1, true);

let a = 1;
a = "foo";

x += 5;
x = x + 5;

"" + ""; // ""

"3" + 4 + 5; // "345"
3 + 4 + "5"; // "75"

123 == "123"; // true
1 == true; // true

123 === "123"; // false
1 === true; // false

const a = 0 && "Hello"; // 0 0 
const b = "Hello" || "world"; // "Hello" "Hello"  "world" 

const name = o && o.getName();

const name = cachedName || (cachedName = getName());

let name = "kittens";
if (name === "puppies") {
  name += " woof";
} else if (name === "kittens") {
  name += " meow";
} else {
  name += "!";
}
name === "kittens meow";

while (true) {
  // 
}

let input;
do {
  input = get_input();
} while (inputIsNotValid(input));

for (let i = 0; i < 5; i++) {
  //  5 
}

for (const value of array) {
  // 
}

for (const property in object) {
  // 
}

switch (action) {
  case "draw":
    drawIt();
    break;
  case "eat":
    eatIt();
    break;
  default:
    doNothing();
}

try {
  buildMySite("./website");
} catch (e) {
  console.error("", e);
}

function buildMySite(siteDirectory) {
  if (!pathExists(siteDirectory)) {
    throw new Error("");
  }
}

try {
  buildMySite("./website");
} catch (e) {
  if (e instanceof RangeError) {
    console.error("", e);
    console.log("...");
    buildMySite("./website");
  } else {
    // 
    throw e;
  }
}

const obj = {
  name: "",
  for: "",
  details: {
    color: "",
    size: 12,
  },
};

// 
obj.name = "";
const name = obj.name;

// 
obj["name"] = "";
const name = obj["name"];

// 
const userName = prompt("");
obj[userName] = prompt("");

obj.details.color; // 
obj["details"]["size"]; // 12

const obj = {};
function doSomething(o) {
  o.x = 1;
}
doSomething(obj);
console.log(obj.x); // 1

const me = {};
const stillMe = me;
me.x = 1;
console.log(stillMe.x); // 1

const a = ["", "", ""];
a.length; // 3

const a = ["", "", ""];
a[100] = "";
console.log(a.length); // 101
console.log(a); // ['', '', '', empty  97, '']

const a = ["", "", ""];
console.log(typeof a[90]); // undefined

const arr = [1, "foo", true];
arr.push({});
// arr = [1, "foo", true, {}]

for (let i = 0; i < a.length; i++) {
  //  a[i]
}

for (const currentValue of a) {
  //  currentValue
}

const babies = ["", "", ""].map((name) => `${name}`);
// babies = ['', '', '']

function add(x, y) {
  const total = x + y;
  return total;
}

add(); // NaN
//  add(undefined, undefined)

add(2, 3, 4); // 5
//  4

function avg(...args) {
  let sum = 0;
  for (const item of args) {
    sum += item;
  }
  return sum / args.length;
}

avg(2, 3, 4, 5); // 3.5

//  { } 
function area({ width, height }) {
  return width * height;
}

//  { } 
console.log(area({ width: 2, height: 3 }));

function avg(firstValue, secondValue, thirdValue = 0) {
  return (firstValue + secondValue + thirdValue) / 3;
}

avg(1, 2); // 1 NaN

// 
const avg = function (...args) {
  let sum = 0;
  for (const item of args) {
    sum += item;
  }
  return sum / args.length;
};

// 
const avg = (...args) => {
  let sum = 0;
  for (const item of args) {
    sum += item;
  }
  return sum / args.length;
};

//  `return`
const sum = (a, b, c) => a + b + c;

(function () {
  // 
})();

function countChars(elm) {
  if (elm.nodeType === 3) {
    // TEXT_NODE
    return elm.nodeValue.length;
  }
  let count = 0;
  for (let i = 0, child; (child = elm.childNodes[i]); i++) {
    count += countChars(child);
  }
  return count;
}

const charsInBody = (function counter(elm) {
  if (elm.nodeType === 3) {
    // TEXT_NODE
    return elm.nodeValue.length;
  }
  let count = 0;
  for (let i = 0, child; (child = elm.childNodes[i]); i++) {
    count += counter(child);
  }
  return count;
})(document.body);

// 
const add = (x) => (y) => x + y;
// 
const babies = ["", "", ""].map((name) => `${name}`);

function parentFunc() {
  const a = 1;

  function nestedFunc() {
    const b = 4; // parentFunc 
    return a + b;
  }
  return nestedFunc(); // 5
}

class Person {
  constructor(name) {
    this.name = name;
  }
  sayHello() {
    return `${this.name}`;
  }
}

const p = new Person("");
console.log(p.sayHello());

const withAuthentication = (cls) =>
  class extends cls {
    authenticate() {
      // 
    }
  };

class Admin extends withAuthentication(Person) {
  // 
}

// 
fs.readFile(filename, (err, content) => {
  // 
  if (err) {
    throw err;
  }
  console.log(content);
});
// 

//  Promise 
fs.readFile(filename)
  .then((content) => {
    // 
    console.log(content);
  })
  .catch((err) => {
    throw err;
  });
// 

// Async/await
async function readFile(filename) {
  const content = await fs.readFile(filename);
  console.log(content);
}

import { foo } from "./foo.js";

// 
const b = 2;

export const a = 1;