Question

The following function uses a Short Circuit Operator to return 0 as default:

function custNumParse(str){
    return str*1||0;
}

I am not sure when this will return the second (I mean, I know that it will when str*1 can’t be evaluated to true), but I am not sure what inputs could produce that output when multiplying by 1.

I know that the falsish values are 0, "", false, null, undefined, NaN, but this doesn’t help me that much.

In other words I am clueless what would happen when I use that function with objects or booleans, etc. I know I can test them all, but I am sure there is an easier way to go

Any ideas on what is the most proper way to find these guys without testing them all?

Answer 1

The * operator is only for numbers, so anything that’s not a number (or can’t be converted to one) will make str*1 return NaN. Also 0*1 is obviously 0.

EDIT: booleans seems to be converted to either 0 or 1.

false * 1 === 0
true * 1 === 1

EDIT 2: Strings with numeric values will also be converted

"12" * 1 === 12
"0" * 1 === 0

EDIT 3: Be careful with arrays (they are converted to strings and then to ints).

[] * 1 === 0
[2] * 1 === 2
[1,2] * 1 === NaN