Angular7: How to eval an arithmetic string

Question

I'm using Angular 7 and I want to evaluate an arithmetic function that I receive as string as follows:

I get a "value" from a service which is a number. I also get a "rule" to apply to this value as @Input parameter, which is a string and can contain ">", "<" and "==". Example:

value = 10;
rule = ">2"

I then want to apply that rule and check if it is true:

if (10 > 2) {
  console.log("this rule applies!");
}

I tried using eval like this:

const strValue: string = value.toString();
const expression = strValue.concat(rule);
if (eval(expression)) {
  console.log("this rule applies!");
}

But I'm getting an error:

Refused to evaluate a string as JavaScript because 'unsafe-eval' is not an allowed source of script in the following Content Security Policy directive: "script-src 'self' 'unsafe-inline'".

So I guess I should not use "eval" to do this. Do you have any idea how I could solve it?

Are you confident that the data you will use eval on is safe? If so, then you can look for a way to enable this or otherwise bypass the unsafe-eval check because it's not unsafe. Otherwise, you need a different way of going about it. — VLAZ
– VLAZ, Commented May 17, 2019 at 6:27

adiga · Accepted Answer · 2019-05-17 06:50:45Z

3

You could use the regex (\D+)(\d+) to get any operator followed by a number. You could add more operators to the switch should you choose to extend it.

function evaluate(value, rule) {
  const [,operator, number] = rule.match(/(\D+)(\d+)/)
  
  switch(operator) {
    case ">" : return value > number
    case "<" : return value < number
    case "==" : return value == number
  }
  
  return null
}

console.log(evaluate(3, ">1"))
console.log(evaluate(3, "<2"))
console.log(evaluate(3, "==3"))

edited May 17, 2019 at 6:50

answered May 17, 2019 at 6:35

adiga

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Nina Scholz · Accepted Answer · 2019-05-17 06:34:52Z

2

You could get the operator and use a function for each operator.

function evaluate(value, rule) {
    const
        operators = {
            '<': (a, b) => a < b,
            '>': (a, b) => a > b,
            '==': (a, b) => a == b
        },
        [op, value2] = rule.match(/^(<|>|==)|.*$/g);

    return operators[op](value, value2);
}

console.log(evaluate(10, '<2'));
console.log(evaluate(10, '>2'));
console.log(evaluate(10, '==2'));
console.log(evaluate(10, '==10'));

answered May 17, 2019 at 6:34

Nina Scholz

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