Evaluators

Normally you run metaesEval in the following way:

metaesEval("2+a", console.log, console.error); // Error

What we already know is to fix the missing a you can provide extra environment value:

metaesEval("2+a", console.log, console.error, { a: 2 }); // 4

But, what if we want to delay a dereferencing util a is hit in the runtime? We'd want use custom Identifier evaluator, meaning we want to tell metaES how to resolve variables in non-standard way. Maybe that interpreter would reach over HTTP, WebSocket etc. and this would take uknown amount of time? It can be done with overriding interpreters field in the config:

const evaluators = createEnvironment(
  {
    Identifier(e, c, cerr) {
      c(44);
    }
  },
  ECMAScriptEvaluators
);
metaesEval("var b=3; 2+a+b", console.log, console.error, { a: 2 }, { evaluators }); // result: 90

Few things to note:

• evaluators is an environment,
• evaluators environment has to reference at some point an outer environment which should be ECMAScriptEvaluators. This environment is provided by metaES itself and contains interpreters to interpret ECMAScript nodes. Otherwise metaES will just throw NotImplementedException for rest of the nodes,
• we actually broke Indentifier - it always returns 44, even if variable is defined.

Let's patch it:

const promised44 = () => Promise.resolve(44);
const evaluators = createEnvironment(
  {
    Identifier(e, c, cerr, env, config) {
      ECMAScriptEvaluators.values.Identifier(e, c, (_) => promised44().then(c).catch(cerr), env, config);
    }
  },
  ECMAScriptEvaluators
);
metaesEval("var b=3; 2+a+b", console.log, console.error, {}, { evaluators }); // 49

Identifier first tries to reach variable in a standard way. When fails, it always falls back with promised44 - a Promise that resolves for any value, in example it's always 44.

This solution is still not ideal - you need to use createEnvironmentOverriding to make sure future configurations will be handled correctly, especially when there are multiple layers of overrides.