add pipe + compose

packages/core/test/util.test.mjs

@@ -1,5 +1,5 @@
 import { strict as assert } from 'assert';
-import { isNote, tokenizeNote, toMidi, mod } from '../util.mjs';
+import { isNote, tokenizeNote, toMidi, mod, compose } from '../util.mjs';
 
 describe('isNote', () => {
   it('should recognize notes without accidentals', () => {
@@ -83,3 +83,16 @@ describe('mod', () => {
     assert.equal(mod(-3, 2), 1);
   });
 });
+
+describe('compose', () => {
+  const add1 = (a) => a + 1;
+  it('should compose', () => {
+    assert.equal(compose(add1, add1)(0), 2);
+    assert.equal(compose(add1)(0), 1);
+  });
+  const addS = (s) => (a) => a + s;
+  it('should compose left to right', () => {
+    assert.equal(compose(addS('a'), addS('b'))(''), 'ab');
+    assert.equal(compose(addS('a'), addS('b'))('x'), 'xab');
+  });
+});

packages/core/util.mjs

@@ -42,3 +42,14 @@ export const getPlayableNoteValue = (event) => {
 
 // rotate array by n steps (to the left)
 export const rotate = (arr, n) => arr.slice(n).concat(arr.slice(0, n));
+
+export const pipe = (...funcs) => {
+  return funcs.reduce(
+    (f, g) =>
+      (...args) =>
+        f(g(...args)),
+    (x) => x,
+  );
+};
+
+export const compose = (...funcs) => pipe(...funcs.reverse());