Asynchronous Javascript Requests

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There are two APIs for making asynchronous requests from HTML5 javascript to MTScript.

The first is a reimplementation of the old XMLHttpRequest(). It is unlikely you'll want to use this directly, but it exists to let older javascript libraries function. Also, the second API is implemented on top of XMLHttpRequest(), so if you need lower level access, this is the way to go.

Assume the following macro is named myMacro and is on a library token called lib:myLib.

[h: broadcast(macro.args)]
[h: val=1d20]
[r: "Arguments: "+macro.args]
[r: "d20: "+val]

The following javascript will execute the macro and spit its result to the chat box.

function getValue(valueCallback) {
    let x = new XMLHttpRequest()"POST", "macro:myMacro@lib:myLib", true) // can use false for synchronous requests, but it is discouraged
    x.onreadystatechange = function() { // runs at each change of readystate, we're done when we hit state 4
      if (x.readyState == 4) {


Often, you'll want to return structured data from the macro, which can be provided like so.

Assume the following macro is named myJson and is on a library token called lib:myLib.

[h: val=1d20]
[h: result = "{}"]
[h: result=json.set(result, "target", macro.args)]
[h: result=json.set(result, "value", val)]
[r: result]

This can easily be retrieved by using the second asynchronous javascript API: fetch()

let x = fetch("macro:myJson@lib:myLib", {method: "POST", body: "someTarget"})
      console.log("Response not valid json")
    console.log("macro invocation failed")

As usual with javascript, if we are happy with async functions, we can clean that up considerably. Here is an example fetching the same macro as a string instead of calling it. (Note that Allow URI access must be enabled on the lib token).

async function getResource() {
  let r = await fetch("lib://myLib/macro/myJson")
  // We cannot ask for the body as json, as it is just a UTF8 string.
  let body = await r.text()

Note that await will throw an exception if the awaited promise fails. Use try/catch if there is a risk of the macro failing in some way.