Why does the JS code not work?

Question

Inserted JS code doesn't work but it there is in JS Challenge. Do I not understand something? : )

https://code.sololearn.com/WUKxGJJDTnOs/?ref=app

Answer 1

Default parameter value doesn't work in old browsers. You may try this...
function x (a, b, c) {
a=a||1;
b=b||5;
c=c||6;
return a+b-c;
}
document.write(x(7,4));
//output 5

Answer 2

Function x have default parameters.
When you call it x(7,4) means a=7 b=4 c=6 and 7+4-6=5
For more information:

https://developer.mozilla.org/th/docs/Web/JavaScript/Reference/Functions/Default_parameters

Answer 3

works fine 7+4-6=5, a=7, b=4, and c uses default of 6.