Bug when evaluating a string in JavaScript

Question

I have a string calc, which represents an operation (for example, "2/2")

I want my code to evaluate the string and return the result.

When the string is invalid and cannot be evaluated (for example, "2.2.2", I want to return null.

This is my code so far:

    const result = eval(calc);
    
    
    if (result === undefined){
      return null;
    } else {
      return result;
    }

My bug is: when the string is invalid as an operation and cannot be evaluated, I don't get null. Instead, the program throws an error.

How can I get null instead of an error?

First, please avoid using eval ( see here why). Secondly use a proper try ... catch to catch an error and set your result accordingly — empiric
– empiric, Commented Aug 17, 2020 at 9:38

I like eating pizza thursdays · Accepted Answer · 2020-08-17 09:37:49Z

3

Use try/catch

try {
    result = eval(calc);
    return result;
} catch (e) {
    return null;
}

answered Aug 17, 2020 at 9:37

I like eating pizza thursdays

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Comments

Mark Baijens · Accepted Answer · 2020-08-17 09:43:00Z

2

Using eval() is often not necessary and can lead to security and performance problems. There is likely a better solution available. I advise you to explain your use case more detailed so we can help you find a better solution.

If you really want to do this you can use a try catch statement since an exception will be thrown when the code can't be evaluated.

console.log(myEval("2.2.2"));
console.log(myEval("2/2"));

function myEval(str) {
  try {
    return eval(str);
  } catch {
    return null
  }
}

answered Aug 17, 2020 at 9:43

Mark Baijens

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Comments

Lucasz · Accepted Answer · 2020-08-17 09:44:47Z

1

eval() will return an error when it cannot parse the command correctly. When you use try and catch you can return null when an error occurred.

try{  
  const result = eval(calc);
  return result;
}
catch (error){
  return null;
}

answered Aug 17, 2020 at 9:44

Lucasz

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