I noticed that the correct
return str.slice(0, res);
returns the same value as the incorrect
var str = "some_string";
return str.slice(str, res);
In this case str is a string and res is a numeric quantity.
My guess is that some how because slice expects a numeric quantity and does not get one ( for the first parameter ), it converts what it finds to a 0.
Is this expected behavior?
JavaScript provides implicit type coercion. That means if an integer is expected (as is the case here), it will convert the value provided to an integer. If no sensible value can be divined, zero is used.
If an integer is provided, great!
If a string is provided and it looks like a integer (e.g. str.slice("5", res)) it will be converted into the expected integer.
If a string is provided and it doesn't look like a number (e.g. str.slice("abc", res)), 0 is used.
My guess is that some how because slice expects a numeric quantity and does not get one ( for the first parameter ), it converts what it finds to a 0.
That's basically what happens. .slice calls ToInteger on the first argument, which in turn calls ToNumber. But if the value cannot be converted to a number (if the result is NaN), it returns 0.
So, if you pass a numeric string as start, .slice would start at the mathematical value of that string. Any other string converts to 0.
strcannot be converted to a value different fromNaN. See es5.github.io/#x9.4.5(as opposed to the integer5) to be a "numeric quantity"?