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

Question

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?

It would need to be new Function("left", "right", "return left.fields.age < right.fields.age"), but that's almost just as bad as using eval() and can be a security risk if these strings are user provided. What is the reason why you need to have the function as a string in the first place? — Nick Parsons
– Nick Parsons, Commented Nov 10, 2022 at 10:34

Guerric P · Accepted Answer · 2022-11-10 10:34:54Z

1

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
  });
}

answered Nov 10, 2022 at 10:34

Guerric P

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Comments

Ömer Can Korkmaz · Accepted Answer · 2022-11-10 10:22:55Z

1

you can use eval() function to run js code as a string, but it has some security issues. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval

answered Nov 10, 2022 at 10:22

Ömer Can Korkmaz

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3 Comments

baitendbidz Over a year ago

yes, but I thought the "never use eval" section shows that Function is a valid alternative

Ömer Can Korkmaz Over a year ago

problem might be the arrow function in functionAsString, have you try the normal function declaration ?

baitendbidz Over a year ago

you mean like so? jsfiddle.net/f0e2wnLj this syntax didn't work for me

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