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JavaScript Java, C, C++ Awk, Perl, Python

JavaScript Unicode Frh ( "early" )

const Frh = "foobar";

JavaScript frh Frh

JavaScript (statement) (;)

: ECMAScript (ASI)JavaScript

JavaScript

C++

// 

/* 
   
 */

*/

/* /*  */ SyntaxError  */

*/

/*  /*  *\/  */

: JavaScript #!/usr/bin/env node

JavaScript

JavaScript 3

var
let
const

JavaScript (_) ($) (09) JavaScript "A" "Z" "a" "z"

Unicode Unicode

Number_hitstemp99_name

var var x = 42
const let let y = 13

const { bar } = foo bar foo

JavaScript

let x = 42 let x = 42 ReferenceError var let undefined

let x;
console.log(x); // "undefined"

let x = 42 let x; x = 42

const undefined

const x; // SyntaxError: Missing initializer in const declaration

let const

if (Math.random() > 0.5) {
  const y = 5;
}
console.log(y); // ReferenceError: y is not defined

var

5 x x if

if (true) {
  var x = 5;
}
console.log(x); // x  5

var var undefined undefined

console.log(x === undefined); // true
var x = 3;

(function () {
  console.log(x); // undefined
  var x = "local value";
})();

var x;
console.log(x === undefined); // true
x = 3;

(function () {
  var x;
  console.log(x); // undefined
  x = "local value";
})();

var

let const ReferenceError

console.log(x); // ReferenceError
const x = 3;

console.log(y); // ReferenceError
let y = 3;

var

window window. globalThis JavaScript

window frame phoneNumber iframe parent.phoneNumber

const ($)

const PI = 3.14;

let

// THIS WILL CAUSE AN ERROR
function f() {}
const f = 5;

// 
function f() {
  const g = 5;
  var g;
}

const

const MY_OBJECT = { key: "value" };
MY_OBJECT.key = "otherValue";

const MY_ARRAY = ["HTML", "CSS"];
MY_ARRAY.push("JAVASCRIPT");
console.log(MY_ARRAY); // ['HTML', 'CSS', 'JAVASCRIPT'];

ECMAScript 8

7
1. true false
2. nullnull JavaScript null Null NULL
3. undefined
4. 42 3.14159
5. 9007199254740992n
6. "Howdy"

JavaScript

let answer = 42;

answer = "Thanks for all the fish!";

JavaScript

'+'

+ JavaScript

x = " " + 42; // " 42"
y = 42 + " "; // "42 "
z = "37" + 7; // "377"

JavaScript

"37" - 7; // 30
"37" * 7; // 259

parseInt

: parseInt

parseInt("101", 2); // 5

+ () Number()

"1.1" + "1.1"; // '1.11.1'
(+"1.1") + (+"1.1"); // 2.2
// :

JavaScript

0 ([]) (length)

3 coffees (length) 3

const coffees = ["French Roast", "Colombian", "Kona"];

: Array Array Array

2 fish

const fish = ["Lion", , "Angel"];

console.log(fish);
// [ 'Lion', <1 empty item>, 'Angel' ]

2 "empty" undefined Array.prototype.map fish[1] undefined

(length) 3 myList[3] myList[1]

const myList = ["home", , "school"];

(length) 4 myList[0] myList[2]

const myList = [, "home", , "school"];

(length) 4 myList[1] myList[3]

const myList = ["home", , "school", ,];

:> git diff

diff

const myList = [
  "home",
  "school",
+ "hospital",
];

JavaScript

undefined

const myList = ["home", /*  */, "school", /*  */, ];

2 true false

: true false Boolean true false

Boolean Boolean

JavaScript 10

-123.4 - 123.4

(BigInt) 10 16 8 2

10 0
0 0o 0O8 8 0 7
0x 0X16 16 (0 9) a f A F 0xa = 0xA = 10 0xf = 0xF = 15
0b 0B2 2 0 1
n (BigInt) 0123n 0 8 0o123n

