'IsNullOrWhitespace' in JavaScript?

For a succinct modern cross-browser implementation, just do:

function isNullOrWhitespace( input ) {
  return !input || !input.trim();
}

Here's the jsFiddle. Notes below.

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The currently accepted answer can be simplified to:

function isNullOrWhitespace( input ) {
  return (typeof input === 'undefined' || input == null)
    || input.replace(/\s/g, '').length < 1;
}

And leveraging falsiness, even further to:

function isNullOrWhitespace( input ) {
  return !input || input.replace(/\s/g, '').length < 1;
}

trim() is available in all recent browsers, so we can optionally drop the regex:

function isNullOrWhitespace( input ) {
  return !input || input.trim().length < 1;
}

And add a little more falsiness to the mix, yielding the final (simplified) version:

function isNullOrWhitespace( input ) {
  return !input || !input.trim();
}

no, but you could write one

function isNullOrWhitespace( str )
{
  // Does the string not contain at least 1 non-whitespace character?
  return !/\S/.test( str );
}

It's easy enough to roll your own:

function isNullOrWhitespace( input ) {

    if (typeof input === 'undefined' || input == null) return true;

    return input.replace(/\s/g, '').length < 1;
}