Before the lab

• Watch the WAT talk by Gary Bernhardt,

Background

• Values, Types and Operators
• Program Structure
• Node.js REPL
• JavaScript functions
• Node.js modules
• CommonJS modules

Questions

• about A2 ?
• about the quiz?

Running JavaScript

In a file

• Create a new file ~/fcshome/cs2613/labs/L08/hello-world.js
• Add some JavaScript code, save

    console.log("Hello world");

Running a script

• open a terminal, find the right directory

  $ node hello-world.js

• commit your file

In a Browser

• Start Firefox

• Open the "Web Console" with Ctrl-Shift-K (or navigate via the "hamburger menu" to "Web Developer -> Web Console" ).

• Select the tab "Console"

• Copy-paste the code in.

In the REPL

• Start up a node repl

  $ node

• Load your code into the repl

    > .load hello-world.js
  

• Evaluate some expressions

    > 1 + 1
    > "Hello " + "world!"
  

Translating Racket expressions into Javascript

• make a new file ~/fcshome/cs2613/labs/L08/expr.js and fill in the JavaScript expressions equivalent to the given Racket expressions.

  • You can test your solutions with the REPL either by pasting in the expressions, or by using .load.

• note that if you want to run your code from the command line, you'll need to use console.log

    //(string-append "Hello\n" "world!")

    //(* (+ 1 2 3) 7)

    //(< 41 (* 6 7))

    //(equal? (* (+ 1 2 3) 7) 42)

    //(equal? "Spongebob" "Squarepants")

    //(and (equal? (* 6 7) 42) (equal? "Spongebob" "Squarepants"))

    //(equal? 42 (if (< 3 5) (* 6 7) "turnip"))

    // (or #t (/ 1 0))

    // (and #f (/ 1 0))

JavaScript equality and type coercion

We remember from earlier Eloquent JavaScript Chapter 1, and the WAT talk by Gary Bernhardt, that type coercion is an important part of evaluating JS expressions. This is sometimes useful

> 42 + 'is a big number'

But just as often a source of errors and confusion.

> "" + 1
> x=""
> x++

One operator where type coercion can be particularly surprising is the standard equality test ==. Not only does type coercion apply:

> "" == 0
> false == 0
> "" == 0

but special rules apply to "undefined" and "null"

> false == undefined
> undefined == null
> undefined == undefined

even though they would normally be considered falsy (considered false in boolean contexts).

   > if (undefined) { console.log("truthy") } else { console.log("falsey") }

NaN is another falsy value not == to the other falsy values, not even itself:

   > NaN == undefined
   > NaN == NaN

To avoid this twisty maze of "helpful" type coercion, you can use the "strict equality" checker ===

   > "" === 0
   > false === 0
   > "" === 0
   > false === undefined
   > undefined === null
   > undefined === undefined

Javascript functions

Reference for this section is JavaScript functions Like Racket, JavaScript has two different ways of defining functions. The first way is by assigning an anonymous function to a variable.

    (define square (lambda (x) (* x x)))

    let square = x => x*x;
    let square2 = function (x) {return x*x};

• Answer the following questions in your journal:

  • How is assignment (=) in javascript different from binding (let, define) in Racket?

  • return is mandatory. What does it mean?

  • what is the difference between the two kinds of anonymous functions in JavaScript? Hint: the first example is an arrow function.

The more compact way of defining functions in both Racket and JavaScript combines the binding and creation of a function/lambda

    (define (square x) (* x x))

    function square(x) { return x*x }

• Start a new file ~/fcshome/cs2613/labs/L08/loop-arith.js, and fill in the following definition for mult using plus and for.

function plus(a,b) {
    for (let i=0; i < a; i++){
        b++;
    }
    return b;
}

function mult(a,b) {
    sum=0;



    return sum;
}

• Test your file manually, using e.g. the node REPL.

Node.js modules

Chapter 10 contains a explanation of the CommonJS module system we will be using, but we don't want to wait that long to use modules. So the extremely quick and dirty guide to to CommonJS modules follows.

Exporting

This is analogous to provide in Racket. The syntax will make more sense when we know about Javascript objects. Add to arithmetic.js (like provide in racket)

    exports.plus = plus;
    exports.mult = mult;

For now, let's put these at the bottom of our files.

Requiring

This is analogous to the form of the same name in Racket. Create a new file client.js

    let arith=require("./loop-arith.js");
    console.log(arith.plus(2,2));