Slack API call to postMessage not working - google-apps-script

Question

I'm just trying to make a simple postMessage call from a google apps script with an image attached, but I get the following response:
"{"ok":false,"error":"invalid_arg_name"}"
Here is the function that creates the payload:
function getPostMessagePayload(fileUrl) {
var content = {
"channel":"#data-vis",
"token": ACCESS_TOKEN,
"text":"Chart update:",
"attachments": [
{
"title": "Chart",
"fallback": "Fallback",
"text": "Testing chart",
"image_url": fileUrl
}
]
};
return content;
}
And here is where I make the request:
var POST_MESSAGE_ENDPOINT = 'https://slack.com/api/chat.postMessage';
function performPostMessage(payload) {
var res = UrlFetchApp.fetch(
POST_MESSAGE_ENDPOINT,
{
method: "post",
payload: JSON.stringify(payload),
muteHttpExceptions: true,
}).getContentText();
return res;
}
It's impossible to tell what the actual problem is. I've tried making my token obviously incorrect, the URL obviously incorrect, and deleting/adding random args and it gives the same response every time.
When I use the webhook to do this rather than the API, it works fine.
My app has the following permissions in Slack:
chat:write:bot
incoming-webhook

Answer 1

Problem
You are sending a JSON object as payload with your POST request, whilst the contentType parameter of the fetch() method is defaulted to application/x-www-form-urlencoded.
Solution 1
In addition to JSON.stringify(), to ensure the payload is sent correctly, wrap it in an encodeURIComponent() built-in function. If the issue persists, continue to solution 2.
Update to solution 1
Nearly forgot how fetch() method treats objects passed to payload with default x-www-form-urlencoded content type. Remove the JSON.stringify() entirely (and add encodeURI() / encodeURIComponent() if needed).
Solution 2
Slack API supports application/json content type of POST requests. In your case it might be easier to send the request with contentType parameter set to application.json (note that you will have to move authorization from payload to headers):
//fetch part;
var res = UrlFetchApp.fetch(
POST_MESSAGE_ENDPOINT,
{
method : 'post',
contentType : 'application/json',
headers : {
Authorization : 'Bearer ' + ACCESS_TOKEN
},
payload : JSON.stringify(payload),
muteHttpExceptions : true,
})
//payload part;
var payload = {
"channel" : "#data-vis",
"text" : "Chart update:",
"attachments" : [
{
"title" : "Chart",
"fallback" : "Fallback",
"text" : "Testing chart",
"image_url" : fileUrl
}
]
};
Useful links
fetch() method reference;
postMessage method reference (Slack API);