feat(adjust): Add module adjust

Applies a function onto given index of an array

src/__tests__/adjust.js

@@ -0,0 +1,12 @@
+import adjust from '../adjust';
+
+test('Core.adjust', () => {
+  const add = x => y => x + y;
+  const list = [0, 1, 2];
+
+  expect(adjust(add(10), 1, list)).toEqual([0, 11, 2]);
+  expect(adjust(add(10), 1, list)).not.toBe(list);
+
+  expect(adjust(add(10), -1, list)).toEqual([0, 1, 12]);
+  expect(adjust(add(10), 10, list)).toBe(list);
+});

src/adjust.js

@@ -0,0 +1,13 @@
+import concat from './concat';
+import length from './length';
+
+export default (fn, i, arr) => {
+  const arrLength = length(arr);
+  if (i >= arrLength || i < -arrLength) return arr;
+
+  const idx = i < 0 ? arrLength + i : i;
+  const list = concat([], arr);
+  list[idx] = fn(list[idx]);
+
+  return list;
+};

src/index.js

@@ -1,3 +1,4 @@
+import adjust from './adjust';
 import allPass from './allPass';
 import anyPass from './anyPass';
 import append from './append';
@@ -33,6 +34,7 @@ import reverse from './reverse';
 import slice from './slice';
 
 export {
+  adjust,
   allPass,
   anyPass,
   append,