CODEWITHSUNDEEP
javascript
learningjavascriptks
learningjavascriptks

Reputation: 325

How do I check the Boolean value on eval() or if an arithmetic operation is false?

I am doing this for a calculator project, I want to check if the operation is valid or not, somehow I cannot check for the Boolean value of eval if it is false? on the console:

Boolean(eval('2+2(9.1-)9'));
Boolean(2+2(9.1-)9); // Both operations return unexpected token

unlike Boolean(2+2) <-- returns true. Help?

Upvotes: 3

Views: 123

Answers (2)

Nebula
Nebula

Reputation: 7151

You actually don't need to evaluate the code in order to see if it's valid - just try creating a Function:

function checkIt() {
  var fn

  try {
    var fn = new Function(document.getElementById("code").value)
    alert("Great, that's a valid piece of code!")
  } catch (e) {
    alert("That's not a valid piece of code.")
  }
}
<input id="code">
<button onclick="checkIt()">Check it</button>

For example, try "123", "valid", and "''not[valid!!!".

Though if you are going to be evaluating it right away, if it's valid, you should probably just check if the error is a syntax error, or otherwise.

function doIt() {
  var fn

  try {
    var result = eval(document.getElementById("code").value)
    alert("The result is: " + result)
  } catch (e) {
    if (e instanceof SyntaxError) {
      alert("That's not a valid piece of code.")
    } else {
      alert(e.message)
    }
  }
}
<input id="code">
<button onclick="doIt()">Check it</button>

For example, try the same things you tried before and see how this behaves.

Upvotes: 1

Kewin Dousse
Kewin Dousse

Reputation: 4027

If I understood your question correctly, the only thing you want to know is "Is that a valid expression". One very simple way to check this is using eval() as you did here, and to enclose it in a try, and see if any error occurs. For example, you could write this :

try {
    eval('2+2(9.1-)9');
    valid = true;
} catch (e) {
    valid = false;
}

Then, the variable valid contains true if the expression is valid, and false if it is not.

Warning with eval() though : Every valid code will pass this test, not only mathematical expressions : Plus, that code will be executed. Be careful what strings you give it.

Upvotes: 0

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