How to convert a equation with variables to a string with variables values?

Question

If a have the following in Java Script (server script):

const a = 5;
const b = 2;
const c = 4;
const d = 6;
const e = 3;

const formula = "a * b - c * (d / e)";
const result = eval(formula); // = 2

I need, for debugging purposes, to have a Java Script function that will return as result a string with the variables values: const variables = "5 * 2 - 4 * (6 / 3)"

I named the variables a,b,c,d,e for simplicity, but they are more complex vars from JSON structures, like for example a = item.tiers.main.low, and similar for b,c,d,e.

I tried to do this: const values = " " + a + " * " + b + " - " + c + " * ( " + d + " / " + e + " )"; which gives the expected result, but there are dozens of formulas to do this with. To call a function like:

const variables = get_var_string(formula);

would be more convenient.

Answer 1

For debugging purposes, you can simply create a function that replaces variable names in the formula with their values. Something like:

function get_var_string(formula, variables) {
  const varNames = Object.keys(variables);
  let result = formula;

  for (const varName of varNames) {
    const varVal = variables[varName];
    const regex = new RegExp('\\b' + varName + '\\b', 'g');
                          //   ^^^ (word boundary)
// to ensure that we are matching the exact variable name and 
// not a substring of another variable name or word.
    result = result.replace(regex, varVal);
  }

  return result;
}

---

Edit:

I tried replicating your original scenario maybe if this helps.

function resolveVariable(path, variables) {
  return path.split('.').reduce((obj, key) => obj[key], variables);
}

function get_var_string(formula, variables) {
  const variablePaths = formula.match(/\b[\w.]+\b/g);
  let result = formula;

  for (const path of variablePaths) {
    const variableValue = resolveVariable(path, variables);
    const regex = new RegExp('\\b' + path + '\\b', 'g');
    result = result.replace(regex, variableValue);
  }

  return result;
}

const item = {
  tiers: {
    main: {
      low: 5,
      high: 2,
    },
  },
  other: {
    value: 4,
  },
};

const formula = "item.tiers.main.low * item.tiers.main.high - item.other.value";
const variables = { item };

const variableString = get_var_string(formula, variables);
console.log(variableString);

Answer 2

I strongly advice anyone that reads it to use something else than eval

But...

You can just replace every letter and use eval on them

const a = 5;
const b = 2;
const c = 4;
const d = 6;
const e = 3;

const formula = "a * b - c * (d / e)";
const result = formula.replace(/[a-z]/g, m => eval(m))
console.log(result)