1  2

let obj = {};

obj.x = 3;
console.log(obj.x); // 3 .
console.log(obj); // { x: 3 } .

const key = "y";
obj[key] = 5;
console.log(obj[key]); // 5 .
console.log(obj); // { x: 3, y: 5 } .

let val = 0;

console.log((val.x = 3)); // 3 .
console.log(val.x); // undefined .
console.log(val); // 0 .

var foo = ["one", "two", "three"];

//   
var one = foo[0];
var two = foo[1];
var three = foo[2];

//   
var [one, two, three] = foo;

//  x  f()  ,
// x = f()    .
let x = f();

//  x  g()  ,
// x = g()    .
x = g(); //  x g()  .

let x;
const y = (x = f()); //  const y = x = f();  .
console.log(y); // x = f()    .

console.log((x = f())); //    .

//      
//     
//         .
console.log([0, (x = f()), 0]);
console.log(f(0, (x = f()), 0));

function f() {
  console.log("F!");
  return 2;
}
function g() {
  console.log("G!");
  return 3;
}
let x, y;

y = x = f();
y = [f(), (x = g())];
x[f()] = g();

let z = (y = x = f());

const var1 = 3;
const var2 = 4;

1 / 2; // 0.5
1 / 2 == 1.0 / 2.0; // 

var a1 = true && true; // t && t true 
var a2 = true && false; // t && f false 
var a3 = false && true; // f && t false 
var a4 = false && 3 == 4; // f && f false 
var a5 = "Cat" && "Dog"; // t && t Dog 
var a6 = false && "Cat"; // f && t false 
var a7 = "Cat" && false; // t && f false 

var o1 = true || true; // t || t true 
var o2 = false || true; // f || t true 
var o3 = true || false; // t || f true 
var o4 = false || 3 == 4; // f || f false 
var o5 = "Cat" || "Dog"; // t || t Cat 
var o6 = false || "Cat"; // f || t Cat 
var o7 = "Cat" || false; // t || f Cat 

var n1 = !true; // !t false 
var n2 = !false; // !f true 
var n3 = !"Cat"; // !t false 

console.log(" " + ""); //  " " 

var mystring = "";
mystring += ""; // "" , mystring "" 

condition ? val1 : val2;

var status = age >= 18 ? "" : "";

var x = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
var a = [x, x, x, x, x];

for (var i = 0, j = 9; i <= j; i++, j--);
//                                  ^
console.log("a[" + i + "][" + j + "]= " + a[i][j]);

delete object.property;
delete object[propertyKey];
delete objectName[index];

delete Math.PI; // false  (    )

const myObj = { h: 4 };
delete myobj.h; // true  (    )

typeof operand;
typeof (operand);

var myFun = new Function("5 + 2");
var shape = "round";
var size = 1;
var foo = ["Apple", "Mango", "Orange"];
var today = new Date();

typeof myFun; // "function" 
typeof shape; // "string" 
typeof size; // "number" 
typeof foo; // "object" 
typeof today; // "object" 
typeof dontExist; // "undefined" 

typeof true; // "boolean" 
typeof null; // "object" 

typeof 62; // "number" 
typeof "Hello world"; // "string" 

typeof document.lastModified; // "string" 
typeof window.length; // "number" 
typeof Math.LN2; // "number" 

typeof blur; // "function" 
typeof eval; // "function" 
typeof parseInt; // "function" 
typeof shape.split; // "function" 

typeof Date; // "function" 
typeof Function; // "function" 
typeof Math; // "object" 
typeof Option; // "function" 
typeof String; // "function" 

void (expression);
void expression;

propNameOrNumber in objectName;

// 
var trees = ["redwood", "bay", "cedar", "oak", "maple"];
0 in trees; // true 
3 in trees; // true 
6 in trees; // false 
"bay" in trees; // false  (   
//    )
"length" in trees; // true  (length Array )

//  
"PI" in Math; // true 
var myString = new String("coral");
"length" in myString; // true 

//   
var mycar = { make: "Honda", model: "Accord", year: 1998 };
"make" in mycar; // true 
"model" in mycar; // true 

objectName instanceof objectType;

var theDay = new Date(1995, 12, 17);
if (theDay instanceof Date) {
  //  
}

this["propertyName"];
this.propertyName;

function validate(obj, lowval, hival) {
  if (obj.value < lowval || obj.value > hival) console.log(" !");
}

<p>18 99   :</p>
<input type="text" name="age" size="3"  />

var a = 1;
var b = 2;
var c = 3;

//  
a + b * c; // 7
//   
a + (b * c); // 7

//  
//    
(a + b) * c; // 9

// ...  
a * c + b * c; // 9

var objectName = new objectType([param1, param2, ..., paramN]);

super([arguments]); //   
super.functionOnParent([arguments]);