title: Why historically was `substr` added when `substring` already existed?
body: <p>I know what the difference is between <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substr" rel="noreferrer"><code>substr(start[, length])</code></a> and <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substring" rel="noreferrer"><code>substring(start[, end])</code></a>. This is addressed e.g. in this earlier question: <a href="https://stackoverflow.com/questions/3745515/what-is-the-difference-between-substr-and-substring">What is the difference between substr and substring?</a></p>
  
  <p>But <em>why</em> did they add <code>substr</code> later when <code>substring</code> was already present? I am looking for information about the history of this decision.</p>
  
  <p>I have found that <code>substring</code> was there from the beginning and then 'suddenly' in Ecmascript 3rd edition they have two different functions to achieve the exact same thing with different arguments. Why?</p>