Javascript to check a checkbox if an other checkbox is checked

I have 2 checkboxes, consider chk1 and chk2. If one checkbox is checked, the second checkbox should be checked automatically and not viceversa. 开发者_JAVA技巧What should be the javascript? Can someone help me? Thank you!!

Here's a simple inline version to demonstrate the general idea. You might want to pull it out into a separate function in real world usage.

If you want chk2 to automatically stay in sync with any changes to chk1, but not do anything when chk2 is clicked go with this.

 <input id="chk1" type="checkbox" onclick="document.getElementById('chk2').checked = this.checked;">
 <input id="chk2" type="checkbox">

This version will only change chk2 when chk1 is checked, but not do anything when ck1 is unchecked.

 <input id="chk1" type="checkbox" onclick="if (this.checked) document.getElementById('chk2').checked = true;">
 <input id="chk2" type="checkbox">

var chk1 = document.getElementById('chk1');
var chk2 = document.getElementById('chk2');

if ( chk1.checked )
      chk2.checked = true;

For the first one:

document.getElementById('chk1').checked = document.getElementById('chk2').checked;

For the second one:

document.getElementById('chk2').checked = document.getElementById('chk1').checked;