Evaluating a String as a Mathematical Expression in JavaScript

Evaluating a string as a mathematical expression in JavaScript

I've eventually gone for this solution, which works for summing positive and negative integers (and with a little modification to the regex will work for decimals too):

function sum(string) {
  return (string.match(/^(-?\d+)(\+-?\d+)*$/)) ? string.split('+').stringSum() : NaN;
}   

Array.prototype.stringSum = function() {
    var sum = 0;
    for(var k=0, kl=this.length;k<kl;k++)
    {
        sum += +this[k];
    }
    return sum;
}

I'm not sure if it's faster than eval(), but as I have to carry out the operation lots of times I'm far more comfortable runing this script than creating loads of instances of the javascript compiler

How to evaluate a math expression given in string form?

With JDK1.6, you can use the built-in Javascript engine.

import javax.script.ScriptEngineManager;
import javax.script.ScriptEngine;
import javax.script.ScriptException;

public class Test {
  public static void main(String[] args) throws ScriptException {
    ScriptEngineManager mgr = new ScriptEngineManager();
    ScriptEngine engine = mgr.getEngineByName("JavaScript");
    String foo = "40+2";
    System.out.println(engine.eval(foo));
    } 
}

Safe evaluation of arithmetic expressions in Javascript

You can try JavaScript Expression Evaluator:

This library is a modified version of
Raphael Graf’s ActionScript Expression
Parser. When I wrote the JavaScript
Function Plotter, I wanted a better
alternative to using JavaScript’s eval
function. There’s no security risk
currently, because you can only run
code in your own browser, but it’s not
as convenient for math (Math.pow(2^x)
instead of 2^x, etc.).

Then your code will be like that:

console.info ( Parser.evaluate( "2 * (3 + 4)" ) ); //prints 14

The source code is on GitHub and it's published on npm as expr-eval. Can be used like so:

import { Parser } from 'expr-eval';

console.log(Parser.evaluate("2 * (3 + 4)")); // 14

Is there a way to evaluate strings as mathematical expressions in HTML/JS?

The function you're in need of is eval().

eval('1+1');
// Output: 2

Or implemented in your code that would be:

var equation = document.getElementById("equation").value;
var answer = document.getElementById("answer");
answer.innerHTML = eval(equation);

For more on the eval() function, read the documentation.

Check if string is a mathematical expression in JavaScript

Here's a regex that can do what I think you're looking for:

/(?:(?:^|[-+_*/])(?:\s*-?\d+(\.\d+)?(?:[eE][+-]?\d+)?\s*))+$/

https://regex101.com/r/w74GSk/4

It matches a number, optionally negative, with an optional decimal number followed by zero or more operator/number pairs.

It also allows for whitespace between numbers and operators.

const re = /(?:(?:^|[-+_*/])(?:\s*-?\d+(\.\d+)?(?:[eE][+-]?\d+)?\s*))+$/;

function test(s) {

  console.log("%s is valid? %s", s, re.test(s));

}

// valid

test(" 1 ");

test("1 + 2");

test(" 1 * 2 * 3 ");

test("-1 * 2 - -3");

test("-1 * 2 - -3e4");

test("-1 * 2 - -3.5E6");

// invalid

test("1 +");

test("1 + foo");

Js - Alternatives to eval mathematical expression with operator as string

implement functions for operators
ops = {
'>': (a, b) => a > b,
'>=': (a, b) => a >= b
};
ops[expressionObject.operator](object[property], expressionObject.number)
if the expression is always valid as expected. then following should faster as no parsing.
eval(${object[property]}${expression})