Function join

join(
    element?: any,
    separator?: null | string,
    filter?: (item?: any) => boolean,
): undefined | string
Returns the joined string with the given element(s) and delimiter
Parameters
- Optionalelement: any
  the source element(s) to inspect
- Optionalseparator: null | string
  the delimiter to append between each element of the given element(s)
- Optionalfilter: (item?: any) => boolean
  the filter to check each element of the given element(s) should be included
Returns undefined | string
the joined string with the given element(s) and delimiter
Author
David Hsing
Example
```
join('foobar');    // 'foobar'
join(['foo', 'bar'], undefined);    // 'foobar'
join(['foo', 'bar', 'world'], undefined, (text => text !== 'world'));    // 'foobar'
```