I would recommend you to use the Text Snippet feature via files.upload method, so you can format the content type properly, in this case javascript.

Send Slack Text Snippet	Run in Fusebit

const testObj = {a: "Hello", b: "World"};
const result = await slackClient.files.upload({
      channels: slackUserId,
      content: testObj,
      title: 'Sample',
      filetype: 'javascript'
});
const {name, pretty_type} = result.file;
ctx.body = { message: `Successfully sent a text snippet of type ${pretty_type} called ${name} to Slack user ${slackUserId}!` };

I found a good working solution using only native node libraries to get nicely formatted JSON file.

Here's the full code, replace contentObj with whatever object you want to convert to JSON. Put your designed channel and bot auth token

const https = require('https')
const contentObj = {a: "hello", b: "world"}

const formObject = {
  channels: "#myChannel",
  content: JSON.stringify(contentObj, null, 2),
  filename: "myFile.json",
  initial_comment:
    "Hello World from JSON HTTPS!",
  title: "myFile.json",
};

const urlencodedForm = new URLSearchParams(
  Object.entries(formObject)
).toString();

const options = {
  hostname: "slack.com",
  port: 443,
  path: "/api/files.upload",
  method: "POST",
  headers: {
    "Content-Type": "application/x-www-form-urlencoded",
    "Content-Length": urlencodedForm.length,
    Authorization: "Bearer " + "Bot Token",
  },
};

const req = https.request(options, function (res) {
  let responseBody = "";
  res.on("data", function (chunk) {
    responseBody += chunk;
  });

  res.on("end", () => {
    const finalResponse = JSON.parse(responseBody);
    if (finalResponse.ok) {
      console.log("successfully sent msg to slack");
    } else {
      console.log("Message Failed");
    }
  });
});

req.on("error", (error) => {
  console.error(error);
});

req.write(urlencodedForm);
req.end();