How does parseInt() in Javascript work?

Answer 1

From Microsoft's docs:

If no prefix of numString can be successfully parsed into an integer, NaN(not a number) is returned.

So yes, only parsable prefixes are returned.

Answer 2

You only get number till where it is meaningful when converting from string to number.

console.log(parseInt('01abbb')) // 1 -> it is started by 01 before chars
console.log(parseInt('31xyz'))  // 31 -> it is started by 31 before chars
console.log(parseInt('zyz31'))  // NaN -> it is not started by numbers
console.log(parseInt('31xyz1')) // 31 -> it is started by 31 before chars

Answer 3

ParseInt reads until it stops seeing a number. Since xyz is not a number, it returns NaN (Not a Number).

Answer 4

For more information please refer to documentation

Note: Only the first number in the string is returned!

Note: Leading and trailing spaces are allowed.

Note: If the first character cannot be converted to a number, parseInt() returns NaN.

Note: Older browsers will result parseInt("010") as 8, because older versions of ECMAScript, (older than ECMAScript 5, uses the octal radix (8) as default when the string begins with "0". As of ECMAScript 5, the default is the decimal radix (10).