Is it possible to evaluate a Mapbox expression directly?

// Here we will later define all functions available for the expression language
const OPERATIONS = {};

// Now, let's set up the evaluator function.
// It obviously must receive the expression it will evaluate,
// but also a context that can be modified by operations.
const evaluate = (expression, context = {}) => {
  // First, we deal with literals by evaluating them as themselves
  if (!(expression instanceof Array)) {
    return expression;
  }

  // Right, now to the real deal:
  // let's find out what operation to run and its parameters.
  const [operationKey, ...rawParameters] = expression;
  const operation = OPERATIONS[operationKey];

  // We handle unknown operations by panicking! AAAH!
  if (operation == null) {
    throw new Error(`Unknown operation ${operationKey}!`);
  }

  // Oh nice, we know this operation! Now, how should we call it?
  // It obviously needs to receive its parameters, as well as the context,
  // in case it is one of those pesky stateful operations. Plus, as we
  // have seen with Mapbox's `let`, operations can create new contexts!
  //
  // I propose the following signature, though you can change it for your
  // particular preference and use-cases:
  //
  // First parameter:
  //      Current context
  // Second parameter:
  //      Array of all of the operation's parameters. This makes for
  //      easy iteration if the operation is variadic, and simpler stuff
  //      can still just use deconstruction to have a "fixed" signature.
  //      We will pass the parameters "raw", not evaluated, so that the
  //      operation can do whatever evil things it wants to do to them.
  // Return value:
  //      Whatever the operation wants to evaluate to!
  return operation(context, rawParameters);
};

// Right, right, we have set up the evaluator, but how do we actually use it?
// We need some operations, let's start with the easy ones to wet our feet:
// Remember how I said above that the parameters array comes in raw?
// We'll need to evaluate them manually inside our operation functions.
OPERATIONS["-"] = (context, [a, b]) => evaluate(a, context) - evaluate(b, context);
OPERATIONS["+"] = (context, [a, b]) => evaluate(a, context) + evaluate(b, context);

// Okay, that was easy, but what if we want
// to accept an arbitrary amount of arguments?
OPERATIONS["*"] = (context, parameters) => parameters
  .map(p => evaluate(p, context))
  .reduce((accumulator, x) => accumulator * x);
  
// Right, now let's implement those arrays we spoke of.
// The solution is simple, have an operation that
// creates the array from its parameters!
OPERATIONS["array"] = (context, parameters) => parameters
  .map(p => evaluate(p, context));

// Cool, cool, but what about the evil spawns of Satan himself? Let and Var?

// Let's start with the lesser of them:
// Easy, we just read whatever was stored in the context for that variable name!
OPERATIONS["var"] = (context, [variable]) => context[variable];

// Now, the "tricky" one, Let, which is both variadic AND changes the context!
// I'll pull out my braces here  because it's gonna be a bit bigger than the
// previous beautiful one-line operations!
OPERATIONS["let"] = (context, [...definitions]) => {
  // Right, we have A context, but we don't want to pollute it outside
  // the Let block! So let's copy it to a new temporary one:
  const innerContext = { ...context
  };

  // Now we need to loop the definitions, remember, they are 2 elements each:
  // A variable name, and its value expression! But first, we need to pick
  // out the last argument which is the expression to be executed in the
  // resulting context:
  const body = definitions.pop()

  // Let's get the obvious stuff out of the way, if we have an odd number of
  // things in our definitions, the user is wrong! Let's throw it on their
  // ugly face! Let's use a cryptic error message just to be evil...
  if (definitions.length % 2 === 1) {
    throw new Error("Unmatched definitions!");
  }

  // Cool, now we get to do the cool stuff which is create those variables:
  for (let i = 0; i < definitions.length - 1; i += 2) {
    const name = definitions[i];
    const value = definitions[i + 1];

    // Here I made the choice that variables in the same block can depend
    // on previous variables, if that's not to your liking, use the parent
    // context instead of the one we're modifying at the moment.
    innerContext[name] = evaluate(value, innerContext);
  }

  // Variables are DONE, now let's evaluate the body!
  return evaluate(body, innerContext);
};

// Bonus points for reading the snippet code:
// Remember that we are not limited to numeric values,
// anything that JSON accepts we accept too!
// So here's some simple string manipulation.
OPERATIONS["join"] = (context, [separator, things]) => evaluate(things, context)
  .flat()
  .join(separator);

// And we're done! That is the basic of evaluating a syntax tree!







// Not really relevant to the question itself, just a quick and dirty REPL

(() => {
  const input = document.getElementById("input");
  const output = document.getElementById("output");

  const runSnippet = () => {
    let expression;

    try {
      expression = JSON.parse(input.value);
    } catch (e) {
      // Let the user type at peace by not spamming errors on partial JSON
      return;
    }

    const result = evaluate(expression);
    output.innerText = JSON.stringify(result, null, 2);
  }

  input.addEventListener("input", runSnippet);

  runSnippet();
})();

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<textarea id="input">
[
  "let",
  "pi", 3.14159,
  "radius", 5,
  [
    "join",
    " ",
    [
      "array",
      "a circle with radius",
      ["var", "radius"],
      "has a perimeter of",
      [
        "*",
        2,
        ["var", "pi"],
        ["var", "radius"]
      ]
    ]
  ]
]

</textarea>
<pre id="output">
</pre>