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Asked: 2026-05-22T13:09:12+00:00

Basically, I have a user input field where a user can enter a number.

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Basically, I have a user input field where a user can enter a number. They would like to also be able to enter equations in the input field as well.

Something like “874.45 * 0.825 + 4000” and have that converted to its real numeric value.

All the solutions I found point to the dreaded eval() method. Especially with this being a user entered field, I’m concerned about just running eval(“874.45 * 0.825 + 4000”) and hoping to get a number out the back end.

I suppose I could do a web service call back to the server (ASP.NET), but I’m afraid a slight delay will create some frustration from the user.

Does anyone know of either a good technique or existing libraries?

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1 Answer

  1. Editorial Team

    What you really need here is an “expression parser”, because you’re trying to allow users to express their values using a small domain-specific language.

    The basic mechanics work like this:

    1. Tokenize their expression into operators and operands.

    2. Based on the order of operations (e.g, where multiplication is evaluated with higher priority than addition), push the operators and operands onto a stack.

    3. Pop the operands from the stack and push intermediate results back onto the stack. Repeat until the stack is empty.

    I’ve done this a gazillion times for different “little languages” throughout my projects. But never in javascript. So I can’t directly recommend a suitable parsing library.

    But a quick googling reveals “PEG.js”. Check it out here:

    http://pegjs.majda.cz/

    All of the examples in their documentation are for exactly the kind of “expression parser” you’re trying to build.

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