Info

This test case compares three alternative isFunction implementations:

• isFunctionA checks the internal [[Class]] name of an object to determine if it is a function. Unfortunately, while this check is the most reliable, it's also less efficient due to the extra call to Object#toString.
• isFunctionB was proposed as an alternative to the typeof and instanceof operators by Garrett Smith. This implementation uses duck-typing, so it may produce incorrect results.
• isFunctionC is currently used in Underscore.js. This implementation uses only basic duck-typing; thus, it is the most likely to produce incorrect results.

Preparation code

<script>
  var getClass = {}.toString,
      hasProperty = {}.hasOwnProperty,
      expression = /Kit/g;
  
  function Test() {}
  
  // Checks the internal [[Class]] name of the object.
  
  
  function isFunctionA(object) {
   return object && getClass.call(object) == '[object Function]';
  }
  
  // Partial duck-typing implementation by Garrett Smith.
  
  
  function isFunctionB(object) {
   if (typeof object != 'function') return false;
   var parent = object.constructor && object.constructor.prototype;
   return parent && hasProperty.call(parent, 'call');
  }
  
  // Pure duck-typing implementation taken from Underscore.js.
  
  
  function isFunctionC(object) {
   return !!(object && object.constructor && object.call && object.apply);
  }
</script>

Test runner

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isFunctionA(Test);
isFunctionA(getClass);
isFunctionA(hasProperty);
!isFunctionA(expression);

isFunctionB(Test);
isFunctionB(getClass);
isFunctionB(hasProperty);
!isFunctionB(expression);

isFunctionC(Test);
isFunctionC(getClass);
isFunctionC(hasProperty);
!isFunctionC(expression);

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Revisions

You can edit these tests or add even more tests to this page by appending /edit to the URL. Here’s a list of current revisions for this page:

• Revision 1: published by Kit Goncharov on 6th April 2011
• Revision 2: published on 29th October 2011
• Revision 3: published on 27th November 2011
• Revision 4: published by Joel Purra on 2nd February 2012
• Revision 6: published on 2nd August 2012
• Revision 7: published on 3rd August 2012
• Revision 8: published by Max on 3rd August 2012
• Revision 9: published by Max on 3rd August 2012
• Revision 10: published on 23rd January 2013
• Revision 11: published by Burakkuhatto on 10th March 2013
• Revision 12: published by Burakkuhatto on 18th March 2013
• Revision 13: published 3 weeks ago

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