Add the once function example.

Author: blakeembrey

once/Readme.md

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+# Once
+
+Write a function that accepts a function as it's only argument and returns a new function that can only ever be executed once.
+
+Once completed, add a second arguments that allows the function to be executed `x` number of times before it stops working.

once/once.js

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+module.exports = function (fn, times) {
+  // Set times to one if nothing is passed through and keep track of the
+  // latest return value to return when we run out of execution times
+  var memo;
+  times = times || 1;
+  // Return another function that will be executed
+  return function () {
+    if (!times) { return memo; }
+    // Set memo to the result of the function and decrement the number of executions
+    memo   = fn.apply(this, arguments);
+    times -= 1;
+    // If there are no more execution times, set the function to `null` so
+    // it can be garbage collected and return the memo
+    times || (fn = null);
+    return memo;
+  };
+};