Returns true if argument is a function (CommonJS)

isFunction

// Safe isFunction()
// Avoid a Chakra JIT bug in compatibility modes of IE 11.

var isFunction = function (expr) {
  return typeof expr === 'function' || false;
};

if (isFunction(/./) || (Uint8Array && !isFunction(Uint8Array))) {
  isFunction = (function (expr) {
    var _toString = Object.prototype.toString;
    return function (expr) {
      return _toString.call(expr) === '[object Function]';
    };
  })();
}

module.exports = isFunction;