CoreJS: Closure

10 Aug 2015

Review of Scope

Predict and explain the output of each program below:

    var x=1, y=1;
    function fun() {
        x=3;
        console.log('x='+x, 'y='+y);
    }
    fun();
    console.log('x='+x, 'y='+y);

    var x=1, y=1;
    function fun(x) {
        x=3;
        console.log('x='+x, 'y='+y);
    }
    fun(2);
    console.log('x='+x, 'y='+y);

    var x=1, y=1;
    function fun(z) {
        x=3;
        console.log('x='+x, 'y='+y);
    }
    fun(2);
    console.log('x='+x, 'y='+y);

    var x=1, y=1;
    function fun(x) {
        x=3, y=3;
        if (x) {
            var y=4;
            console.log('x='+x, 'y='+y);
        }
        console.log('x='+x, 'y='+y);
    }
    fun(2);
    console.log('x='+x, 'y='+y);

    var x=1, y=1;
    function showXY(x,y) {
        console.log('x='+x, 'y='+y);
    }
    function fun(x) {
        x=3;
        showXY(x,y);
    }
    fun(2);
    showXY(x,y);

    var x=1, y=1;
    function showXY() {
        console.log('x='+x, 'y='+y);
    }
    function fun(x) {
        x=3;
        showXY();
    }
    fun(2);
    showXY();

    var x=1, y=1;
    function fun(x) {
        function showXY() {
            console.log('x='+x, 'y='+y);
        }
        x=3;
        showXY();
    }
    fun(2);
    showXY();

Combined accessor function

Combine the getter and setter functions into a single function which can do both. The accessor function accessFn should be the only way of modifying or retrieving the variable privateVal.

function makeAccessor() {
    var privateVal;
    function accessFn(val) {
        //...
    }
    return accessFn;
}
// Example of use:
var cache = makeAccessor();
cache(7);
cache(); //-->7
cache(9);
cache(); //-->9

Practice: Factory with personal instance methods

function factory(id) {
    var instance = {};
    instance.id = id;
    instance.getID = function() {
        return this.id;
    }
    return instance;
}

var thing1 = factory(1);
console.log(thing1.getID());

Rewrite the factory above to use closure to persist and retrieve the value of id instead of putting an id property on the instance.

Practice: Using Closure for Private Variables

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.

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

Hoisting

Here is an excellent article on hoisting.

Samples

scratchpad

Diagrams

scope examples 1-3

scope examples 4-6

scope example 7; makeObject

makeFunction; makeGetter; makeSetter

factory w. closures, personal methods

factory w. shared methods