How to access global object property from relative property key using Joi expression

Question

I'm trying to set a default value of a property based on a global context passed through at validate and an existing property of the object being validated. I just can't seem to use the value of the property of the object as a key for the global context.

const context = {
    letters: {
        a: 1,
        b: 2,
        c: 3,
    },
};

const s = Joi.object().keys({
    letter: Joi.string().valid(...['a', 'b', 'c']),
    num: Joi.number().default(Joi.expression('{$letters}.{letter}'))
})

const test = {
    letter: 'a',
}

console.log(s.validate(test, { context }));

Simple example above. I have an object called letters that I place into the context. The schema will look for letter, then try to set a default value for num using the global letters as the object to pull from and the letter passed in as the key. The closest I can get is { value: { letter: 'a', num: '[object Object].a' } }

Answer 1

Some of the below examples that would work

Joi.expression(`{$letters[.a]}`)
Joi.expression(`{{$letters.b}}`)
Joi.expression(`{{$letters[.c]}}`)

But the problem here is we can't use variable like letter in place of a or b or c.

But we can achieve the what is required with custom function

// https://stackoverflow.com/a/6491621/14475852
Object.byString = function(o, s) {
  s = s.replace(/\[(\w+)\]/g, '.$1'); // convert indexes to properties
  s = s.replace(/^\./, ''); // strip a leading dot
  var a = s.split('.');
  for (var i = 0, n = a.length; i < n; ++i) {
    var k = a[i];
    if (k in o) {
      o = o[k];
    } else {
      return;
    }
  }
  return o;
};

function customEvaluator(expression) {
  return function(parent, helpers) {
    let global = expression[0] == '$';
    let exp = '';
    for (var i = global ? 1 : 0, n = expression.length; i < n; ++i) {
      if (expression[i] == '#') {
        let part = expression.slice(i + 1);
        let end = -1;
        [' ', '.', '[', ']', '{', '}'].forEach(i => {
          let tmp = part.indexOf(i);
          if (end == -1 && tmp != -1) {
            end = tmp;
          }
        });

        end = end == -1 ? part.length : end;
        exp += parent[part.slice(0, end)];
        i += end;
      } else {
        exp += expression[i];
      }
    }

    return Object.byString(global ? helpers.prefs.context : parent, exp);
  };
}

const Joi = joi; // exported as joi in the browser bundle

const s = Joi.object().keys({
    letter: Joi.string().valid(...["a", "b", "c"]),
    num: Joi
        .number()
        .default(customEvaluator('$letters.#letter')),
});

const context = {
    letters: {
        a: 1,
        b: 2,
        c: 3,
    },
};

console.log(s.validate({ letter: "a" }, { context }));
console.log(s.validate({ letter: "b" }, { context }));
console.log(s.validate({ letter: "a", num: 4 }, { context }));

<script src="https://cdn.jsdelivr.net/npm/joi@17.4.2/dist/joi-browser.min.js"></script>