how is the transpiling language of a script chosen by the browser

Question

Recently i have been working with code that imports babel as a script

<script src="https://unpkg.com/@babel/standalone@7.9.3/babel.js"></script>

and then uses the type text/babel in a script below to be able to write jsx.

<script type="text/babel">

 // some jsx code

</script>

I wonder how that type is recognized by the browser who decides to use the previously imported babel to transpile the code.

I always assumed the browser had a set of default script types that it knew how to handle -- and it does -- but text/babel is clearly not one of them.

So how does this work?

Answer 1

I just looked inside the babel.js file and it became obvious.

var scriptTypes = ['text/babel','text/jsx'];

[....]

function runScripts(transformFn, scripts) {
    headEl = document.getElementsByTagName("head")[0];

    if (!scripts) {
      scripts = document.getElementsByTagName("script");
    }

    var jsxScripts = [];

    for (var i = 0; i < scripts.length; i++) {
      var script = scripts.item(i);
      var type = script.type.split(";")[0];

      if (scriptTypes.indexOf(type) !== -1) {
        jsxScripts.push(script);
      }
    }

    if (jsxScripts.length === 0) {
      return;
    }

    console.warn("You are using the in-browser Babel transformer. Be sure to precompile " + "your scripts for production - https://babeljs.io/docs/setup/");
    loadScripts(transformFn, jsxScripts);
  }

the babel script runs before the text/babel script and it finds all text/babel scripts, transpiles them and runs them. So the browser doesn't need to be aware of that type.

Pretty handy to write your own types and transpilers