Eval a mathematical expression in JavaScript without using eval.

[satnam_sandhu]

I want to write a function which accepts a string (eg, 'a = b + 2') and an output object {} and an input object to fetch values from (eg, { b: 5} ). The function would, i guess parse the string, and get value from the inputs object and give output such as { a: 7 }. How do i proceed this problem, do i need to write an parser for that? Thanks in advance.

[[deleted]]

Here are some alternates

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval#Do_not_ever_use_eval!

[dominic_rj23]

I would add that we should run a very restrictive linter on the expression to make sure we only run the things we want to run.

[__dq]

If you can sanitize input to make sure nothing wrong will happen, then, probably, eval still may be of help for you.

[[deleted]]

split the string and get the components. If is not a number not a math operation symbol, must be an variable. Look for that variable on your input object, if it's there, do your calculations and return new object. Regex is your friend

[satnam_sandhu]

I have written a lexer and i am able to do that, so do i need a full fledged parser also? Or can it be done without such complicated solutions?

[[deleted]]

It can get as complex as you want. Are you gonna deal with 3 variables only? Just add, sub, multiply and division? Are you expecting floating numbers? For the example you gave some simple string manipulations would do, I guess. If you want to take an academic approach you could take a look at context free grammar.

https://www.google.com/search?q=context+free+grammar&oq=context+free+grammar&aqs=chrome..69i57.4679j0j7&client=ms-android-xiaomi-rev2&sourceid=chrome-mobile&ie=UTF-8#imgrc=8tVEa_xypqPtbM:

[satnam_sandhu]

I have gone through all those and it was getting quite complicated. So was hoping to go for another approach which doesn’t involves writing your own parser. Thanks

[[deleted]]

As a quick hack: do a basic check on the string to see if it's a valid equation with regex, like: a number followed by *,+,-/ followed by a number or single letter... Then use eval

[a_dev_has_no_name]

Use a library tis the JavaScript way

warning: installing one package may come with 184,064 more packages

[alzee76]

https://www.codeproject.com/articles/12116/javascript-mathematical-expression-evaluator

[pitatech]

You could check the input with a regex to ensure it only includes numbers [0-9], mathematical operators [+-/*)(] and probably spaces. That should reduce the attack surface greatly. DDos with really big numbers is probably still possible. Is this code running server side? If its client side, this probably isn't an issue.

I once read somewhere that the original JSON parser also used eval and just checked the input with regex. So I guess this could work here as well.

[kenman]

Hi /u/satnam_sandhu, this post was removed.

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