Why is this 2

Question

https://code.sololearn.com/W37tfqtHuB40/?ref=app

Answer 1

arrayName.filter(callbackFunction(){
//...
return booleanResultTrue;
}

is a method for the array object that return the element only if that's a true condition for his callback. What means: the boolean false is equal to 0, boolean true is every number not equal to 0; said so, your callback is supposed to be a "filter" for your array so it will return a result only if a condition is true -> your var a = c = [1,2,3,4] so suppose to write
var newArray = a%2-1;
return newArray;
inside the callback.
what will newArray contain? newArray = [0, -1, 0, -1] because a%2 is [1,0,1,0] (the rest for each element of your array a).
Now the method filter will pass to your var b only the elements of c that will return a true condition, so the elements that correspond to newArray[1] and newArray[3], so b = [2, 4], so b[0] = 2.

if you try to write only return a%2 your var b will be equals to [1,3]

Answer 2

thank you for your response