exercise 21 into the right folder

Author: ElviraQDP

exercises/21-Filter-an-array/README.es.md

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+Otra función sorprendente para los arreglos es el filtro. Repite toda el arreglo original y solo devuelve los valores que coinciden con una condición particular.
+
+[Aquí está la documentación de la función de filtro en w3school](https://www.w3schools.com/jsref/jsref_filter.asp)
+
+Por ejemplo, este algoritmo filtra el arreglo allNumbers y devuelve un nuevo arreglo con solo los números impares:
+
+```js
+var allNumbers = [23,12,35,5,3,2,3,54,3,21,534,23,42,1];
+
+var onlyOdds = allNumbers.filter(function(number){
+	return (number % 2 > 0)
+});
+
+console.log(onlyOdds);
+```
+
+Instrucciones
+Completa el código para que llene el arreglo resultante de Nombres con solo los nombres que comienzan con la letra R
+Usa la función Array.filter
+
+Pista:
+Aquí hay un video de 2:29min explicando array.filter
+https://www.youtube.com/watch?v=0qsFDFC2oEE

exercises/21-Filter-an-array/README.md

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+# `21` Filter an Array
+
+Another amazing function for arrays is `filter`. 
+It loops the entire original array and only returns the values that match a particular condition.
+
+[Here is the documentation of the filter function in w3school](https://www.w3schools.com/jsref/jsref_filter.asp)
+
+For example, this algorithm filters the `allNumbers` array and returns a new array with only the odds numbers:
+
+```js
+var allNumbers = [23,12,35,5,3,2,3,54,3,21,534,23,42,1];
+
+var onlyOdds = allNumbers.filter(function(number){
+	return (number % 2 > 0)
+});
+
+console.log(onlyOdds);
+```
+
+# 📝Instructions
+- Complete the code to make it fill the `resultingNames` array with only the names that start with letter R
+- Use the `array.filter` function
+
+## 💡Hint
+Here is a 2:29min video explaining array.filter
+https://www.youtube.com/watch?v=0qsFDFC2oEE

exercises/21-Filter-an-array/app.js

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+let allNames = ["Romario","Boby","Roosevelt","Emiliy", "Michael", "Greta", "Patricia", "Danzalee"];
+
+//your code here
+
+console.log(resultingNames);

exercises/21-Filter-an-array/test.js

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+const fs = require('fs');
+const path = require('path');
+const rewire = require("rewire");
+
+let _log = console.log;
+let _buffer = '';
+global.console.log = console.log = jest.fn((text) => _buffer += text + "\n");
+
+const app_content = fs.readFileSync(path.resolve(__dirname, './app.js'), 'utf8');
+
+test("You should create the variable resultingNames.", function(){
+  const file = rewire("./app.js");
+  const myVar = file.__get__('resultingNames');
+  expect(myVar).toBeTruthy();
+});
+
+test('You have to use the console.log function to print the correct output.', function () {
+    const app = require('./app.js');
+    expect(console.log.mock.calls.length > 0).toBe(true);
+});
+
+test('You should use the console.log to call the variable resultingNames', function () {
+  const file = rewire("./app.js");
+  const myVar = file.__get__('resultingNames');
+  expect(console.log).toHaveBeenCalledWith(myVar);
+ });
+
+test('The output in the console should match the one in the instructions!', function () {
+    const _app = rewire('./app.js');
+    
+    let _allNames = ["Romario","Boby","Roosevelt","Emiliy", "Michael", "Greta", "Patricia", "Danzalee"];
+    let _test = _allNames.filter(item=>item[0] === "R")
+    expect(_buffer).toMatch(_test.map(n => n).join(","));
+  });
+
+
+
+