title: Javascript: How to have value in string represented by %s and then replaced with a value
body: <p>I know this a really stupid question.</p>
  
  <p>I've had a good few years experience with javascript but this one thing seems to have skipped my mind, my head has gone blank and I can't remember what it's called and how I would go about doing it.</p>
  
  <p>Basically what I'm looking for is when you have a string variable such as:</p>
  
  <pre><code>var error_message = "An account already exists with the email: %s"
  </code></pre>
  
  <p>And you then pass a string somehow into this and it replaces the %s.</p>
  
  <p>I probably sound really idiotic, but I'd really appreciate the help / reminding!</p>
  
  <p>Thanks guys.</p>