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How to parse a string to a function that expects parameters and returns a value?

November 10, 2022

It is possible to configure functions as strings to parse them to functions during runtime.

The following example functionAsString expects input and deals with it, I only know that it MUST return a boolean ( I’m expecting that )

const x = {
  fields: {
    age: 0
  }
};
const y = {
  fields: {
    age: 1
  }
};

const functionAsString = "(left, right) => left.fields.age < right.fields.age";
const compareFunction = new Function(functionAsString);

const isXLessThanY = compareFunction(x, y);

if (isXLessThanY === undefined) {
  console.error("it should not be undefined...");
} else {
  console.log({
    isXLessThanY
  });
}

isXLessThanY is undefined. Do you know how to setup a valid function based on a string?

>Solution :

The Function constructor actually takes more structured data than eval, it takes the parameter names, then the body. Also it does generate a regular function (not an arrow function) so you have to explicitly return.

const x = {
  fields: {
    age: 0
  }
};
const y = {
  fields: {
    age: 1
  }
};

const functionAsString = "return left.fields.age < right.fields.age";
const compareFunction = new Function('left', 'right', functionAsString);

const isXLessThanY = compareFunction(x, y);

if (isXLessThanY === undefined) {
  console.error("it should not be undefined...");
} else {
  console.log({
    isXLessThanY
  });
}