理解_.js中下划线的声明？

Question

这是从注释的_.js源开始.尽管我可以尝试,但我的JavaScript能力还不够高,无法理解这里发生了什么.我希望有人能给出真正的一步一步的解释.我真的不知道下面的代码除了以某种方式设置_使用之外还做了什么,尽管我理解每个单独的表达.

 var _ = function(obj) {
    if (obj instanceof _) return obj;
    if (!(this instanceof _)) return new _(obj);
    this._wrapped = obj;
  };

  if (typeof exports !== 'undefined') {
    if (typeof module !== 'undefined' && module.exports) {
      exports = module.exports = _;
    }
    exports._ = _;
  } else {
    root._ = _;
  }

Answer 1

var _ = function(obj) {
    // Either serve as the identity function on `_` instances,
    // ... or instantiate a new `_` object for other input.

    // If an `_` instance was passed, return it.
    if (obj instanceof _) return obj;
    // If someone called `_(...)`, rather than `new _(...)`,
    // ... return `new _(...)` to instantiate an instance.
    if (!(this instanceof _)) return new _(obj);

    // If we are instantiating a new `_` object with an underlying,
    // ... object, set that object to the `_wrapped` property.
    this._wrapped = obj;
};

// If there is an exports value (for modularity)...
if (typeof exports !== 'undefined') {
    // If we're in Node.js with module.exports...
    if (typeof module !== 'undefined' && module.exports) {
        // Set the export to `_`
        exports = module.exports = _;
    }
    // Export `_` as `_`
    exports._ = _;
} else {
    // Otherwise, set `_` on the global object, as set at the beginning
    // ... via `(function(){ var root = this; /* ... */ }())`
    root._ = _;
}