add array reduce

Author: praffn

implementations/reduce.js

@@ -0,0 +1,49 @@
+/**
+ *
+  reduce() exectues a provided *reducer* function on each element of
+  the array returning a final single output value.
+
+  reduce() has two parameters:
+
+  - the reducer callback function
+  - an initial value. If no initial value is supplied, the value
+    of the first element in the array is used, and the initial
+    iteration is skipped.
+    (which will be the initial value of the accumulator)
+
+  The reducer function takes four arguments:
+
+  - the accumulator
+  - the value of the current element
+  - the index of the current element
+  - the Array object being traversed
+
+  The return value of the reducer will be the new accumulator argument
+  in the next iteration, and ultimately the final, single resulting value.
+
+  Calling reduce on an empty array without an initial value will trow a TypeError
+*/
+
+Array.prototype.myReduce = function myReduce(reducer, initialValue) {
+  let accumulator = initialValue;
+  let i = 0;
+
+  // initival value check
+  if (typeof initialValue === 'undefined') {
+    if (this.length === 0) {
+      // no reduce on empty array without and initial value
+      throw new TypeError('reduce on empty array without initial value');
+    }
+
+    // no initial value, so accumulator is set to first element,
+    // and first iteration is skipped
+    [accumulator] = this;
+    i = 1;
+  }
+
+  for (; i < this.length; i += 1) {
+    accumulator = reducer(accumulator, this[i], i, this);
+  }
+
+  return accumulator;
+};