PSA: Don't use es6 default parameters for Array.reduce() accumulator function

reduce.js

// PSA: Don't use es6 default parameters for Array.reduce() accumulator function
a = [1, 2, 3];
a.reduce((sum = 0, x) => sum + x); // = 6
a.reduce((sum, x) => sum + x, 0); // = 6
// seems to work...

c = a.reduce((sum = 3, x) => sum + x); // = 6
// but! other initial value doesn't have an effect

// even better
b = [{i: 1}, {i: 2}, {i: 3}];
b.reduce((sum, x) => x.i + sum, 0); // = 6
b.reduce((sum = 0, x) => sum + x.i); // = [object Object]23
// ffffuuuuuuu

// -> default parameters don't work as you'd expect with Array.reduce, 
// because Array.reduce uses the first array element as accumulated 
// value ("sum"), if you don't supply a second argument..