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1-js/06-advanced-functions/03-closure/4-closure-sum/solution.md

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+For the second brackets to work, the first ones must return a function.
+
+Like this:
+
+```js run
+function sum(a) {
+
+  return function(b) {
+    return a + b; // takes "a" from the outer lexical environment
+  };
+
+}
+
+alert( sum(1)(2) ); // 3
+alert( sum(5)(-1) ); // 4
+```
+

1-js/06-advanced-functions/03-closure/4-closure-sum/task.md

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+importance: 4
+
+---
+
+# Sum with closures
+
+Write function `sum` that works like this: `sum(a)(b) = a+b`.
+
+Yes, exactly this way, via double brackets (not a mistype).
+
+For instance:
+
+```js
+sum(1)(2) = 3
+sum(5)(-1) = 4
+```
+