Expressions and Statement

JavaScript 两个最重要的概念表达式和语句

表达式概念：An expression is a phrase of JavaScript that a JavaScript interpreter can evaluate to produce a a value.

表达式的分类:

1.Primary Expressions

"hi word"
/pattern/

//some of JavaScript reserved word are primary expressions
true
false
null
this //Evaluates to the "current" object

//finally the third type of primary expressions is the bare variable reference
I
sum
undefined // undefined is a global variable ,not like null.

2.Object and Array Initalizers

// Array initializer
 [1,5,2,6,5]

// Object initializer
{x:2,y3}

3. Function Definition Expressions

//*function expression
function(x) { return x*x }

//* function statement 
function foo(){
 console.log("bar");
};

4.Property Access expression & Invoication expression & Object Creation expression

//Property Access expression
var o={x:1,y:{z:3}}
var a=[o,1,4,[5,6]]

o.x //property x of expression o
o["y"] //property x of expression o
a[1] //element at index 1 of object o
a[0].x // property x of expression a[0]

//Invocation expression
foo(1,2)
a.sort()
Math.max(x,y,z)

//object creation expression
new Object()
new Point(2,3)

5.Arithmetic Expression & Logical Expression & Assignment Expression

//Arithmetic 
1+1
2*5
3-4
6%9
5--
6++

//Logical
7&8
7<<2
^0x0f
// Assignment
I=6
o.x=2

x +=x+1
.......

语句概念：statements are JavaScript sentences or commands；Expressions are evaluated to produce a value, but statements are
executed to make something happen.

语句的分类:

Statement	Syntax	Purpose
break	break [label];	Exit from the   innermost loop or switch or from named enclosing statement
case	case expression:	Label a statement   within a switch
continue	continue   [label];	Begin next iteration   of the innermost loop or the named loop
debugger	debugger;	Debugger breakpoint
default	default:	Label the default   statement within a switch
do/while	do statement while   (expression);	An alternative to the   while loop
empty ;	Do nothing
for	for(init; test; incr)   statement	An easy-to-use loop
for/in   for	(var in object)   statement	Enumerate the   properties of object
function	function   name([param[,...]]) { body }	Declare a function   named name
if/else	if (expr) statement1   [else statement2]	Execute statement1 or   statement2
label	label: statement	Give statement the   name label
return	return   [expression];	Return a value from a   function
switch	switch (expression) {   statements }	Multiway branch to   case or default: labels
throw	throw   expression;	Throw an exception
try	try { statements   }	Handle exceptions
	[catch { handler   statements }]
	[finally{ cleanup   statements }]
use   strict	use strict;	Apply strict mode   restrictions to script or function
var	var name [ = expr] [   ,... ];	Declare and   initialize one or more variables
while	while (expression)   statement	A basic loop   construct
with	with (object)   statement	Extend the scope   chain (forbidden in strict mode)

明确了概念后，我们需要非常清晰的分出来JavaScript的表达式和语句; 在后面的学习中你会发现表达实在是太重要了，它的作用域特殊，它的执行特殊；

/*确认下面code到底是什么?*/
var a="123";   //?
var a=1,b=2;//?
var a,b//?

var f=function(x){ return x+1; } //?
function() {} //?
function foo(){}//?

i++;//?
a+b;//?
(x+y);//?

{x:2}//?
var a={x:2}//?
a.x//?

/*block*/
while(x<10){
    x++;
}             //?

var x=1;
{
    var x=2;
}             //?

if(x>1){} //?