0, 117, 123456789123456789n             10 
015, 0001, 0o777777777777n              8 
0x1123, 0x00111, 0x123456789ABCDEFn     16 
0b11, 0b0011, 0b11101001010101010101n   2

10
(.)
10

e E + - 1 e E

[digits].[digits][(E|e)[(+|-)]digits]

3.1415926
.123456789
3.1E+12
.1e-23

0 ({})

: {

car myCar "Saturn" 2 getCar (carTypes("Honda")); 3 special (sales)

const sales = "";

function carTypes(name) {
  return name === "" ? name : `${name}`;
}

const car = { myCar: "", getCar: carTypes(""), special: sales };

console.log(car.myCar); // 
console.log(car.getCar); // 
console.log(car.special); //

const car = { manyCars: { a: "", b: "" }, 7: "" };

console.log(car.manyCars.b); // 
console.log(car[7]); //

JavaScript

(.)

const unusualPropertyNames = {
  "": "",
  "!": "",
};
console.log(unusualPropertyNames.""); // SyntaxError: Unexpected string
console.log(unusualPropertyNames.!); // SyntaxError: Unexpected token !

([])

console.log(unusualPropertyNames[""]); // 
console.log(unusualPropertyNames["!"]); // !

foo: foo super

const obj = {
  // __proto__
  __proto__: theProtoObj,
  //  'handler: handler' 
  handler,
  // 
  toString() {
    // 
    return `d ${super.toString()}`;
  },
  // 
  ["prop_" + (() => 42)()]: 42,
};

const re = /ab+c/;

0 (") (') ()

'foo'
"bar"
'1234'
'one line \n another line'
"Joyo's cat"

String String String

String JavaScript String String length

// 
console.log("Joyo's cat".length); //  10

(`) (grave accent)

Perl Python

// 
`JavaScript  '\n' `;

// 
`JavaScript 
 
 `;

// 
const name = "Lev",
  time = "today";
`Hello ${name}, how are you ${time}?`;

print print [object Object]

const formatArg = (arg) => {
  if (Array.isArray(arg)) {
    // Print a bulleted list
    return arg.map((part) => `- ${part}`).join("\n");
  }
  if (arg.toString === Object.prototype.toString) {
    // This object will be serialized to "[object Object]".
    // Let's print something nicer.
    return JSON.stringify(arg);
  }
  return arg;
};

const print = (segments, ...args) => {
  // For any well-formed template literal, there will always be N args and
  // (N+1) string segments.
  let message = segments[0];
  segments.slice(1).forEach((segment, index) => {
    message += formatArg(args[index]) + segment;
  });
  console.log(message);
};

const todos = [
  "Learn JavaScript",
  "Learn Web APIs",
  "Set up my website",
  "Profit!",
];

const progress = { javascript: 20, html: 50, css: 10 };

print`I need to do:
${todos}
My current progress is: ${progress}
`;

// I need to do:
// - Learn JavaScript
// - Learn Web APIs
// - Set up my website
// - Profit!
// My current progress is: {"javascript":20,"html":50,"css":10}

print(["I need to do:\n", "\nMy current progress is: ", "\n"], todos, progress);

console.log

console.log("I need to do:\n%o\nMy current progress is: %o\n", todos, progress);

"one line \n another line";

JavaScript


`\0`
`\b`
`\f`
`\n`
`\r`
`\t`
`\v`
`\'`
`\"`
`\\`	(\)
`\XXX`	Latin-1 3 8 `XXX` `0` `377` `\251`
`\xXX`	Latin-1 2 16 `XX` `00` `FF` `\xA9`
`\uXXXX`	Unicode 4 16 `XXXX` `\u00A9` Unicode
`\u{XXXXX}`	Unicode `\u{2F804}` Unicode `\uD87E\uDC04`

const quote = "He read \"The Cremation of Sam McGee\" by R.W. Service.";
console.log(quote);

He read "The Cremation of Sam McGee" by R.W. Service.

c:\temp

const home = "c:\\temp";

const str =
  "this string \
is broken \
across multiple \
lines.";
console.log(str); //

JavaScript

MDN

MDN 2026317

GitHub