Why does parseInt('1 year old') return 1 but parseInt('I am 1 year old') returns NaN?

Question

The code ran in developer console is shown below. Why does parseInt return the number present in [0]th index of a string but not return the number present in any other index in the string?

parseInt('i am 1 year old')
// output NaN
parseInt('1 year old')
// output 1

What is the reason why it behaves like this?

Answer 1

The parseInt function reads the string to a point it keeps getting numbers. So, 10 is a number returns 10 because the function stops at third character, which is a space. But in case of i am 1 year old, it stops at i because it is not a number and returns NaN instead.

If parseInt encounters a character that is not a numeral in the specified radix, it ignores it and all succeeding characters and returns the integer value parsed up to that point. parseInt truncates numbers to integer values. Leading and trailing spaces are allowed.

Reference: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/parseInt