What does "let _self = this" mean in Javascript/Typescript? : <p>In this <a href="https://github.com/MurhafSousli/ng2-disqus/blob/master/src/disqus/disqus.component.ts" rel="noreferrer">code snippet</a>, why is it that <code>this.identifier</code> doesn't work but <code>_self.url</code> works?</p>

<pre><code>  getConfig() {
    let _self = this;
    return function () {
      this.page.url = this.url || window.location.href;
      this.page.identifier = _self.identifier;
      this.page.category_id = this.categoryId;
      this.language = this.lang;
    };
}
</code></pre>

<p>So does <code>let _self = this</code> actually do?</p>
