An empty string is a string containing zero characters, written as two quotation marks: "". It can represent missing text without being the same as a number or another kind of value. section 6.2.1
Removing characters from a phone number
Replacing parentheses and hyphens with "" removes them from the phone number while leaving the digits. Calling replace() once for each unwanted character produces a consistent, character-free value. section 4.3.4
Testing an edge case
A longest_word function given ['', ''] should return ''. The test checks behavior when every candidate contains no characters. section 6.2.1
