Websites Performance tunning

1. Page Load:
1) External SCRIPT tags have a negative impact on page performance because of their blocking
behavior. So script should be laoded load dynamic after page load.(script dom element/script defer)

2) Inline script also block the resource load, so we should move inline script to the bottom.(script defer)

2. Efficient Script:
1) Use local variables.(scope chain: local variable < array < object < global)

2) Condition:
   Use the if statement when:
      There are no more than two discrete values for which to test.
      There are a large number of values that can be easily separated into ranges.
   Use the switch statement when:
      There are more than two but fewer than 10 discrete values for which to test.
      There are no ranges for conditions because the values are nonlinear.
   Use array lookup when:
      There are more than 10 values for which to test.
      The results of the conditions are single values rather than a number of actions
to be taken.

3) Loop: reference: [http://flynndang.iteye.com/blog/676977]

4) String: using + is perferred.

5) String trim:

text.replace(/^\s+/, "").replace(/\s+$/, "");

6) setTimeout delay between 50 and 100 milliseconds.

3. Image:
1)JPEG for photos, GIF for animations, and PNG for everything else. Strive for PNG8 whenever possible.(png8 < png24 < png32)

2)Don’t scale images in HTML, resize image first.

4. Domains:
1) Two domain is a good rule.

5. Iframe:
1) Iframe block the onload event.So we should void using iframe.