How to calculate an expression given in string? JS

Question

I exported a large excel spreadsheet as html and I want to calculate some cells' values in JavaScript. I smuggled the expressions used in excel as data-xxx info in every P I intend to fill.

Is there a way of translating the data-xxx to JavaScript code so that it'll give me the wanted value?

Example strings:

06A02
min(06C02;06C03)
max(03B04;03B05;03B06)
max(min(07A01;07A02;08F01);min(09A01;09A02))
max(min(03B01;03B02;03B03);08C02;09B01;09C02)
max(min(07A03;07A04;08F02);min(09A03;09A04);09C02)
max(09B05;min(07A01;07A02;08F01);min(09A01;09A02))
max(09B05;min(07A03;07A04;08F02);min(09A03;09A04);09B01;09C02)

06A02 and similar are just keys of values stored in localStorage, saved via another page's form.

I thought I could just use ifs and elses and slice the string piece by piece, but that would make a giant tree I could make too many mistakes in.

I thought of regular expressions to shorten the tree, but I'm not sure if it's that much better.

There may be a way I don't know yet.

Example HTML:

<TR>
<TD>Prolonged lack of access...</TD>
<TD>A</TD>
<TD>A</TD>
<TD><p id="ar1_11e" data-eks="AC.E.Inc"></p></TD>
<TD>1</TD>
<TD><p id="ar1_11za" data-zap="03D01"></p></TD>
<TD><p id="ar1_11i" data-imp="ar1"></p></TD>
<TD><p id="ar1_11og" data-ogr="min(03D02;03D03)"></p></TD>
<TD><p id="ar1_11la" data-lag="04A05"></p></TD>
<TD><p id="Par1_11"></p></TD>
<TD><p id="War1_11"></p></TD>
<TD><p id="Rar1_11"></p></TD>
<TD><p id="Oar1_11"></p></TD>
</TR>

jdaz · Accepted Answer · 2020-07-25 04:11:33Z

3

@hev1 has provided an excellent solution. If you wanted to use regex you could do something like this:

function evalExcel(rawString) {
  // match expected format, prevent easy eval injection
  const safeMatch = /^(min|max|[();]|[A-F0-9]{5})*$/;

  if (rawString.match(safeMatch) && window.localStorage) {
    const regexNum = /[A-F0-9]{5}/g;
    const jsCode = rawString.replace(/;/g, ", ")
      .replace(/min/g, "Math.min")
      .replace(/max/g, "Math.max")
      .replace(regexNum, "+localStorage.getItem(\"$&\")");
    return eval(jsCode);
  }
  return null;
}

Demo

edited Jul 25, 2020 at 4:11

answered Jul 9, 2020 at 19:33

jdaz

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4 Comments

x1n13y84issmd42 Over a year ago

I like this one. I'd change safeMatch to /^(min|max|[A-F0-9]{5})/ though, because single value is a valid expression as well (see OP's example & HTML).

jdaz Over a year ago

I’m not sure what you mean, safeMatch already matches a single value.

x1n13y84issmd42 Over a year ago

Ah, figured the issue - the ; is missing in the linked demo regexp (the [()] part), but I thought it was not working because of $.

Warxuaroz Over a year ago

This is great. Somehow all I get in my Ps are zeroes, but I believe it's a defect in the rest of the code somewhere. Thanks a lot, that's exactly what I needed!

Unmitigated · Accepted Answer · 2020-07-09 23:23:09Z

1

You can use a simple recursive inner function.

function getValue(str){
    let idx = 0;
  function helper(){
    let op = Math[str.slice(idx,idx+3)];
    let opened = 1, closed = 0;
    let args = [];
    for(idx += 3; idx < str.length && opened != closed; idx++){
        if(str.indexOf("min", idx) == 0 || str.indexOf("max", idx) == 0){
        args.push(helper());
      } else {
        let delimIdx = str.indexOf(";", idx);
        if(delimIdx != -1){
            args.push(+localStorage.getItem(str.slice(idx, delimIdx)));
        } else {
            args.push(+localStorage.getItem(str.slice(idx,str.indexOf(')', idx))));
        }
      }
    }
    return op(...args);
  }
  return str.startsWith("min") || str.startsWith("max") ? helper(): +localStorage.getItem(str);
}

Demo

edited Jul 9, 2020 at 23:23

answered Jul 9, 2020 at 19:19

Unmitigated

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9 Comments

Warxuaroz Over a year ago

I'm trying out this function and it seems to have a mistake in "return op(...args);" Console says op is not a function and I don't understand it either. Why is helper() being pushed to args, why is it on the return?

Warxuaroz Over a year ago

The demo does nothing when I try to run it and the code pasted in my .js file raises an error in he line I mentioned above.

Unmitigated Over a year ago

@Warxuaroz Does the demo itself work? Could you provide a reproducible example of it not working?

Unmitigated Over a year ago

@Warxuaroz Could you put your code in a JSFiddle for me?

Unmitigated Over a year ago

@Warxuaroz I've fixed it. Try it out.

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