In the codebase I'm working on, I encountered code like this:

try {
    price = parseFloat(price);
} catch (err) {
    console.log(err);
}

I know that in most cases where price cannot be turned into a number, it'll simply get the value of NaN instead. My question is: are there cases where it will throw an error, making the try-catch-construction necessary?

In the codebase I'm working on, I encountered code like this:

try {
    price = parseFloat(price);
} catch (err) {
    console.log(err);
}

I know that in most cases where price cannot be turned into a number, it'll simply get the value of NaN instead. My question is: are there cases where it will throw an error, making the try-catch-construction necessary?

• javascript
• parsefloat

• 1 Depends on what price is. Try with var price = {valueOf() { throw new Error; }} – Bergi Commented Oct 31, 2016 at 10:59

3 Answers 3

are there cases where it will throw an error, making the try-catch-construction necessary?

Yes. Apart from reference errors (because price was not declared) or parseFloat was overwritten with something that's not a function or the like, the builtin parseFloat can also throw exceptions.

It does however never throw an error when you pass in a string. It will only throw when trying to convert the argument to a string fails with an exception. Examples for that include:

• passing a symbol
• passing an object without [Symbol.toPrimitive], .valueOf or .toString methods
• passing an object where one of these methods throws
• passing an object where none of these methods return a primitive

Short answer: No.

from MDN

A floating point number parsed from the given string. If the first character cannot be converted to a number, NaN is returned

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/parseFloat

parseFloat() itself only returns and never throws an error. However if the variable price has not been declared in the context you will get an error like:

Uncaught ReferenceError: price is not defined

It explains the reason why there is a try/catch block around it.

So to answer your question: Yes there are cases when it is necessary if you can't trust previous js execution/context (due to adblockers or similar for instance) but no it's not the function parseFloat() that throws the error it's the interpreter.