How to return an error message when the number is a fraction in NodeJS/Express?

Question

I am in the process of building a picture framing calculator API using Node JS & Express. The JavaScript I am using is:

app.post("/api/chatfuel/calculator/triplematapi", (req,res) => {
    if (!(req.body.FrameWidth > -1)) {
        res.status(422).json({ messages: "Please insert a positive value" });
    }
    let FrameWidth = 1 * req.body.FrameWidth.split("/").reduce((a, denom) => a.split(" ").reduce((int, numer) => 1 * numer + int * denom) / denom);
    if (!(req.body.FrameHeight > -1)) {
        res.status(422).json({ messages: "Please insert a positive value" });
    }
    let FrameHeight = 1 * req.body.FrameHeight.split("/").reduce((a, denom) => a.split(" ").reduce((int, numer) => 1 * numer + int * denom) / denom);
    if (!(req.body.PictureWidth > -1)) {
        res.status(422).json({ messages: "Please insert a positive value" });
    }
    let PictureWidth = 1 * req.body.PictureWidth.split("/").reduce((a, denom) => a.split(" ").reduce((int, numer) => 1 * numer + int * denom) / denom);
    if (!(req.body.FrameHeight > -1)) {
        res.status(422).json({ messages: "Please insert a positive value" });
    }
    let PictureHeight = 1 * req.body.PictureHeight.split("/").reduce((a, denom) => a.split(" ").reduce((int, numer) => 1 * numer + int * denom) / denom);
    if (!(req.body.MiddleMat > -1)) {
        res.status(422).json({ messages: "Please insert a positive value" });
    }
    let MiddleMat = 1 * req.body.MiddleMat.split("/").reduce((a, denom) => a.split(" ").reduce((int, numer) => 1 * numer + int * denom) / denom);
    if (!(req.body.BottomMat > -1)) {
        res.status(422).json({ messages: "Please insert a positive value" });
    }
    let BottomMat = 1 * req.body.BottomMat.split("/").reduce((a, denom) => a.split(" ").reduce((int, numer) => 1 * numer + int * denom) / denom);
    let TopMatWidth = ((FrameHeight)-(PictureHeight))/2-(MiddleMat);
    let TopMatHeight = ((FrameWidth)-(PictureWidth))/2-(MiddleMat);
    let MiddleMatWidth = ((FrameHeight)-(PictureHeight))/2;
    let MiddleMatHeight = ((FrameWidth)-(PictureWidth))/2;
    let BottomMatWidth = ((FrameHeight)-(PictureHeight))/(2)+(BottomMat);
    let BottomMatHeight = ((FrameWidth)-(PictureWidth))/(2)+(BottomMat);
    res.json({"messages": [{"text": "Place the parallel mat guide at the following inch mark, then make the respective width and height cut starting with the Top Mat, Middle Mat, then Bottom Mat:"},{"text": `Top Mat Width Cut = ${new Fraction(TopMatWidth).toString()} inches`},{"text": `Top Mat Height Cut = ${new Fraction(TopMatHeight).toString()} inches`},{"text": `Middle Mat Width Cut = ${new Fraction(MiddleMatWidth).toString()} inches`},{"text": `Middle Mat Height Cut = ${new Fraction(MiddleMatHeight).toString()} inches`},{"text": `Bottom Mat Width Cut = ${new Fraction(BottomMatWidth).toString()} inches`},{"text": `Bottom Mat Height Cut = ${new Fraction(BottomMatHeight).toString()} inches`},{"buttons": [{"title": "Go to I Was Framed!","type": "web_url","url": "https://iwasframed.com/"}]}]});
});

Therefore, an example POST request would be:

{
    "FrameWidth": "16",
    "FrameHeight": "20",
    "PictureWidth": "11",
    "PictureHeight": "17",
    "MiddleMat": "1",
    "BottomMat": "1/2"
}

The problem I'm running into is that before I added a check to verify that the input value is positive, then the return response would make the calculation. However, by adding this code (I had to use -1 because 0 is a valid, positive input):

if (!(req.body.BottomMat > -1)) {
    res.status(422).json({ messages: "Please insert a positive value" });
}

Then any fraction I input such as 1/2 - it throws the error:

{
    "messages": "Please insert a positive value"
}

It should not be throwing a error message because these fractions are positive inputs. Can I get some guidance on how to prevent an error message when the input is a positive fraction as well?

Answer 1

Your input "1/2" is string at the time you are checking it in your condition, so you need to convert it to number

If you are directly trying to convert the fraction string into number using Number("1/2") method it will throw NaN. so use eval("1/2")

It will solve your problem

if (!(eval(req.body.BottomMat) > -1)) {
        res.status(422).json({ messages: "Please insert a positive value" });
    }

If you need secure method you can add regex to validate the input is valid number

if (!(req.body.BottomMat.match(/^[0-9\/\.]+$/) != null && eval(req.body.BottomMat) > -1)) {
  ...
}

So if the input contains any text other than 0-9 and / and . it will return false so it doesn't run eval for malicious inputs

Answer 2

the problem is with your condition req.body.BottomMat > -1 BottomMat is a string and you compare string with number "1/2" > -1 therefore it returns false, you need to convert to number before check

Answer 3

First Step

Don't send your data as 1/2, but as 0.5, because you will be able to parse it as Number with parseFloat().

Second Step

Now you can easily add validations:

app.post("/api/chatfuel/calculator/triplematapi", (req,res) => {
  const FrameWidth = parseFloat(req.body.FrameWidth);
  const FrameHeight = parseFloat(req.body.FrameHeight);
  const PictureWidth = parseFloat(req.body.PictureWidth);
  const PictureHeight = parseFloat(req.body.PictureHeight);
  const MiddleMat = parseFloat(req.body.MiddleMat);
  const BottomMat = parseFloat(req.body.BottomMat);
 
  if (FrameWidth < 0) res.status(422).json({ messages: "Please insert a positive value" });
  if (FrameHeight < 0) res.status(422).json({ messages: "Please insert a positive value" });
  if (PictureWidth < 0) res.status(422).json({ messages: "Please insert a positive value" });
  if (PictureHeight < 0) res.status(422).json({ messages: "Please insert a positive value" });
  if (MiddleMat < 0) res.status(422).json({ messages: "Please insert a positive value" });
  if (BottomMat < 0) res.status(422).json({ messages: "Please insert a positive value" });

  ...

});