Undefined , . x Undefined .
var x;
Null . :
var x = null;
JavaScript . , . . :
myArr[3] = Hello!;
var x = myArr[3];
, :
var emptyArr = [];
var filledArr = ["Hello", "World"];
. :
var myArr = [[1,2],[3,4],[5,6]];
alert(myArr[2][1]);
// 32 ,
, , 6.
, :
var myArr = new Array();
JavaScript ; , .
{ }.
:
var n = j = k = 2;
JavaScript ,
Java: +, -, *, /, %, <<+=>>, << -=>>,...
" + " ,
s = "str1" + "str2"
++, 1 , -- 1 .
JavaScript :
<, <=, >, >=, !=, == ; === ( ); !== .
:
var x = 0;
var y = false;
alert(x==y);
// true -
alert(x===y);
// false, .
JavaScript :
&& ; || ; ! .
?
? , , , :
? 1: 2;
:
var x = 6; var y = 9;
var res = x < y? "x y": "x y";
|
|
|
alert(res);
// x y
typeof()
typeof() , : " number ", " string ", " boolean ", " object ", " function " " undefined ".
if
:
if ()
Else
else .
:
while ()
Do
while ();
for :
for ( ; ; )
for:
<script type="text/javascript">
for (var i = 0, j = 7; i < j; i++, j--) {
alert(i + ' ' + j);
}
// : 0 7, 1 6, 2 5, 3 4
</script>
forin :
for (_ in )
- :
<p id="myP">test</p>
<script type="text/javascript">
var objProp;
for (objProp in document.getElementById("myP").style) {
alert(objProp + ' = ' + document.getElementById("myP").style[objProp]);
}
</script>
style .
continue break, , .
with
with :
with (object) {
}
with:
<p id="myP">test</p>
<script type="text/javascript">
with (document.getElementById("myP").style) {
color = "red";
fontSize = "20px";
fontFamily = "Arial";
letterSpacing = "5px";
}
</script>
switch
switch () {
case 1:
break;
case 2:
break;
...
case N:
break;
default:
}
default , .
eval()
JavaScript , .
:
<body>
<input type="text" id="calc" />
<input type="button" value="Calculate!"
onclick=
"alert(eval(document.getElementById('calc').value))" />
</body>
JavaScript :
function _(_) {
;
return ()
}
. :
function hello() {
alert(Hello, World!)
}
hello();
// Hello, World!
, , JavaScript undefined.
|
|
|
return.
function sumIt(arg1, arg2, arg3) {
var res = arg1 + arg2;
if (arg3) res = res + arg3;
return res;
}
var x = 1; var y = 2; var z = 3;
alert(sumIt(x,y,z));
// 6
alert(sumIt(x,y));
// 3, , . ,
