Question

I ran into this issue the other day and couldn’t figure out what exactly is happening under the hood. What are the rules for coercion of a String into a Number type? and why does it fail in the instance of ‘5.0.1’?

var numStr = '5.0';
var floatStr = '5.0.1';

//Passes
if (numStr >= 4) {
    alert('5 > 4');
}

//Fails
if (floatStr >= 4) {
    alert('5.0.1 > 4');
}

console.log(parseInt(numStr)); //5
console.log(parseInt(floatStr)); //5

console.log(Number(numStr)); //5
console.log(Number(floatStr)); //NaN

Answer 1

Well, for one, "5.0.1" is not a valid String representation of a floating pointer number. I would have expected parseInt to have failed as well.

Edit: However, as opposed to type casting the value, parseInt is a function that was designed to extract a value with more tolerance for noise.

The particular implementation details are defined in: EMCA-262

For instance, when applying a cast Number([value]), the toNumber function is used to perform the conversion — described in section 9.3.1. The behavior of parseInt is described in section 15.1.2.2.