JavaScript is often the subject of parody, for its seemingly unexpected results.
var a = []+{} // [Object object]
var b = {}+[] // 0
However there is consistency in its madness, and I suspected the parseInt behavior must have some reason behind it.
Getting to the bottom of what's happening
I first thought of debugging parseInt, but since couldn't debug a native function, I thought of wrapping it around another function that basically does the same thing.
var a = ['10','10','10','10']
var intParse = function (x) {
return parseInt(x);
};
console.log(a.map(parseInt)); // [10, NaN, 2, 3, 4]
console.log(a.map(intParse)); // [10,10,10,10]
Ok so it seems like everything is working fine
But just for the sake of brevity I decided to try some more observations
var a;
(a = Array(13).join('10,').split(',')).pop() // try array of 13 '10's
a.map(parseInt); // [10, NaN, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]
(a = Array(10).join('100,').split(',')).pop() // try array of 10 '100's
a.map(parseInt); // [100, NaN, 4, 9, 16, 25, 36, 49, 64, 81]
(a = Array(10).join('7,').split(',')).pop() // try array of 10 '6's
a.map(parseInt); // [7, NaN, NaN, NaN, NaN, NaN, NaN, 6, 6, 6]
Maybe it's not that weird after all
At this point as weird as the results may seem, they are consistent (in some way), there certainly seems to be a pattern.
It then hit me.
Array.map(callback) the callback takes 3 parameters, (key, index, array), so what if parseInt doesn't just take one parameter but 2 instead.
That would certainly had an effect on its results
Turns out The parseInt() function parses a string argument and returns an integer of the specified radix or base.
Syntax
parseInt(string, radix);
the radix is the base of the number
parseInt("10", 0) // 10, zero meant decimal
parseInt("10", 1) // NaN, since only 0 is allowed in base 1
parseInt("10", 2) // 2, '10' in binary
parseInt("10", 3) // 3, '10' in ternary
//...
Since the second argument in map's callback is the index the radix kept changing according to the index.
This explains why my intParse function worked.
I had specifically defined that it uses 'parseInt' with just x.
I thought this was what's happening inside map
var intParse = function (x) { return parseInt(x);}
When in fact this is what was happening
var intParse = function (x, r, array) { return parseInt(x, r);}
What I should've done when wrapping the function was
to not assuming the arguments that where being passed like so
var a = ['10','10','10','10']
var intParse = function () {
return parseInt.apply(this, arguments);
}
console.log(a.map(parseInt)); // [10, NaN, 2, 3, 4]
console.log(a.map(intParse)); // [10, NaN, 2, 3, 4]
Lessons learned
This was a nice exploration, I think I wound up learning a bit more than I thought I would about parseInt.
More importantly tho I was reminded that when programs act in an unexpected way, it is most likely for a reason.
Finally, if one wants to properly wrap a function use .apply(this, arguments)
