js中小数的格式化

因为精度问题，JS中小数运算经常结果会变的很长，不简洁，这个时候推荐toFixed()；

具体例子
var numObj = 12345.6789;

numObj.toFixed(); // Returns ‘12346’: note rounding, no fractional part
numObj.toFixed(1); // Returns ‘12345.7’: note rounding
numObj.toFixed(6); // Returns ‘12345.678900’: note added zeros
(1.23e+20).toFixed(2); // Returns ‘123000000000000000000.00’
(1.23e-10).toFixed(2); // Returns ‘0.00’
2.34.toFixed(1); // Returns ‘2.3’
2.35.toFixed(1); // Returns ‘2.4’. Note it rounds up
2.55.toFixed(1); // Returns ‘2.5’. Note it rounds down – see warning above
-2.34.toFixed(1); // Returns -2.3 (due to operator precedence, negative number literals don’t return a string…)
(-2.34).toFixed(1); // Returns ‘-2.3’ (…unless you use parentheses)