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(Variables)

JavaScript let, const var .

let . " " .

let a;
let name = "Simon";

// myLetVariable   **

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

// myLetVariable   **

const . " " .

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

const .

const Pi = 3.14;
Pi = 1; //        .

const . .

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

var (: ) , JavaScript .

, undefined. , const .

let const , (temporal dead zone) . .

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

foo(1, true);

, const x = 2 x x "1" "2" . JavaScript console.log " 'x' ." . let .

JavaScript . ( ) . let .

let a = 1;
a = "foo";

JavaScript +, -, *, /, %( ), **( ) . = , += -= . x = x y .

x += 5;
x = x + 5;

++ -- . .

+ .

"hello" + " world"; // "hello world"

( ) . .

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

JavaScript <, >, <= >= , . , . , .

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

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

!= !== .

JavaScript . "" .

const a = 0 && "Hello"; // 0 "falsy" , 0.
const b = "Hello" || "world"; // "Hello" "world"  "truthy", "Hello" .

&& || . , . null .

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

( )

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

. .

JavaScript C . .

, .
// /* */ . # Perl, Python, and Bash .
JavaScript . . Python .

JavaScript .

JavaScript C . if else , .

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

JavaScript elif , else if if else .

JavaScript while do...while . , .

while (true) {
  // !
}

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

JavaScript for C Java . , .

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

JavaScript for . iterables( ) for...of, enumerable for...in.

for (const value of array) {
  // value  .
}

for (const property in object) {
  //    .
}

switch .

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

C , case labels , break "". . , case , . === .

Rust , JavaScript . , const a = if (x) { 1 } else { 2 } .

JavaScript try...catch .

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

(Error) throw . .

function buildMySite(siteDirectory) {
  if (!pathExists(siteDirectory)) {
    throw new Error("Site directory does not exist");
  }
}

, . throw . Error . TypeError RangeError Error , . JavaScript . , instanceof , case .

try {
  buildMySite("./website");
} catch (e) {
  if (e instanceof RangeError) {
    console.error("Seems like a parameter is out of range:", e);
    console.log("Retrying...");
    buildMySite("./website");
  } else {
    //      .
    //          .
    throw e;
  }
}

try...catch , .

(Objects)

JavaScript - (name-value pairs) . JavaScript .

Python Dictionaries.
Perl Ruby Hashes.
C C++ Hash tables.
Java HashMaps.
PHP (Associative arrays).

JavaScript (hashes). , JavaScript . , , , . strings (symbols). .

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

(.) ([]) . , . .

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

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

//      .
const userName = prompt("what is your key?");
obj[userName] = prompt("what is its value?");

obj.details.color; // orange
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

, . .

( ) , . .

(Arrays)

JavaScript . ( [] ) , length . .

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

JavaScript . . "" length .

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

(sparse array) . . !

. undefined .

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

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

C for .

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

, C++/Java for (int x : arr) for...of .

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

. . , map() .

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

(Functions)

, JavaScript . .

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

JavaScript 0 . . return . ( ), JavaScript undefined .

. undefined . .

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

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

. , rest Python *args (JS , **kwargs ).

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

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

args .

(rest parameter) , . , function avg(firstValue, ...args) firstValue args .

, (spread syntax) spread . , avg(...numbers)

JavaScript . , .

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

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

, ( undefined ) .

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

avg(1, 2); // NaN , 1.

JavaScript ( ) . , .

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

avg() . , function avg() {} .

//     .
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;

. .

. (IIFE, Immediately Invoked Function Expressions) .

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

IIFE .

JavaScript . DOM .

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);

. , . , .

JavaScript . (tail-call optimization) , JavaScriptCore (Safari ) . , .

(first-class objects)

JavaScript (first-class objects). , . , JavaScript (closures) .

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

JavaScript JavaScript , .

(Inner functions)

JavaScript . JavaScript .

function parentFunc() {
  const a = 1;

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

. , . .

JavaScript Java class .

class Person {
  constructor(name) {
    this.name = name;
  }
  sayHello() {
    return `Hello, I'm ${this.name}!`;
  }
}

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

JavaScript new . , . . , , . . , (mixin) .

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

class Admin extends withAuthentication(Person) {
  // 
}

static . Private hash(#) (private ) . . (Python # _ .) , Private . (derived classes) .

guide page .

JavaScript . , . , (polling) .

JavaScript .

(: setTimeout())
Promise
Promise (syntactic sugar) async/await

, JavaScript .

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

//  (Promise-based)
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);
}

, , . , . , , .

, . , (Promise) , then() . , await , () . Promise "" . Promise . . , Promise then() (monads) (, . , Promise<Promise<T>> ).

, Non-Blocking IO Node.js , . , JavaScript CPU ( ) . workers .

, promises JavaScript .

JavaScript . URL . import export .

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

// export         .
const b = 2;

export const a = 1;

Haskell, Python, Java JavaScript . URL , "" .

, JavaScript . Math Intl . JavaScript , .

. , Node.js npm , , Deno URL HTTP URL .

, " " " " .

JavaScript . . . , API( console.log()) , JavaScript .

JavaScript () , . , , , . JavaScript (DOM API ), Node.js( API ) . JavaScript ( ) , , , , , . JavaScript , . , API .

JavaScript . , JavaScript JavaScript .

, .

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