How to detect string which contains only spaces?

The fastest solution would be using the regex prototype function test() and looking for any character that is not a space, tab, or line break \S :

if (!/\S/.test(str)) {
  // Didn't find something other than a space which means it's empty
}

In case you have a super long string, it can make a significant difference as it will stop processing as soon as it finds a non-space character.

To achieve this you can use a Regular Expression to remove all the whitespace in the string. If the length of the resulting string is 0, then you can be sure the original only contained whitespace. Try this:

var str = "    ";
if (!str.replace(/\s/g, '').length) {
  console.log('string only contains whitespace (ie. spaces, tabs or line breaks)');
}

Similar to Rory's answer, with ECMA 5 you can now just call str.trim().length instead of using a regular expression. If the resulting value is 0 you know you have a string that contains only spaces.

if (!str.trim().length) {
  console.log('str is empty!');
}

You can read more about trim here.

Edit: After looking at this a few years later I noticed this could be simplified further. Since the result of trim will either be truthy or falsy you can also do the following:

if (!str.trim()) {
     console.log('str is empty!');
}