I am trying to declare a variable to match Contentful posts that match true on a particular boolean field

I have built a blog with gatsby and Contentful and have a boolean field in the contentful posts to select if a particular post is "featured" or not. How can I declare the variable featuredPost to match posts which have the featured boolean set as true. PLEASE NOTE; in the code below in the declaration I have put ???????? to highlight what/where my question refers to. I would greatly appreciate your advice:

...

class BlogIndex extends React.Component {
  render() {
    const { data } = this.props
    const siteTitle = data.site.siteMetadata?.title || `Title`
    const posts = data.allContentfulPost.edges
    const featuredPost = ???????????

    return (
      <Layout location={this.props.location}>
        <SEO title="FieldPro Blog" keywords={[]} />
        <Header />
        <FeaturedPost>
          {featuredPost.map(({ node }) => {
            const title = node.title || node.slug
            return (
              <div key={node.slug}>
                <FeaturedImage>
                  <Link style={{ boxShadow: `none` }} to={node.slug}>
                    <Img className="Image" fluid={node.image.fluid} />
                  </Link>
                </FeaturedImage>
                <FeaturedText>
                  <FeaturedTitle>
                    <Link style={{ boxShadow: `none` }} to={node.slug}>
                      {title}
                    </Link>
                  </FeaturedTitle>
                  <Labels>
                    <Caption>{/* node.tags */}</Caption>
                  </Labels>
                </FeaturedText>
              </div>
            )
          })}
        </FeaturedPost>
        <PostGroup>
          {posts.map(({ node }) => {
            const title = node.title || node.slug
            return (
              <Post key={node.slug}>
                <PostImage>
                  <Link style={{ boxShadow: `none` }} to={node.slug}>
                    <Img className="Image" fluid={node.image.fluid} />
                  </Link>
                </PostImage>
                <PostText>
                  <Title>
                    <Link style={{ boxShadow: `none` }} to={node.slug}>
                      {title}
                    </Link>
                  </Title>
                  <Labels>
                    <Caption>{/* node.tags */}</Caption>
                  </Labels>
                </PostText>
              </Post>
            )
          })}
        </PostGroup>
      </Layout>
    )
  }
}

export default BlogIndex

export const pageQuery = graphql`
  query {
    site {
      siteMetadata {
        title
      }
    }
    allContentfulPost(sort: { fields: createdAt, order: DESC }) {
      edges {
        node {
          title
          slug
          featured
          image {
            fluid {
              ...GatsbyContentfulFluid
            }
          }
        }
      }
    }
  }
`
...

You can't declare it outside the loop since it's a dependant variable of each element of the loop.

One built-in option you can do is to filter your GraphQL query on the fly to get only the featured posts. You can create an alias to get both types of posts:

export const pageQuery = graphql`
  query {
    site {
      siteMetadata {
        title
      }
    }
    featuredPosts: allContentfulPost(filter: { featured: { eq: "true" } }, sort: { fields: createdAt, order: DESC }) {
      edges {
        node {
          title
          slug
          featured
          image {
            fluid {
              ...GatsbyContentfulFluid
            }
          }
        }
      }
    }
  }
`

Note: you will need to add the other query for the non-featured posts.

In this case, your data will be in props.data.featuredPosts.

The other option, as I said, is to get the featured property of each element while looping through them:

<FeaturedPost>
  {featuredPost.map(({ node }) => {
    const title = node.title || node.slug;
    if(node.featured){
    return (
      <div key={node.slug}>
        <FeaturedImage>
          <Link style={{ boxShadow: `none` }} to={node.slug}>
            <Img className="Image" fluid={node.image.fluid} />
          </Link>
        </FeaturedImage>
        <FeaturedText>
          <FeaturedTitle>
            <Link style={{ boxShadow: `none` }} to={node.slug}>
              {title}
            </Link>
          </FeaturedTitle>
          <Labels>
            <Caption>{/* node.tags */}</Caption>
          </Labels>
        </FeaturedText>
      </div>
    )
    }
  })}
</FeaturedPost>

Your property is inside node.featured and may differ for each element so must be treated inside the loop.