JavaScript Basics
Link to external Javscript file
<head>
<title>Test</title>
<meta http-equiv="content-type" content="text/html;charset=utf-8" />
<script src="spectra.js" type="text/javascript" ></script>
</head>
Types
The built-in types are string, number, boolean, null, undefined, object.
Use typeof some_var to get the type.
The following are considered false: false, 0, "", null, undefined, NaN.
Strings
" Hello ".trim() // "Hello"
"How are you?".split(" ") // ["How", "are", "you?"]
Arrays
Arrays are special objects that get their methods from Array.prototype.
var my_array = []; // make a new empty array
var my_array = [1, 4, 2];
my_array.forEach( function(element){
// do something with element
}
my_array.filter(function(i){return i % 2 == 0}); // [4, 2] only keeps even numbers
my_array.map(function(i){return i*i}); // [1, 16, 4]
my_array.reduce(function(total, currentValue, index, array){ return total += currentValue;})
// 7
my_array.concat([2]); // [1, 4, 2, 2]
my_array.join('-'); // "1-4-2"
var x = my_array.pop(); // x is 2, my_array is [1, 4]
my_array.push(x); // my_array is [1, 4, 2]
my_array.reverse(); // my_array is [2, 4, 1]
x = my_array.shift(); // x is 2, my_array is [4, 1]
my_array.unshift(x); // my_array is [2, 4, 1]
my_array = [1,2,3,4,5,6,7];
my_array.slice(2); // [3, 4, 5, 6, 7]
my_array.slice(2, 4) // [3, 4]
my_array.slice(2, -1); // [3, 4, 5, 6]
x = my_array.splice(2, 2); // x is [3, 4], my_array is [1, 2, 5, 6, 7]
my_array = [4, 2, 3, 1]
my_array.sort(); // my_array is [1, 2, 3, 4]
Objects
var new_object = {}; // make a new empty object
new_object.name = "Bill"; // create a property
new_object["name"] = "Bill"; // same thing
for (var key in my_object){ // iterate over an object
// do something with key
// value is my_object[key]
}
Functions
Functions are special objects that get their methods from Function.prototype. Javascript doesn't care about function signatures, so each name must be unique.
var adder = {
x: 1, y: 2,
add: function(){return this.x + this.y;}
}
adder.add() // 3
adder.add.apply({x:2, y:4}) // 6
addx10 = adder.add.bind({x:10, y:20})
addx10() // 30
Constructors
function Fruit (theColor, theSweetness, theFruitName, theNativeToLand) {
this.color = theColor;
this.sweetness = theSweetness;
this.fruitName = theFruitName;
this.nativeToLand = theNativeToLand;
this.showName = function () {
console.log("This is a " + this.fruitName);
}
this.nativeTo = function () {
this.nativeToLand.forEach(function (eachCountry) {
console.log("Grown in:" + eachCountry);
});
}
}
var mango = new Fruit ("Yellow", 8, "Mango", ["South America", "Central America", "West Africa"]);
Counter Example
function Counter() {
this.things = {}; // name -> count
this.add = function (name){
if (name in this.things){
this.things[name]++;
} else {
this.things[name] = 1;
};
};
this.getCount = function(name){
if (name in this.things){
return this.things[name];
} else {
return 0;
};
};
};
var myCounter = new Counter();
Prototypes
Every object has a prototype property. This can be used to add attributes or methods after the constructor has been defined:
Fruit.prototype.peeled = false;
Fruit.prototype.peel = function() {
this.peeled = true;
}
Closures
Offers a way to hide data inside an object:
function createIncrementer(){
var i = 0; // i is not accessible outside
return {
increment: function(){
return ++i;
}
};
}
obj = createIncrementer();
obj.increment(); obj.increment(); obj.increment(); // 1, 2, 3
This is a useful way to create a singleton page controller object.
Exceptions
function noEven(num){
if (num % 2 == 0){
throw {
code: 'even_number',
message: 'This function cannot be called with even numbers'
};
}
}
function passNumber(num){
try {
noEven(num);
} catch (e){
console.log(e.code+':'+e.message);
noEven(num+1);
}
}
Timing
setTimeout(someFunction, 1000); // someFunction will execute after 1 second
setInterval(someFunction, 1000); // someFunction will execute every 1 second