truncate  source npm
_.truncate([string=''], [options])
截断字符串,如果字符串超出了限定的最大值。
被截断的字符串后面会以 omission 代替,omission 默认是 "..."。
参数
- [string=''] (string) 要截断的字符串 
- [options] (Object) 选项对象 
- [options.length=30] (number) 允许的最大长度 
- [options.omission='...'] (string) 超出后的代替字符 
- [options.separator] (RegExp|string) 截断点 
返回值 (string)
返回截断后的字符串
示例
_.truncate('hi-diddly-ho there, neighborino');
// => 'hi-diddly-ho there, neighbo...'
_.truncate('hi-diddly-ho there, neighborino', {
  'length': 24,
  'separator': ' '
});
// => 'hi-diddly-ho there,...'
_.truncate('hi-diddly-ho there, neighborino', {
  'length': 24,
  'separator': /,? +/
});
// => 'hi-diddly-ho there...'
_.truncate('hi-diddly-ho there, neighborino', {
  'omission': ' [...]'
});
// => 'hi-diddly-ho there, neig [...]'