Variables are like memory cells. You can store any information in them, e.g. a number of lives, resources or experience, an NPC's name, etc.
Variables can also contain more complex data, like a list of inventory items or a deck of cards.
You can declare new variables with a
var keyword, and assign them values with an equation sign:
var maxHealth; maxHealth = 100; var health; health = maxHealth; // You can copy a value from one variable (maxHealth) to another (health) var mana, maxMana; // You can combine a number of declarations in one 'var' keyword maxMana = mana = 100; // You can assign a value (100) to different values at once var name; name = 'ct.js'; // Text values, or Strings, are enclosed in quotation marks var title = 'The Almighty Cat'; // You can combine both declaration process and assignment var invincible = true, // These are Boolean values stunned = false, bleed = false; // You can combine multiple declarations and assignments with comma!
Declaration process tells ct.js that we want to create a new variable. Without it, ct.js will throw an error, because we can't store information in a place that doesn't exist.
Assignment writes a new value to a variable. When declared, variables are
undefined. They will store useful information only after assignment. You can assign new variables many times.
Variables are great for temporal values, but they disappear after a ct.js event completes (e.g., after 'Destruction', 'Frame start'). This makes variables useful for quick operations but unusable for a long run. We should use properties to store information so that we can use it later.
You can use properties in the same way you use variables, but they may only exist inside Objects. Objects are very abstract things, and they include all the Copies and Rooms. You will create your own Objects soon, too. But for now, let's look how to create and use properties:
this.maxHealth = 100; this.health = this.maxHealth; this.maxMana = this.mana = 100; this.name = 'ct.js'; this.title = 'The Almighty Cat'; this.invincible = true; this.stunned = false; this.bleed = false;
As you can see, the most significant difference is that we don't need to declare properties. We can start writing values to them directly.
There is also a new keyword:
this stands for the current object which calls the code. If you write a code for an 'Frame start' event of a Copy, then
this will point to this exact copy. Writing
this.health = 100; means that we store a property
health inside the current copy, with a value
We can use a
this keyword inside a room's code. In this case, values will be stored in a room.
Operations with values, properties and variables
Variables and properties are quite useless “as is”. With conditional statements, loops and operations they become a powerful mechanism defining your game's logic. We will talk about loops and conditionals later. For now, let's look at operations.
Operations with numbers are familiar to everyone. They are like arithmetic equations:
this.level = 10; this.health = this.level * 4; // 40 this.health = this.health - 5; // 35 this.inventoryCapacity = (5 + 10) * 8; // 120 this.magicPower = 5 + 10 * 8; // 85 this.magicDamage = this.magicPower + this.level * 5; // 135 this.magicResistance = this.magicPower / 10; // 8.5 /* What if we need to get a remainder of division? */ this.remainder = 11 % 4; // 3, because 11 / 4 = 2 and 3 as a remainder;
When we need to change a variable or property, we can unite assignment and the needed operation:
this.health = 10; this.health += 5; // health is now 15 this.health /= 5; // 3 this.health *= 10; // 30 this.health -= 20; // 10
There are also two fancy operators which modify a variable's value by
this.counter = 10; this.counter++; // this.counter is now 11 this.counter++; // 12 this.counter--; // 11 again
Strings have their operators too. We will use a plus sign to concatenate Strings:
this.name = 'ct.js'; this.title = 'Almighty Cat'; this.title = 'The ' + this.title; // 'The Almighty Cat' this.name += ', '; // 'ct.js, ' this.name += this.title; // 'ct.js, The Almighty Cat'
We can even add numbers to strings:
var score = 1000, drawText = 'Score: ' + score; // 'Score: 1000' var power = 42, powerInfo = power + ' of power'; // '42 of power'
Warning! Things get weird when we mix number-like strings and math operators:
var money = 100, price = '5'; var case1 = money - price, // 95 case2 = money + price; // 1005 (!)
So the rule of thumb is to store numerical values as Numbers, not Strings. If you need to convert a String into a Number, use
Boolean values and comparisons
Boolean is these variables and properties which values are either
false. Note that we don't use quotation marks here.
Boolean values can be retrieved by using comparisons, and they also have their own operators. Numbers have comparisons like what we can find in Math, while Strings are either equal or not:
var health = 63, maxHealth = 100, mana = 100, maxMana = 100; health < maxHealth; // true mana > maxMana; // false mana >= maxMana; // true health <= maxHealth; // true health === maxHealth; // are they equal? false health !== maxHealth; // they aren't equal, right? true var cat = 'Albert', dog = 'Snowball'; cat === dog; // false cat !== dog; // true /* There are also non-strict comparisons, which compare values but transform variables' templates */ 5 === '5'; // false, because '5' is String, not a Number 5 == '5'; // true 5 === parseFloat('5') // true, because parseFloat returns a Number, and so is 5
Boolean values have their operators, too. The most basic one is
!, which negates a value next to it.
!true; // false !false; // true var health = 50, alive = !(health <= 0); // true var dead = !alive; // false
There are also
||. The first one is referred to as "AND", and the second one is "OR". They are used to combine different Boolean values.
this.onGround = true; var keyUp = ct.keyboard.down['up'], // it will be 'true' if an upper arrow key is held down canJump = this.onGround && keyUp; this.powerFromLeft = false; this.powerFromRight = true; this.poweredOn = this.powerFromLeft || this.powerFromRight; // true
In the next part we will talk about conditional statements and loops. For now, I recommend you to read about default properties of Copies and Rooms.