Check all child checkboxes

Question

I need to check all child checkboxes when a user check one parent checkbox. For example, if a user checks the checkbox with id ='s9' then checkboxes with id s15 , s16 and s116 must be checked, because they have parentId='s9' .

<span class='font-bold'><input id='s1'  type='checkbox'/>bla</span><br/>
<span class='left-indent'><input id='s2' parentId='s1' type='checkbox'/>bla11</span><br/>
<span class='left-indent'><input id='s3' parentId='s1' type='checkbox'/>bla12><br/>
<span class='left-indent'><input id='s4' parentId='s1' type='checkbox'/>bla13</span><br/>
<span class='left-indent'><input id='s5' parentId='s1' type='checkbox'/>bla14</span><br/>
<span class='left-indent'><input id='s6' parentId='s1' type='checkbox'/>bla15</span><br/>


<span class='font-bold'><input id='s8'  type='checkbox'/>bla2</span><br/>
<span class='font-bold'><input id='s9'  type='checkbox'/>bla3</span><br/>
<span class='left-indent'><input id='s15' parentId='s9' type='checkbox'/>bla31</span><br/>
<span class='left-indent'><input id='s16' parentId='s9' type='checkbox'/>bla32</span><br/>
<span class='left-indent'><input id='s116' parentId='s9' type='checkbox'/>bla32</span><br/>

<span class='font-bold'><input id='s10'  type='checkbox'/>bla4</span><br/>
<span class='font-bold'><input id='s11'  type='checkbox'/>bla5</span><br/> 

UPDATE

If some child are checked, parent must be checked too.

If all childs are unchecked, parent must be unchecked too.

Answer 1

You shouldn't use parentId as a custom attribute as it will invalidate your HTML. Use classnames instead:

<input id="s9" type="checkbox" />
<input id="s15" class="parent-s9" type="checkbox" />
<input id="s16" class="parent-s9" type="checkbox" />

Example:

$('input[type=checkbox]').change(function() {
    // get id of the current clicked element
    var id = $(this).attr('id');
    // find elements with classname 'parent-<id>' and (un)check them
    var children = $('.parent-' + id).attr('checked', $(this).attr('checked'));
});

Live example here.