title: Finding a substring and inserting another string
body: <p>Suppose I have string variable such as:</p>
  
  <pre><code>var a = "xxxxxxxxhelloxxxxxxxx";
  </code></pre>
  
  <p>or:</p>
  
  <pre><code>var a = "xxxxhelloxxxx";
  </code></pre>
  
  <p>I want to insert <code>"world"</code> after <code>"hello"</code>. </p>
  
  <p>I can't use <code>substr()</code> because the position is not known ahead of time. How can I do this in JavaScript or jQuery?</p>