Develop Reference

chineseFood[array[0]] = array[array.length-1];

I don't understand the purpose of this = sign on the sixth line in the code block below. I understand how the argument grabs each index number of the array, I just don't understand why chineseFood[array[0]] = array[array.length-1]; In other words, I don't get the purpose of the equal sign as if it were almost comparing each other to be stored in the empty object that is stored in the variable chineseFood. Could someone please clarify? It would be much appreciated.
function transformFirstAndLast(array) {
var chineseFood = {};
//takes 1st element (at index 0) and sets it to the last element (nth index): array(length-1)
chineseFood[array[0]] = array[array.length - 1];
return chineseFood;
}
console.log( transformFirstAndLast(['Orange', 'Lemon', 'Pork', 'Chicken']) );
Output Below
{Orange: "Chicken"}

The equals sign is not comparison, it is assignment. chineseFood is an object, which means that it can be treated like a dictionary, and its properties can be accessed using the [] operator instead of the . operator:
myObj = {
foo: "bar"
};
console.log(myObj["foo"]); // bar
console.log(myObj.foo); // bar
Likewise, you can also assign properties this way:
myObj = {};
myObj["foo"] = 3;
console.log(myObj["foo"]); // 3
console.log(myObj.foo); // 3
This is what your code is doing. It is retrieving the value of array[array.length-1], which is "Chicken". Then it is assigning this value to the property of chineseFood that has the name represented by array[0], which happens to be "Orange". Thus, the property named Orange on chineseFood is set to array[array.length - 1], which is why chineseFood evaluates to {Orange: "Chicken"}.
This method of accessing properties is especially useful when you don't know the name of the property you will be changing in advance, as is the case with this code, or when you want to create properties that have names that would otherwise be illegal:
myObj = {
".you can't usually use with spaces or start w/ periods": false
};
myObj[".you can't usually use with spaces or start w/ periods"] = true;
console.log(myObj[".you can't usually use with spaces or start w/ periods"]);
// there is no way to read this property the normal way

Basically what is does is:
your object is :
var obj = {Orange: "Chicken"};
And Your array is :
var arr = ['Orange','Lemon','Pork','Chicken']
What this line says is pick first element of the array and check for this prop in object and change its value to last element of array, here:
arr[0] = "orange";
So this line :
obj[arr[0]] can be seen as obj['orange'].
After that you change its value:
Obj[arr[0]] = arr[arr.length-1] which can be written as obj['orange'] = 'chicken'

How to write to multiple arrays based off numbers?

Say I have two arrays and an integer.
var int = 1; //or 0 depending on other circumstances
var ar0 = [0]
var ar1= [1]
Is there a way to use the integer to determine which array to write to? Like if int = 1 then I could have something along the lines of
'ar'+ int
that would choose the correct array? Or do I need a bunch of if-statements?
I'd like to be able to identify and edit the array that I need to edit by a number that was given to me.

What you are trying to do is to set up an environment. However, you cannot access variables in what is called the Variable Environment in JavaScript by name such as with 'ar' + int. If it is in the global scope that is kind of possible by using window['ar'+int] but this is bad practice and also assumes that the variable is global.
What you should do is wrap those in an object and then use the reference in the object to locate the array.
var int = 1;
var environmentObject = {};
environmentObject['ar0'] = [0];//string notation assignment example
environmentObject.ar1 = [1];//dot notation assignment example
and now you can easily access your array by name
var myarr = environmentObject[`ar`+int];

You could use an eval
var int = 1;
var ar0 = [0];
var ar1 = [1];
eval('ar' + int).push(2);
console.log(ar1); // 1, 2
But it is a bad practice. For your case, if you have only two integers(0, 1) better to use if statement:
if(int) { // 0 is falsy, 1 is truth
ar1.push(...);
} else {
ar0.push(...);
}
Or as Oriol mentioned, using ternary operator:
(int ? ar1 : ar0).push(...);

How to access the properties of a JavaScript object?

while review a javascript coding, i saw that
var detailInf = {
"hTitle":"Results",
"hMark":"98"
};
What's the concept behind this js coding. While give alert for the variable its shows as "[object Object]". So this is an object, then how can we access the variable and reveal the data from this object.

Try doing this:
alert(detailInf['hTitle']);
alert(detailInf.hTitle);
Both will alert "Results" - this is a Javascript object that can be used as a dictionary of sorts.
Required reading: Objects as associative arrays
As a footnote, you should really get Firebug when messing around with Javascript. You could then just console.log(detailInf); and you would get a nicely mapped out display of the object in the console.

