Greatest Common Divisor - Javascript Problem

I am stuck explaining a solution to myself and I need help.

Question:

Write a function gcd that takes two natural numbers and calculates their gcd.
Example: gcd (6, 15) should return 3.

Answer:

function gcd (a,b) {

if ((typeof a !== 'number') || (typeof b !== 'number'))
return false;

a = Math.abs(a);
b = Math.abs(b);

while (b) {
let c = b;
b = a % b;
a = c;
}
return a;

};

Explanation:

we declare a function 'gcd'
it has 2 parameters: 'a' and 'b'
we first open an if statement to check if our condition is true
we want to know if typeof a or typeof b is not a number
if our condition is true, meaning if typeof a & typeof b are not a number and a string, boolean or any other data type, then we return false and end the function
if our condition is false, which it is, as both (typeof a and typeof b) are numbers then we want to execute the following code

we want to convert our two parameters 'a' & 'b' to absolute positive values
we do that by using Math.abs(a) and Math.abs(b)
we initialize the variable a & b by the output we get from using the Math.abs method on a & b
we then open a while statement
this while loop will loop through a block of code as long as a specified condition is true
if the specified condition is false then the statement after the while loop will be executed
our condition is to check if b is a truthy or falsy value
if it is true then we want to run the block of code

we create a temporary variable 'c'
we initialize it with the value of 'b'
we then use the modulo / remainder operator by dividing a by b
we store the result in b