Background

JavaScript Objects

Time: 10 minutes
Activity: Demo / discussion

JavaScript objects are quite different from Java objects; in particular it is possible (and idiomatic) to use JavaScript objects without any concept of class. JavaScript objects are conceptually similar to (mutable) hashtables in Racket.

> x={ apple: 1}
> x['pear']=2
> x
> x.apple === x['apple']

The last line demonstrates so called property notation that we have already been using. A method in javascript is just a property that is assigned to a function

> obj = {}
> obj.hello = function () { console.log("hello world\n") }
> obj.hello()
> obj['hello']()

Deep Comparison

Time: 20 minutes
Activity: Small groups

Follow the Deep Comparison Exercise to write a function deepEqual that passes the following tests. Jasmine uses something very similar for the toEqual matcher used below. Coming from Racket this is a bit disappointing, since the built in equal? function can already do these kinds of comparisons.

describe("equal", function () {
    let obj = {here: {is: "an"}, object: 2};
    it("self", function () {
        expect(deepEqual(obj,obj)).toBe(true);
    });
    it("null", function () {
        expect(deepEqual(null,null)).toBe(true);
    });

    it("different", function () {
        expect(deepEqual(obj, {here: 1, object: 2})).toBe(false);
    });
    it("equivalent", function () {
        expect(deepEqual(obj, {here: {is: "an"}, object: 2})).toBe(true);
    });
});

JavaScript Arrays

Time: 20 minutes
Activity: Small groups

JavaScript supports arrays. They are actually implimented as objects, which leads to some slightly surprising behaviour:

> x=[]
> x[10]=1
> x
> x["10"] === x[10]

Follow the (first part of the) Sum of a Range Exercise to write a range function that passes the following tests.

describe("range", function () {
    it("empty", function () {
        expect(array.range(2,1)).toEqual([]);
    });
    it("single", function () {
        expect(array.range(2,2)).toEqual([2]);
    });
    it("multiple", function () {
        expect(array.range(42,50)).toEqual([42,43,44,45,46,47,48,49,50]);
    });
});

Arrays Continued

Time: 20 minutes
Activity: Small Groups

Follow the (second part of the) Sum of a Range Exercise to write a sum function that passes the following tests.

describe("sum", function () {
    it("empty", function () {
        expect(array.sum([])).toBe(0);
    });
    it("single", function () {
        expect(array.sum([2])).toBe(2);
    });
    it("multiple", function () {
        expect(array.sum(array.range(1,10))).toBe(55);
    });    
});

Variadic Functions

Aside: 10 minutes
Activity: Demo/Discussion

Traditional JavaScript supports the arguments object, which is something like an array of all the arguments passed to the function.

function makemap (){
    let map={};
    if (arguments.length %2 != 0)
        return map;

    for (let i=0; i<arguments.length; i+=2) {

    }

    return map;

}

ES2015 supports rest arguments similar to those in Racket. These are more general than the arguments object, and generally preferred.

function brag(name,...args){
    console.log(name + " has");
    for (let i=0; i<args.length; i++) {
        console.log("\t"+args[i]);
    }
}

ES2015 also supports default values for function parameters

function win(person,thing="a brick"){
    console.log(person + " won " + thing);
}

Adding step argument to range.

Time: 20 minutes
Activity: Small groups

Using the method of your choice, add an optional step argument to the range function so that the following tests pass. The original tests should keep passsing, that's a big part of the point of having unit tests.

    it("step 2", function () {
        expect(array.range(1,10,2)).toEqual([1,3,5,7,9]);
    });        
    it("step -1", function () {
        expect(array.range(5,2,-1)).toEqual([5,4,3,2]);
    });