- 只要意在保存对象的变量还没有真正保存对象,就应该让该变量保存null值。
- Number()函数的转换规则
- 如果是null,返回0;
- 如果是undefined,返回NaN;
- 如果是字符串,转换数字形式字符串。字符串为空,转换为0,其他转换为NaN;
- 如果是对象,则调用对象的valueOf(),然后按照3转换。如果转换为NaN,则调用toString(),然后按照3转换。
- parseInt()
- 忽略字符串前面的空格,直找到第一个非空格字符。
- 如果第一个字符不是数字字符或者负号,parseInt()返回NaN;
- parseInt()转换空字符会返回NaN,(Number()对空字符返回0)
- parseInt()会解析,直到遇到一个非数字字符。
- parseFloat()
- parseFloat()从第一个字符(位置0)开始解析,直到遇到一个无效的浮点数字字符(第一个小数点有效,第二个无效),或最后。
- 转换为字符串
- 数值,布尔值,对象和字符串值都有toString()。但是null和undefined没有这个方法
- 相等操作符
- null==undefined 会返回true,因为它们是类似的值。但是null===undefined返回false,因为它们是不同的类型的值。
- for-in语句
- for-in循环来显示BOM中window对象的所有属性,每次执行循环都会将window对象中存在的一个属性名赋值给变量propName。
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)
- 如果表示要迭代的对象的变量值为null或undefined,for-in会抛错误;后更正为不执行循环体。
- 通过for-in循环输出的属性名的顺序是不可预测的,但是属性都会被返回一次。
- 如果定义了二个名字相同的函数,则该名字只属于后定义的函数
posted on
2016-04-06 17:08
宜荷
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