how do you know when something is done sorting javascript code example

Example: in compare method why we taking a and b for sorting in javascript

for compare and sort method:-
  
function compare(a,b) {
    return a - b;
}
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var numbers = [1,5,3.14];
compare(1,5);     // Returns -4, a is less than b
compare(1,3.14);  // Return -2.14, a is less than b
compare(5,3.14);  // returns 1.86, a is greater than b