1、js语句以；号结束

var num = 1;

2、标识符(变量名)

标识符由字母、美元符号($)、下划线、数字组成，其中不能以数字开头。

var sum_account = 0;

注意保留关键字是不可以用于命名变量的：

abstract	arguments	boolean	break	byte
case	catch	char	class*	const
continue	debugger	default	delete	do
double	else	enum*	eval	export*
extends*	false	final	finally	float
for	function	goto	if	implements
import*	in	instanceof	int	interface
let	long	native	new	null
package	private	protected	public	return
short	static	super*	switch	synchronized
this	throw	throws	transient	true
try	typeof	var	void	volatile
while	with	yield