How to create your own data type and set the cell values by using Range.valuesAsJson property. ScriptLab demo.

Excel Custom Data Type Add-in Demo.EXCEL.yaml

name: Excel Custom Data Type Add-in Demo
description: >-
  How to create your own data type and set the cell values by using
  Range.valuesAsJson property. ScriptLab demo.
host: EXCEL
api_set: {}
script:
  content: |
    $("#run").click(() => tryCatch(run));

    async function run() {
      await Excel.run(async (context) => {
        // Get the keyword from the active cell
        const fnd = context.workbook.getActiveCell();
        fnd.load("values");
        await context.sync();
        var keyword = fnd.values[0][0];

        // Starting from here, it is exactly the same code with the Custom Function

        var lookin = "title"; // or body to search in the product article body

        keyword = "?" + lookin + "=" + keyword;

        const url = "https://demo.batcoder.com/api/book/" + keyword;

        const response = await fetch(url);
        if (!response.ok) {
          throw new Error(response.statusText);
        }

        const books = await response.json();

        var bookEntities = [];

        for (var i = 0; i < books.length; i++) {
          var book = books[i];

          var authors = [];

          for (var j = 0; j < book.authors.length; j++) {
            var author = book.authors[j];

            var authorBooks = [];

            for (var k = 0; k < author.books.length; k++) {
              var authorBook = author.books[k];

              var authorBookEntity = {
                type: "String",
                basicValue: authorBook
              };
              authorBooks.push([authorBookEntity]);
            }

            var authorEntity = {
              type: "Entity",
              text: author.name,
              properties: {
                Name: {
                  type: "String",
                  basicValue: author.name
                },
                WebPage: {
                  type: "String",
                  basicValue: author.url
                },
                Website: {
                  type: "String",
                  basicValue: author.website
                },
                Image: {
                  type: "WebImage",
                  address: author.image
                },
                Books: {
                  type: "Array",
                  elements: authorBooks
                }
              }
            };
            authors.push([authorEntity]);
          }

          var bookEditions = [];
          for (var m = 0; m < book.editions.length; m++) {
            var bookEdition = book.editions[m];

            var bookEditionEntity = {
              type: "String",
              basicValue: bookEdition
            };
            bookEditions.push([bookEditionEntity]);
          }

          var entity = {
            type: "Entity",
            text: book.title,
            properties: {
              Title: {
                type: "String",
                basicValue: book.title
              },
              PublishDate: {
                type: "FormattedNumber",
                basicValue: book.published_on_numeric,
                numberFormat: "mm/dd/YYYY"
              },
              ISBN: {
                type: "String",
                basicValue: book.isbn
              },
              URL: {
                type: "String",
                basicValue: book.url
              },
              CoverImage: {
                type: "WebImage",
                address: book.image
              },
              Authors: {
                type: "Array",
                elements: authors
              }
            }
          };

          if (bookEditions.length > 0) {
            entity.properties["Editions"] = {
              type: "Array",
              elements: bookEditions
            };
          }

          bookEntities.push([entity]);
        }

        // The following is the addition to the Custom Function code instead of returning the entity
        if (bookEntities.length > 0) {
          // Prepare the target range starting from the search cell
          var rng = fnd.getResizedRange(bookEntities.length - 1, 0);
          rng.valuesAsJson = bookEntities;
        } else {
          console.log("no books");
        }

        // Done!
        await context.sync();
      });
    }

    async function tryCatch(callback) {
      try {
        await callback();
      } catch (error) {
        console.error(error);
      }
    }
  language: typescript
template:
  content: |
    <button id="run" class="ms-Button">
        <span class="ms-Button-label">Run</span>
    </button>
  language: html
style:
  content: |-
    section.samples {
        margin-top: 20px;
    }

    section.samples .ms-Button, section.setup .ms-Button {
        display: block;
        margin-bottom: 5px;
        margin-left: 20px;
        min-width: 80px;
    }
  language: css
libraries: |
  https://appsforoffice.microsoft.com/lib/beta/hosted/office.js
  @types/office-js-preview

  office-ui-fabric-js@1.4.0/dist/css/fabric.min.css
  office-ui-fabric-js@1.4.0/dist/css/fabric.components.min.css

  core-js@2.4.1/client/core.min.js
  @types/core-js

  jquery@3.1.1
  @types/jquery@3.3.1