JAVASCRIPT基础学习篇（8）－－ECMAScript Basic4（EcmaScript 表达式）

 第四章 表达式

    1、The if statement

if (condition) statement1 else statement2

The condition can be any expression条件可为任意一表达式，js会计算出结果，若为true则statement1...

if (condition1) statement1 else if (condition2) statement2 else statement3

    2、Iterative statements迭代

do {

      statement

} while (expression);

while(expression) statement

for (initialization; expression; post-loop-expression) statement

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

alert(i);

}

for-in

The for-in statement is a strict iterative statement. It is used to enumerate the properties of an object.

Syntax:

for (property in expression) statement

For example:

for (sProp in window) {

alert(sProp);

}

Labeled statements

It is possible to label statements for later use with the following syntax:

label: statement

For example:

start: var iCount = 10;

The break and continue statements

Both the break and continue statements can be used in conjunction with labeled statements to return

to a particular location in the code. This is typically used when there are loops inside of loops, as in the

following example:

var iNum = 0;

outermost:

for (var i=0; i < 10; i++) {

for (var j=0; j < 10; j++) {

if (i == 5 && j == 5) {

break outermost;

}

iNum++;

}

}

alert(iNum); //outputs “55”

The with statement

The with statement is used to set the scope of the code within a particular object. Its syntax is the following:

      with (expression) statement;

For example:

var sMessage = “hello world”;

with(sMessage) {

alert(toUpperCase()); //outputs “HELLO WORLD”

}

The switch statement

switch (expression) {

case value: statement

break;

case value: statement

break;

case value: statement

break;

...

case value: statement

break;

default: statement

}

Two big differences exist between the switch statement in ECMAScript and Java

In ECMAScript, the switch statement can be used on strings, and it can indicate case by nonconstant values:

在java中switch(expression)其中expression必须是int类型或能够转化为int类型的如char,bit,short int等，显然不能是字符串，而在js中是可以的，在c中也是可以的。

var BLUE = “blue”, RED = “red”, GREEN = “green”;

switch (sColor) {

case BLUE: alert(“Blue”);

break;

case RED: alert(“Red”);

break;

case GREEN: alert(“Green”);

break;

default: alert(“Other”);

}