Evaluating a string as a mathematical expression in JavaScript

Question

How do I parse and evaluate a mathematical expression in a string (e.g. \'1+1\') without invoking eval(string) to yield its numerical value?

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Answer 1

Somebody has to parse that string. If it's not the interpreter (via eval) then it'll need to be you, writing a parsing routine to extract numbers, operators, and anything else you want to support in a mathematical expression.

So, no, there isn't any (simple) way without eval. If you're concerned about security (because the input you're parsing isn't from a source you control), maybe you can check the input's format (via a whitelist regex filter) before passing it to eval?

Answer 2

Simple and elegant with Function()

function parse(str) {
  return Function(`'use strict'; return (${str})`)()
}

parse("1+2+3"); 

Answer 3

I believe that parseInt and ES6 can be helpful in this situation

let func = (str) => {
  let arr = str.split("");
  return `${Number(arr[0]) + parseInt(arr[1] + Number(arr[2]))}`
};

console.log(func("1+1"));

The main thing here is that parseInt parses the number with the operator. Code can be modified to the corresponding needs.

Answer 4

I've recently done this in C# (no Eval() for us...) by evaluating the expression in Reverse Polish Notation (that's the easy bit). The hard part is actually parsing the string and turning it into Reverse Polish Notation. I used the Shunting Yard algorithm, as there's a great example on Wikipedia and pseudocode. I found it really simple to implement both and I'd recommend this if you haven't already found a solution or are looking for alternatives.