Implement Array.prototype.reduce

Write Array.prototype.myReduce(cb, initial) so it matches the built-in reduce. The grader disables the native map, filter, reduce and concat while your code runs.

Rules

• Call cb(accumulator, value, index, array) for each element, skipping holes. A callback that isn't a function throws TypeError.
• Use initial whenever that argument was passed, even as undefined. Otherwise start from the first element that exists and continue after it.
• With no initial value and nothing to reduce, throw TypeError.
• Define the method with function, not an arrow, so this is the array.

[1, 2, 3].myReduce((a, v) => a + v, 10)         // 16
[1, 2].myReduce((a, v) => String(a) + v, undefined)   // "undefined12"

Tests write a hole as "<hole>"; the grader turns it into a real empty slot. Callbacks are named in each test's input and supplied by the grader.