title: How to use the pageContext in SPFx?
body: <p>I am trying to get a value from the current page by using the pageContext but I am getting either undefined or 404. 
  This is the situation:
  In the Site pages library there are several news pages. Each news page has some tags attached to them. This tags lives in a custom column in the Site Pages library. 
  There are news that have 1 tag and other several tags. It can be the situation where two or more news share the same tag(s). 
  The goal is when I open a news page the tags that are attached to that news are also visible. </p>
  
  <p>Until now I am using @pnp/pnpjs and the code looks like this:</p>
  
  <pre><code>var result: any = await sp.web.lists.getByTitle("Site Pages")
    .items.getById(15)
    .select("Tags")
    .get();
  
  return await result.Tags;
  </code></pre>
  
  <p>And it is giving me 404 error</p>
  
  <p>I also tried this one:</p>
  
  <pre><code>this.context.pageContext.list('Site Pages').listItem['Tags'].get().then((items: any[]) =&gt; {
    console.log(items);
  });
  </code></pre>
  
  <p>But it giving me Cannot read property 'list' of undefined</p>
  
  <p>Du you have an idea how can get the value of the Tags column asociated with the current news?</p>
  
  <p><strong>Here is an update</strong>
  Now I am getting the right tag. The question now is how to show it in the screen? </p>
  
  <pre><code>import * as React from 'react';
  import styles from './ReadTags.module.scss';
  import { IReadTagsProps } from './IReadTagsProps';
  import { sp } from '@pnp/pnpjs';
  
  export default class ReadTags extends React.Component&lt;IReadTagsProps, {}&gt; {
    constructor(props: IReadTagsProps) {
      super(props);
  
    }
  
    private async getTags() {
        var id = this.props.context.pageContext.listItem.id;
        var result: any = await sp.web.lists.getByTitle("Site Pages")
          .items.getById(id)
          .select("Tags")
          .get();
  
        return await result.Tags;
    }
  
    public render(): React.ReactElement&lt;IReadTagsProps&gt; {
      console.log(this.getTags());
      return (
        &lt;div className={ styles.readTags }&gt;
          &lt;div className={ styles.container }&gt;
            &lt;div className={ styles.row }&gt;
              &lt;div className={ styles.column }&gt;
              &lt;/div&gt;
            &lt;/div&gt;
          &lt;/div&gt;
        &lt;/div&gt;
      );
    }
  }
  </code></pre>
  
  <p>Regards
  Amerco</p>