feat: add thenP (#551)

Ref #35

src/allP.js

@@ -1,6 +1,5 @@
 import { bind } from 'ramda';
 
-/* eslint-disable max-len */
 /**
  * Composable shortcut for `Promise.all`.
  *
@@ -24,7 +23,6 @@ import { bind } from 'ramda';
  * RA.allP([Promise.resolve(1), Promise.resolve(2)]); //=> Promise([1, 2])
  * RA.allP([1, Promise.reject(2)]); //=> Promise(2)
  */
-/* eslint-enable */
 const allP = bind(Promise.all, Promise);
 
 export default allP;

src/index.d.ts

@@ -721,7 +721,7 @@ declare namespace RamdaAdjunct {
             (high: number, value: number): boolean;
             (high: number): (value: number) => boolean;
         };
-        
+
         /**
          * Returns whether or not an object has an own property with the specified name at a given path.
          */
@@ -777,6 +777,15 @@ declare namespace RamdaAdjunct {
          */
         rejectP<T>(value?: T): Promise<T>;
 
+        /**
+         * Composable shortcut for `Promise.then`.
+         * The thenP function returns a Promise. It takes two arguments: a callback function for the success of the Promise
+         * and the promise instance itself.
+         */
+        thenP<T>(onFulfilled: Function, thenable: Promise<T>): Promise<T>;
+        thenP<T>(onFulfilled: Function): (thenable: Promise<T>) => Promise<T>;
+
+
         /**
          * Runs the given list of functions in order with the supplied object, then returns the object.
          * Also known as the normal order sequencing combinator.

src/index.js

@@ -90,6 +90,7 @@ export { default as curryRight } from './curryRight';
 export { default as allP } from './allP';
 export { default as resolveP } from './resolveP';
 export { default as rejectP } from './rejectP';
+export { default as thenP } from './thenP';
 export { default as Y } from './Y';
 export { default as seq } from './seq';
 export { default as sequencing } from './seq';

src/thenP.js

@@ -0,0 +1,27 @@
+import { invoker } from 'ramda';
+
+/**
+ * Composable shortcut for `Promise.then`.
+ * The thenP function returns a Promise. It takes two arguments: a callback function for the success of the Promise
+ * and the promise instance itself.
+ *
+ * @func thenP
+ * @memberOf RA
+ * @since {@link https://char0n.github.io/ramda-adjunct/2.8.0|v2.8.0}
+ * @category Function
+ * @sig (a -> Promise b | b) -> Promise b
+ * @param {Function} onFulfilled A Function called if the Promise is fulfilled. This function has one argument, the fulfillment value
+ * @param {Promise} promise Any Promise or Thenable object
+ * @return {Promise} A Promise in the pending status
+
+ * @see {@link RA.resolveP|resolveP}, {@link RA.rejectP|rejectP}, {@link RA.allP|allP}
+ * @example
+ *
+ * const promise = Promise.resolve(1);
+ * const add1 = v => v + 1;
+ *
+ * RA.thenP(add1, promise); // => Promise(2)
+ */
+const thenP = invoker(1, 'then');
+
+export default thenP;

test/thenP.js

@@ -0,0 +1,51 @@
+import { assert } from 'chai';
+import sinon from 'sinon';
+import * as R from 'ramda';
+
+import * as RA from '../src/index';
+
+describe('thenP', function() {
+  context('when applied on Promise', function() {
+    specify('should call then method with onFulfilled function', function() {
+      const promise = RA.resolveP(1);
+      const expected = RA.thenP(R.add(1), promise);
+
+      assert.eventually.strictEqual(expected, 2);
+    });
+  });
+
+  context('when applied on Thenable', function() {
+    specify('should call then method with onFulfilled function', function() {
+      const thenable = { then: fn => RA.resolveP(fn(1)) };
+      const expected = RA.thenP(R.add(1), thenable);
+
+      assert.eventually.strictEqual(expected, 2);
+    });
+  });
+
+  context('when applied on non-thenable', function() {
+    specify('should throw TypeError', function() {
+      assert.throws(() => RA.thenP(R.identity, {}), TypeError);
+    });
+  });
+
+  it('should call `then` method on thenable', function(done) {
+    const then = sinon.stub().returns(RA.resolveP(1));
+    const thenable = { then };
+    const expected = RA.thenP(R.identity, thenable);
+
+    expected.then(() => {
+      assert.isTrue(then.calledOnce);
+      done();
+    });
+  });
+
+  it('should have arity of 2', function() {
+    assert.strictEqual(RA.thenP.length, 2);
+  });
+
+  it('should be curried', function() {
+    assert.eventually.strictEqual(RA.thenP(R.identity, RA.resolveP(1)), 1);
+    assert.eventually.strictEqual(RA.thenP(R.identity)(RA.resolveP(1)), 1);
+  });
+});