CoreJS: Closure Examples; IIFEs

12 Aug 2015

Practice: Using Closure for Private Variables

Loan Manager

var loanManager = {
    assets: 1000,
    debt: 1000,
    rate: 0,
    borrow: function(amount,rate) {
        this.assets += amount;
        this.debt += amount;
        this.rate = rate;
    }
    compoundInterest: function() {
        this.debt *= (1+this.rate);
    },
    repay: function(amount) {
        this.assets -= amount;
        this.debt -= amount;
    }
}

Rewrite the code above using a closure, so that the values for assets, debt, and rate can only be changed by the methods borrow, compoundInterest, and repay. You'll need to group the methods together within an object and build that object in a factory.

Authenticator

Write a (toy) password-authentication system! Start with a function storePassword(passwd). It should return a function which you can use to check whether a submitted password matches the stored one (passwd).

function storePassword(passwd) {
    //...code here...
}

// Use it like this:
var verifyPassword = storePassword("sekrit");

verifyPassword("password"); // false
verifyPassword("12345"); // false
verifyPassword("sekrit"); // true

Practice: Using Closure for Persistence

Write a function that generates the next number each time it's called:

sequence(); //=> 0
sequence(); //=> 1
sequence(); //=> 2

Write a function counter that returns your sequence generator:

var sequence1 = counter();
var sequence2 = counter();
sequence1(); //=> 0
sequence1(); //=> 1
sequence2(); //=> 0
sequence1(); //=> 2
sequence2(); //=> 1

Allow your counter to start at any number, for instance, counter(5).
Allow your counter to be reset:

var sequence1 = counter();
var sequence2 = counter();
sequence1.next(); //=> 0
sequence1.next(); //=> 1
sequence2.next(); //=> 0
sequence1.next(); //=> 2
sequence1.reset(); //=> void
sequence1.next(); //=> 0
sequence2.next(); //=> 1
sequence1.reset(5); //=> void
sequence1.next(); //=> 5

Practice: Counting Factory

Consider the following factory:

function makeThing(_id) {
    // thing needs no id propery; instead has personal getter:
    var thing = {
        // personal method
        id: function() {
            return _id;
        },
        // shared method:
        color: makeThing.color
    };
    return thing;
}
makeThing.color = function() {
    return (this.id()%2) ? 'red': 'blue';
}

Modify the factory to eliminate the _id parameter. Instead, the factory should auto-increment a counter and give each thing a unique id.

// some code here...

    function makeThing() { //<-- no parameter
        //...
        return thing;
    }

// more code here...

You'll be able to use the factory like so:

var thing0 = makeThing(),
    thing1 = makeThing(),
    thing2 = makeThing();
thing0.id(); //0
thing1.id(); //1
thing2.id(); //2

IIFEs

Wrap the following marble factory in an IIFE:

function marble(size,color) {
  return {
    size:size,
    color:color,
    isBigger:marble.isBigger
  }
}
marble.isBigger = function (other) {
  return this.size > other.size;
}

Samples

scratchpad

Diagrams

loan manager

counting factory in IIFE