CS142 Project 2: JavaScript Calisthenics

Due: Thursday, April 15, 2021 at 11:59 PM


Although this project has you run code in your browser, you need to have Node.js installed on your system to run the code quality checker. If you haven't already installed Node.js and the npm package manager, follow the installation instructions now.

Once you have Node.js installed, create a directory project2 and extract the contents of this zip file into the directory. The zip file contains the files cs142-test-project2.html and cs142-test-project2.js that act as a testing framework for the code you write in this project.

You can fetch the code quality tool, JSHint, by running the following command in the project2 directory:

npm install
This will fetch JSHint into the node_modules subdirectory. You will be able to run it on all the JavaScript files in project2 directory by running the command:
npm run jshint
The code you submit should start with "use strict"; in each JavaScript file and running JShint should not output any warnings. Any warnings will be used to deduct style points.

To run the assignment open the file cs142-test-project2.html in your browser. The web page will load the JavaScript and run a few tests. Since Node.js also contains a JavaScript virtual machine you can also run the tests without a browser using the command:

npm test
The JavaScript development environment is much better in the browser so we suggest you do your development using it.

Note: These tests don't cover all the edge cases. They are there to help guide you and let you know when you have the basic functionality. It is your responsiblity to handle everything stated in the following specs that aren't explicitly tested in the test file we give you.

In this project we ask you to write or modify some JavaScript functions. The problems in this assignment are of a practical nature and functionality you develop will be useful in completing later class projects. Given the availability of JavaScript libraries to solve or help solve pretty much any JavaScript tasks you would be assigned, it is likely you could solve these with a couple of lines to call some library routine. Since the goal of the project is to learn JavaScript, we forbid you to use any JavaScript libraries in your solutions. Functions built-in to JavaScript Arrays and Date objects are acceptable.

Problem 1: Generate a Multi Filter Function (10 points)

In your project2 directory, make a new file named cs142-make-multi-filter.js.The code for your Multi Filter Function will go in this file.

Declare a global function named cs142MakeMultiFilter that takes an array (originalArray) as a parameter and returns a function that can be used to filter the elements of this array. The returned function (arrayFilterer) internally keeps track of a notion called currentArray. Initially, currentArray is set to be identical to originalArray. The arrayFilterer function takes two functions as parameters. They are:

  1. filterCriteria - A function that takes an array element as a parameter and returns a boolean. This function is called on every element of currentArray and currentArray is updated to reflect the results of the filterCriteria function. If the filterCriteria function returns false for an element, that element should be removed from currentArray. Otherwise, it is left in currentArray. If filterCriteria is not a function, the returned function (arrayFilterer) should immediately return the value of currentArray with no filtering performed.
  2. callback - A function that will be called when the filtering is done. callback takes the value of currentArray as an argument. Accessing this inside the callback function should reference the value of originalArray. If callback is not a function, it should be ignored. callback does not have a return value.
The arrayFilterer function should return itself unless the filterCriteria parameter is not specified in which case it should return the currentArray. It must be possible to have multiple arrayFilterer functions operating at the same time.

The following code shows how one might make use of the functions you define in this problem:

// Invoking cs142MakeMultiFilter() with originalArray = [1,2,3] returns a function, saved in the variable arrayFilterer1,
// that can be used to repeatedly filter the input array
var arrayFilterer1 = cs142MakeMultiFilter([1,2,3]);

// call arrayFilterer1 (with callback function) to filter out all the numbers not equal to 2
arrayFilterer1(function (elem) {
  return elem !== 2; // check if element is not equal to 2
}, function (currentArray) {
  console.log(this); // printing 'this' within the callback function should print originalArray which is [1,2,3]
  console.log(currentArray); // prints [1, 3]

// call arrayFilterer1 (without callback function) to filter out all the elements not equal to 3
arrayFilterer1(function (elem) {
  return elem !== 3; // check if element is not equal to 3

// calling arrayFilterer1 with no filterCriteria should return the currentArray
var currentArray = arrayFilterer1();
console.log('currentArray', currentArray);   // prints [1] since we filtered out 2 and 3

