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curriculum/challenges/english/10-coding-interview-prep/rosetta-code/accumulator-factory.md

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+---
+title: Accumulator factory
+id: 594810f028c0303b75339ace
+challengeType: 5
+forumTopicId: 302222
+---
+
+## Description
+<section id='description'>
+A problem posed by <a href='https://en.wikipedia.org/wiki/Paul_Graham_(programmer)' target='_blank'>Paul Graham</a> is that of creating a function that takes a single (numeric) argument and which returns another function that is an accumulator. The returned accumulator function in turn also takes a single numeric argument, and returns the sum of all the numeric values passed in so far to that accumulator (including the initial value passed when the accumulator was created).
+</section>
+
+## Instructions
+<section id='instructions'>
+Create a function that takes a number $n$ and generates accumulator functions that return the sum of every number ever passed to them.
+<strong>Rules:</strong>
+Do not use global variables.
+<strong>Hint:</strong>
+Closures save outer state.
+</section>
+
+## Tests
+<section id='tests'>
+
+```yml
+tests:
+  - text: <code>accumulator</code> should be a function.
+    testString: assert(typeof accumulator === 'function');
+  - text: <code>accumulator(0)</code> should return a function.
+    testString: assert(typeof accumulator(0) === 'function');
+  - text: <code>accumulator(0)(2)</code> should return a number.
+    testString: assert(typeof accumulator(0)(2) === 'number');
+  - text: Passing in the values 3, -4, 1.5, and 5 should return 5.5.
+    testString: assert(testFn(5) === 5.5);
+
+```
+
+</section>
+
+## Challenge Seed
+<section id='challengeSeed'>
+
+<div id='js-seed'>
+
+```js
+function accumulator(sum) {
+
+}
+```
+
+</div>
+
+
+### After Test
+<div id='js-teardown'>
+
+```js
+const testFn = typeof accumulator(3) === 'function' && accumulator(3);
+if (testFn) {
+  testFn(-4);
+  testFn(1.5);
+}
+```
+
+</div>
+
+</section>
+
+## Solution
+<section id='solution'>
+
+
+```js
+function accumulator(sum) {
+  return function(n) {
+    return sum += n;
+  };
+}
+
+```
+
+</section>