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var - JavaScript | MDN

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var

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var , .

In this article

var x = 1;

if (x === 1) {
  var x = 2;

  console.log(x);
  // Expected output: 2
}

console.log(x);
// Expected output: 2

js

    var varname1 [= value1 [, varname2 [, varname3 ... [, varnameN]]]];

varnameN: . .
valueN: . .

. var , , .

( ) . :

(execution context) . .

js

function x() {
  y = 1; // strict  ReferenceError .
  var z = 2;
}

x();

console.log(y); //  "1" .
console.log(z); // ReferenceError: z is not defined outside x .

. .

js

console.log(a); // ReferenceError .
console.log("still going..."); //   .

js

var a;
console.log(a); //    "undefined"  "" .
console.log("still going..."); //  "still going..." .

(execution context) . . (e.g .)

js

var a = 1;
b = 2;

delete this.a; // strict  TypeError .    .
delete this.b;

console.log(a, b); // ReferenceError .
// 'b'  ,   .

, . , . ECMAScript 5 strict mode, .

var (hoisting)

( ) , . . "(hoisting)" .

js

bla = 2;
var bla;
// ...

//       :

var bla;
bla = 2;

, ( ) . () , .

js

var a = 0,
  b = 0;

js

var a = "A";
var b = a;

//  :

var a,
  b = (a = "A");

:

js

var x = y,
  y = "A";
console.log(x + y); // undefinedA

, x y , . "x = y" , y ReferenceError 'undefined' . , x undefined . , y 'A' . , , x === undefined && y === 'A', .

js

var x = 0;

function f() {
  var x = (y = 1); // x  . y !
}
f();

console.log(x, y); // 0, 1
// x  
// y leaked outside of the function, though!

.

js

var x = 0; // x  , 0 .

console.log(typeof z); // undefined, z   .

function a() {
  // a   ,
  var y = 2; // y  a  , 2 .

  console.log(x, y); // 0 2

  function b() {
    // b  ,
    x = 3; //   x 3 ,   var   .
    y = 4; //   y 4 ,   var   .
    z = 5; //   z   5  .
  } // (strict mode ReferenceError .)

  b(); //  b   z .
  console.log(x, y, z); // 3 4 5
}

a(); //  a  b .
console.log(x, z); // 3 5
console.log(typeof y); // undefined y function a  .

Specification
ECMAScript 2027 LanguageSpecification # sec-variable-statement

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