// Since arrayFilterer returns itself, calls can be chained
function filterTwos(elem) { return elem !== 2; }
function filterThrees(elem) { return elem !== 3; }
var arrayFilterer2 = cs142MakeMultiFilter([1,2,3]);
var currentArray2 = arrayFilterer2(filterTwos)(filterThrees)();
console.log('currentArray2', currentArray2);   // prints [1] since we filtered out 2 and 3

// Multiple active filters at the same time
var arrayFilterer3 = cs142MakeMultiFilter([1,2,3]);
var arrayFilterer4 = cs142MakeMultiFilter([4,5,6]);
console.log(arrayFilterer3(filterTwos)());	// prints [1,3]
console.log(arrayFilterer4(filterThrees)());	// prints [4,5,6]

Problem 2: Template Processor (5 points)

In your project2 directory, make a new file named cs142-template-processor.js. The code for your Template Processor will go in this file.

Create a template processor class (Cs142TemplateProcessor) that is constructed with a string parameter template and has a method fillIn. When invoked with an argument of a dictionary object, fillIn returns a string with the template filled in with values from the dictionary object. Cs142TemplateProcessor should be written using the standard JavaScript constructor and prototype structure.

The fillIn method returns the template string with any text of the form {{property}} replaced with the corresponding property of the dictionary object passed to the function.

If the template specifies a property that is not defined in the dictionary object, the property should be replaced with an empty string. If the property is between two words, you'll notice that replacing the property with an empty string will result in two consecutive whitespaces. Example: "This {{undefinedProperty}} is cool" -> "This is cool". This is fine. You do not have to worry about getting rid of the extra whitespace.

Your system need only handle properly formatted properties. Its behavior can be left undefined in the following cases as we will not be checking explicitly for them.

  • nested properties - {{foo {{bar}}}} or {{{{bar}}}} or {{{bar}}}
  • unbalanced brackets - {{bar}}}
  • stray brackets in any property string - da{y or da}y

The following code shows how one might make use of the functions you define in this problem:

var template = 'My favorite month is {{month}} but not the day {{day}} or the year {{year}}';
var dateTemplate = new Cs142TemplateProcessor(template);

var dictionary = {month: 'July', day: '1', year: '2016'};
var str = dateTemplate.fillIn(dictionary);

assert(str === 'My favorite month is July but not the day 1 or the year 2016');

//Case: property doesn't exist in dictionary
var dictionary2 = {day: '1', year: '2016'};
var str = dateTemplate.fillIn(dictionary2);

assert(str === 'My favorite month is  but not the day 1 or the year 2016');

Problem 3: Fix cs142-test-project2.js to not pollute the global namespace (5 points)

The test JavaScript file we give you (cs142-test-project2.js) declares numerous symbols in the global JavaScript namespace. For example, after the script is loaded the symbol p1Message appears in the global namespace. Another JavaScript file would then be able to access and change p1Message. Change cs142-test-project2.js to use the standard JavaScript module pattern using an anonymous function to hide symbols in the global namespace yet keep the same checking functionality.

Style Points (5 points)

These points will be awarded if your JavaScript code for the problems above is clean, readable, and JSHint warning-free.

Useful Hints

  • In JavaScript, the convention for checking equality/inequality is === and !==, instead of == and !=.
  • When running JSHint, make sure the .jshintrc (hidden file) is present, or JSHint will throw an error. You can use the command ls -a (dir /a in a Windows command line) to view all the files in your current directory, including hidden files.
  • When moving files around, make sure to do that using the command line. Dragging/dropping files usually does not move hidden files which is a common reason for students missing their .jshintrc file.
  • The requirement that your solution to Problem 2 support multiple simultaneous uses of cs142MakeMultiFilter means that you should not be using global variables to store the currentArray state.
  • In problem 3, you only need to address symbols created by the test file. Also, you will need to leave the variable cs142Project2Results as a global (i.e., property on the window object) so that you can continue to run the tests using the run-tests-using-node.js script.


Use the standard class submission mechanism to submit the project2 directory. This directory should include the starter files we gave you along with code files you created or modified for the problems including cs142-make-multi-filter.js, cs142-template-processor.js, cs142-test-project2.js, and cs142-test-project2.html. Make sure your submission runs npm run jshint and npm test with no errors.