你可能不知道的10个JavaScript小技巧

1

var

myVar   =

"3.14159"

,

2

str     =

""

+ myVar,

//  to string

3

int     = ~~myVar,

//  to integer

4

float   = 1*myVar,

//  to float

5

bool    = !!myVar,

/*  to boolean - any string with length

6

and any number except 0 are true */

7

array   = [myVar];

//  to array

1

(int).toString(16);

// converts int to hex,eg 12 =>"C" 

2

(int).toString(8);

// converts int to octal,eg. 12 =>"14" 

3

parseInt(string,16)

// converts hex to int,eg. "FF" =>255 

4

parseInt(string,8)

// converts octal to int,eg. "20" =>16

1

0xFF;

// Hex declaration,returns 255 

2

020;

// Octal declaration,returns 16 

3

1e3;

// Exponential,same as 1 * Math.pow(10,3),returns 1000 

4

(1000).toExponential();

// Opposite with previous,returns 1e3 

5

(3.1415).toFixed(3);

// Rounding the number,returns "3.142"

01

var

JS_ver  = [];

02

(Number.prototype.toFixed)?JS_ver.push(

"1.5"

):

false

;

03

([].indexOf && [].forEach)?JS_ver.push(

"1.6"

):

false

;

04

((

function

(){

try

{[a,b] = [0,1];

return

true

;}

catch

(ex) {

return

false

;}})())?JS_ver.push(

"1.7"

):

false

;

05

([].reduce && [].reduceRight && JSON)?JS_ver.push(

"1.8"

):

false

;

06

(

""

.trimLeft)?JS_ver.push(

"1.8.1"

):

false

;

07

JS_ver.supports =

function

()

08

{

09

if

(arguments[0])

10

return

(!!~

this

.join().indexOf(arguments[0] +

","

) +

","

);

11

else

12

return

(

this

[

this

.length-1]);

13

}

14

alert(

"Latest Javascript version supported: "

+ JS_ver.supports());

15

alert(

"Support for version 1.7 : "

+ JS_ver.supports(

"1.7"

));

01

// BAD: This will cause an error in code when foo is undefined

02

if

(foo) {

03

　　doSomething();

04

}

05

// GOOD: This doesn't cause any errors. However,even when 

06

// foo is set to NULL or false,the condition validates as true 

07

if

(

typeof

foo !=

"undefined"

) {

08

　　doSomething();

09

}

10

// BETTER: This doesn't cause any errors and in addition

11

// values NULL or false won't validate as true

12

if

(window.foo) {

13

　　doSomething();

14

}

1

// UGLY: we have to proof existence of every

2

// object before we can be sure property actually exists

3

if

(window.oFoo && oFoo.oBar && oFoo.oBar.baz) {

4

　　doSomething();

5

}

1

function

doSomething(arg0,arg1,arg2,arg3,arg4) {

2

　　...

3

}

4

doSomething(

''

,

'foo'

,5,[],

false

);

01

function

doSomething(){

02

// Leaves the function if nothing is passed

03

if

(!arguments[0]) {

04

return

false

;

05

　　}

06

var

oArgs   = arguments[0]

07

　　arg0    = oArgs.arg0 ||

""

,

08

　　arg1    = oArgs.arg1 ||

""

,

09

　　arg2    = oArgs.arg2 || 0,

10

　　arg3    = oArgs.arg3 || [],

11

　　arg4    = oArgs.arg4 ||

false

;

12

}

13

doSomething({

14

　　arg1    :

"foo"

,

15

　　arg2    : 5,

16

　　arg4    :

false

17

});

01

function

createList(){

02

var

aLI = [

"first item"

,

"second item"

,

"third item"

,

03

"fourth item"

,

"fith item"

];

04

// Creates the fragment

05

var

oFrag   = document.createDocumentFragment();

06

while

(aLI.length) {

07

var

oLI = document.createElement(

"li"

);

08

// Removes the first item from array and appends it

09

// as a text node to LI element

10

　　　　oLI.appendChild(document.createTextNode(aLI.shift()));

11

　　　　oFrag.appendChild(oLI);

12

　　}

13

　　document.getElementById(

'myUL'

).appendChild(oFrag);

14

}

01

var

sFlop   =

"Flop: [Ah] [Ks] [7c]"

;

02

var

aValues = {

"A"

:

"Ace"

,

"K"

:

"King"

,7:

"Seven"

};

03

var

aSuits  = {

"h"

:

"Hearts"

,

"s"

:

"Spades"

,

04

"d"

:

"Diamonds"

,

"c"

:

"Clubs"

};

05

sFlop   =sFlop.replace(/\[\w+\]/gi,

function

(match) {

06

　　match   = match.replace(match[2],aSuits[match[2]]);

07

　　match   = match.replace(match[1],aValues[match[1]] +

" of "

);

08

return

match;

09

});

10

// string sFlop now contains:

11

// "Flop: [Ace of Hearts] [King of Spades] [Seven of Clubs]"

01

outerloop:

02

for

(

var

iI=0;iI<5;iI++) {

03

if

(somethingIsTrue()) {

04

// Breaks the outer loop iteration

05

break

outerloop;

06

　　}

07

　　innerloop:

08

for

(

var

iA=0;iA<5;iA++) {

09

if

(somethingElseIsTrue()) {

10

// Breaks the inner loop iteration

11

break

innerloop;

12

　　}

13

　　}

14

}