JS 中的 with 关键字

with (expression) statement;

var qs = location . search . substring ( 1 );

var hostName = location . hostname ;

var url = location . href ;

with ( location ){

var qs = search . substring ( 1 );

var hostName = hostname ;

var url = href ;

}

function func () {

console . time ( "func" );

var obj = {

a : [ 1 , 2 , 3 ]

};

for ( var i = 0 ; i < 100000 ; i ++ ) {

var v = obj . a [ 0 ];

}

console . timeEnd ( "func" ); //0.847ms

}

func ();

function funcWith () {

  console . time ( "funcWith" );

var obj = {

a : [ 1 , 2 , 3 ]

};

var obj2 = { x : 2 };

with ( obj2 ) {

console . log ( x );

         for ( var i = 0 ; i < 100000 ; i ++ ) {  

var v = obj . a [ 0 ];

}

}

console . timeEnd ( "funcWith" ); //84.808ms

}

funcWith ();

function funcWith () {

console . time ( "funcWith" );

var obj = {

a : [ 1 , 2 , 3 ]

};

with ( obj ) {

for ( var i = 0 ; i < 100000 ; i ++ ) {

var v = a [ 0 ];

}

}

console . timeEnd ( "funcWith" ); //88.260ms

}

funcWith ();

function foo ( obj ) {

with ( obj ) {

a = 2 ;

}

}

var o1 = {

a : 3

};

var o2 = {

b : 3

};

foo ( o1 );

console . log ( o1 . a ); // 2

foo ( o2 );

console . log ( o2 . a ); // undefined

console . log ( a ); // 2

var obj = {

x : 10 ,

foo : function () {

with ( this ) {

var x = 20 ;

var y = 30 ;

console . log ( y ); //30

}

}

};

obj . foo ();

console . log ( obj . x ); //20

console . log ( obj . y ); //undefined

var obj = {

x : 10 ,

foo : function () {

var x ; //声明局部变量x

var y ; //声明局部变量y

with ( obj ) {

x = 20 ; //访问变量x，在obj上找到x，则修改为20

y = 30 ; //访问变量y，在bojg上找不到y，则进一步查找，找到局部变量y，修改为30

console . log ( y ); //30//直接输出局部变量y，

}

}

};

obj . foo ();

console . log ( obj . x ); //20，obj.x已被修改为20

console . log ( obj . y ); //undefined，obj不存在y属性，则为undefined

({

x : 10 ,

foo : function () {

function bar () {

console . log ( x );

console . log ( y );

console . log ( this . x );

}

with ( this ) {

var x = 20 ;

var y = 30 ;

bar . call ( this );

}

}

}). foo ();