Array.prototype.reduce

README.md

@@ -50,6 +50,7 @@ Polyfills are also available over a CDN, for example
 |[Array.prototype.forEach](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach?v=control#Polyfill)|328|
 |[Array.prototype.includes](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Array/includes?v=control#Polyfill)|346|
 |[Array.prototype.some](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/some#Polyfill)|346|
+|[Array.prototype.reduce](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/reduce#Polyfill)|492|
 |[String.prototype.includes](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/String/includes#Polyfill)|153|
 |[String.prototype.repeat](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/repeat#Polyfill)|504|
 |[String.prototype.startsWith](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/String/startsWith#Polyfill)|117|

package.json

@@ -24,6 +24,7 @@
     "firstElementChild",
     "includes",
     "some",
+    "reduce",
     "repeat",
     "startsWith",
     "endsWith",
@@ -47,6 +48,7 @@
     "Array.prototype.findIndex",
     "Array.prototype.includes",
     "Array.prototype.some",
+    "Array.prototype.reduce",
     "String.prototype.includes",
     "String.prototype.repeat",
     "String.prototype.startsWith",
@@ -87,6 +89,7 @@
     "Array.prototype.forEach.js",
     "Array.prototype.includes.js",
     "Array.prototype.some.js",
+    "Array.prototype.reduce.js",
     "CustomEvent.js",
     "Element.prototype.classList.js",
     "Element.prototype.closest.js",

rollup.build.js

@@ -34,6 +34,7 @@ rollup(input('./src/Array.prototype.findIndex/index.js')).then(output('./Array.p
 rollup(input('./src/Array.prototype.forEach/index.js')).then(output('./Array.prototype.forEach.js'));
 rollup(input('./src/Array.prototype.includes/index.js')).then(output('./Array.prototype.includes.js'));
 rollup(input('./src/Array.prototype.some/index.js')).then(output('./Array.prototype.some.js'));
+rollup(input('./src/Array.prototype.reduce/index.js')).then(output('./Array.prototype.reduce.js'));
 rollup(input('./src/Array.of/index.js')).then(output('./Array.of.js'));
 rollup(input('./src/String.prototype.repeat/index.js')).then(output('./String.prototype.repeat.js'));
 rollup(input('./src/String.prototype.startsWith/index.js')).then(output('./String.prototype.startsWith.js'));

src/Array.prototype.reduce/index.js

@@ -0,0 +1,63 @@
+// Production steps of ECMA-262, Edition 5, 15.4.4.21
+// Reference: http://es5.github.io/#x15.4.4.21
+// https://tc39.github.io/ecma262/#sec-array.prototype.reduce
+if (!Array.prototype.reduce) {
+    Object.defineProperty(Array.prototype, 'reduce', {
+        value(callback /*, initialValue*/) {
+            if (this === null) {
+                throw new TypeError('Array.prototype.reduce ' +
+            'called on null or undefined');
+            }
+            if (typeof callback !== 'function') {
+                throw new TypeError(callback +
+            ' is not a function');
+            }
+
+            // 1. Let O be ? ToObject(this value).
+            var o = Object(this);
+
+            // 2. Let len be ? ToLength(? Get(O, "length")).
+            var len = o.length >>> 0;
+
+            // Steps 3, 4, 5, 6, 7
+            var k = 0;
+            var value;
+
+            if (arguments.length >= 2) {
+                value = arguments[1];
+            } else {
+                while (k < len && !(k in o)) {
+                    k++;
+                }
+
+                // 3. If len is 0 and initialValue is not present,
+                //    throw a TypeError exception.
+                if (k >= len) {
+                    throw new TypeError('Reduce of empty array ' +
+              'with no initial value');
+                }
+                value = o[k++];
+            }
+
+            // 8. Repeat, while k < len
+            while (k < len) {
+                // a. Let Pk be ! ToString(k).
+                // b. Let kPresent be ? HasProperty(O, Pk).
+                // c. If kPresent is true, then
+                //    i.  Let kValue be ? Get(O, Pk).
+                //    ii. Let accumulator be ? Call(
+                //          callbackfn, undefined,
+                //          « accumulator, kValue, k, O »).
+                if (k in o) {
+                    value = callback(value, o[k], k, o);
+                }
+
+                // d. Increase k by 1.
+                k++;
+            }
+
+            // 9. Return accumulator.
+            return value;
+        }
+    });
+}