gatsby-node.js

const fetch = require('node-fetch')
const crypto = require('crypto')
const path = require('path')
const fs = require('fs');
const _ = require('lodash');

exports.sourceNodes = async ({ actions }) => {
  const { createNode } = actions

  const createProduct = (var1, var2) => {
    // do stuff to create the product object
    return product
  }

  var fetchNow = function() {
    // fetch data
    createNode(createProduct(var1, var2))
  }
  
  return
}

// Implement the Gatsby API “createPages”. This is called once the
// data layer is bootstrapped to let plugins create pages from data.
exports.createPages = ({ actions, graphql }) => {
  const { createPage } = actions;

  return new Promise((resolve, reject) => {
    // Query for products to use in creating pages.
    resolve(
      graphql(
        `
          {
            allProduct {
              edges {
                // query
              }
            }
          }
        `
      ).then(result => {
        if (result.errors) {
          console.log(result.errors);
          reject(result.errors);
        }

        const allProducts = result.data.allProduct.edges
        // Create pages for each product.
        allProducts.forEach(({ node }) => {
          const decodedSlug = decodeURIComponent(node.slug)
          const id = node.id
          
          createPage({
            path: `/product/${decodedSlug}`,
            component: path.resolve(`src/templates/product-detail.js`),
            context: {
              data1,
              data2,
              data3 // ...etc
            },
          });
          
        });
      })
    );
  });
};

const productQuery = `{
  allProduct {
    edges {
      // query
    }
  }
}`;

product-detail.js

import React from 'react'

const ProductDetailTemplate = ({ pageContext, data, location }) => {
  console.log(pageContext.data1, pageContext.data2, pageContext.data3)
}