lang/isFunction.js

define(['./isNative', '../internal/root'], function(isNative, root) {

  /** `Object#toString` result references. */
  var funcTag = '[object Function]';

  /** Used for native method references. */
  var objectProto = Object.prototype;

  /**
   * Used to resolve the `toStringTag` of values.
   * See the [ES spec](https://people.mozilla.org/~jorendorff/es6-draft.html#sec-object.prototype.tostring)
   * for more details.
   */
  var objToString = objectProto.toString;

  /** Native method references. */
  var Uint8Array = isNative(Uint8Array = root.Uint8Array) && Uint8Array;

  /**
   * Checks if `value` is classified as a `Function` object.
   *
   * @static
   * @memberOf _
   * @category Lang
   * @param {*} value The value to check.
   * @returns {boolean} Returns `true` if `value` is correctly classified, else `false`.
   * @example
   *
   * _.isFunction(_);
   * // => true
   *
   * _.isFunction(/abc/);
   * // => false
   */
  function isFunction(value) {
    // Avoid a Chakra JIT bug in compatibility modes of IE 11.
    // See https://github.com/jashkenas/underscore/issues/1621 for more details.
    return typeof value == 'function' || false;
  }
  // Fallback for environments that return incorrect `typeof` operator results.
  if (isFunction(/x/) || (Uint8Array && !isFunction(Uint8Array))) {
    isFunction = function(value) {
      // The use of `Object#toString` avoids issues with the `typeof` operator
      // in older versions of Chrome and Safari which return 'function' for regexes
      // and Safari 8 equivalents which return 'object' for typed array constructors.
      return objToString.call(value) == funcTag;
    };
  }

  return isFunction;
});