Function by string in javascript and SyntaxError

Question

I create a let calc = new Function(return ${strExpression};); I want to check syntax of calc before run or caught SyntaxError raised by calc();

I tried:

function compute(strExpression){
    let calc = new Function(`return ${strExpression};`);
    try {
            result = calc();
            return result;
        } catch (error) {
            return null;
        } 
}

For some invaild strExpression I could get an error and in this case I see SyntaxError on console but catch doesn't work.

The syntax error from invalid code is thrown by the Function call, not the calc call. Put everthing within the try block. — Bergi
– Bergi, Commented Jan 20, 2018 at 14:22

Nina Scholz · Accepted Answer · 2018-01-20 13:44:51Z

1

You see an error from

let calc = Function("some code");

because the given string is not valid code.

You could move the line into the try block and catch the error.

var result = function() {
    try {
        let calc = Function("some code");
        result = calc();
        return result;
    } catch (error) {
        return null;
    }
}();

console.log(result);

answered Jan 20, 2018 at 13:44

Nina Scholz

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2 Comments

connexo Over a year ago

I used to think it's gotta be new Function(...). No need to explicitly call the constructor using new?

Nina Scholz Over a year ago

it works both, according to MDN

Hanif · Accepted Answer · 2018-01-20 15:17:28Z

1

I changed my answer based your updated question. Then completely code block from where may arise exception should keep inside try block like following:

function compute(strExpression){
  try {
      let calc = new Function(`return ${strExpression};`);
      result = calc();
      return result;
  } catch (error) {
      return null;
  }

}

But you should also keep in mind few things. What will be happen if I call above function with empty parameter like following?

console.log(compute());

There you expecting should go in catch block but reality is NO. Because parameter is undefined so this is also one kind of datatype in JS so no in catch block. So you should also these type issue when you using try catch.

edited Jan 20, 2018 at 15:17

answered Jan 20, 2018 at 13:48

Hanif

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