That form of a JavaScript object is called an object literal, just like there are array literals. For example, the following two array declarations are identical:
var a = [1, 2, 3]; // array literal
var b = new Array(1, 2, 3); // using the Array constructor
Just as above, an object may be declared in multiple ways. One of them is object literal in which you declare the properties along with the object:
var o = {property: "value"}; // object literal
Is equivalent to:
var o = new Object; // using the Object constructor
o.property = "value";
Objects may also be created from constructor functions. Like so:
var Foo = function() {
this.property = "value";
};
var o = new Foo;
Adding methods
As I said in a comment a few moments ago, this form of declaring a JavaScript object is not a JSON format. JSON is a data format and does not allow functions as values. That means the following is a valid JavaScript object literal, but not a valid JSON format:
var user = {
age : 16,
// this is a method
isAdult : function() {
// the object is referenced by the special variable: this
return this.age >= 18;
}
};
Also, the name of the properties need not be enclosed inside quotes. This is however required in JSON. In JavaScript we enclose them in brackets where the property name is a reserved word, like class, while and others. So the following are also equivalent:
var o = {
property : "value",
};
var o = {
"property" : "value",
};
Further more, the keys may also be numbers:
var a = {
0 : "foo",
1 : "bar",
2 : "abz"
};
alert(a[1]); // bar
Array-like objects
Now, if the above object would have also a length property, it will be an array like object:
var arrayLike = {
0 : "foo",
1 : "bar",
2 : "baz",
length : 3
};
Array-like means it can be easily iterated with normal iteration constructs (for, while). However, you cannot apply array methods on it. Like array.slice(). But this is another topic.
Square Bracket Notation
As Paolo Bergantino already said, you may access an object's properties using both the dot notation, as well as the square bracket notation. For example:
var o = {
property : "value"
};
o.property;
o["property"];
When would you want to use one over the other? People use square bracket notation when the property names is dynamically determined, like so:
var getProperty = function(object, property) {
return object[property];
};
Or when the property name is a JavaScript reserved word, for example while.
object["while"];
object.while; // error

That's an object in JSON format. That's a javascript object literal. Basically, the bits to the left of the :'s are the property names, and the bits to the right are the property values. So, what you have there is a variable called detailInf, that has two properties, hTitle and hMark. hTitle's value is Results, hMark's value is 98.
var detailInf = { "hTitle":"Results", "hMark":"98"};
alert(detailInf.hTitle); //should alert "Results"
alert(detailInf.hMark); //should alert "98
Edit Paolo's answer is better :-)

As Dan F says, that is an object in JSON format. To loop through all the properties of an object you can do:
for (var i in foo) {
alert('foo[' + i + ']: ' + foo[i]);
}

What does ':' (colon) do in JavaScript?

I'm learning JavaScript and while browsing through the jQuery library I see : (colon) being used a lot. What is this used for in JavaScript?
// Return an array of filtered elements (r)
// and the modified expression string (t)
return { r: r, t: t };

var o = {
r: 'some value',
t: 'some other value'
};
is functionally equivalent to
var o = new Object();
o.r = 'some value';
o.t = 'some other value';

And also, a colon can be used to label a statement. for example
var i = 100, j = 100;
outerloop:
while(i>0) {
while(j>0) {
j++
if(j>50) {
break outerloop;
}
}
i++
}

You guys are forgetting that the colon is also used in the ternary operator (though I don't know if jquery uses it for this purpose).
the ternary operator is an expression form (expressions return a value) of an if/then statement. it's used like this:
var result = (condition) ? (value1) : (value2) ;
A ternary operator could also be used to produce side effects just like if/then, but this is profoundly bad practice.

The ':' is a delimiter for key value pairs basically. In your example it is a Javascript Object Literal notation.
In javascript, Objects are defined with the colon delimiting the identifier for the property, and its value so you can have the following:
return {
Property1 : 125,
Property2 : "something",
Method1 : function() { /* do nothing */ },
array: [5, 3, 6, 7]
};
and then use it like:
var o = {
property1 : 125,
property2 : "something",
method1 : function() { /* do nothing */ },
array: [5, 3, 6, 7]
};
alert(o.property1); // Will display "125"
A subset of this is also known as JSON (Javascript Object Notation) which is useful in AJAX calls because it is compact and quick to parse in server-side languages and Javascript can easily de-serialize a JSON string into an object.
// The parenthesis '(' & ')' around the object are important here
var o = eval('(' + "{key: \"value\"}" + ')');
You can also put the key inside quotes if it contains some sort of special character or spaces, but I wouldn't recommend that because it just makes things harder to work with.
Keep in mind that JavaScript Object Literal Notation in the JavaScript language is different from the JSON standard for message passing. The main difference between the 2 is that functions and constructors are not part of the JSON standard, but are allowed in JS object literals.

It is part of the object literal syntax. The basic format is:
var obj = { field_name: "field value", other_field: 42 };
Then you can access these values with:
obj.field_name; // -> "field value"
obj["field_name"]; // -> "field value"
You can even have functions as values, basically giving you the methods of the object:
obj['func'] = function(a) { return 5 + a;};
obj.func(4); // -> 9

It can be used to list objects in a variable. Also, it is used a little bit in the shorthand of an if sentence:
var something = {face: 'hello',man: 'hey',go: 'sup'};
And calling it like this
alert(something.man);
Also the if sentence:
function something() {
(some) ? doathing() : dostuff(); // if some = true doathing();, else dostuff();
}

These are generally the scenarios where colon ':' is used in JavaScript
1- Declaring and Initializing an Object
var Car = {model:"2015", color:"blue"}; //car object with model and color properties
2- Setting a Label (Not recommended since it results in complicated control structure and Spaghetti code)
List:
while(counter < 50)
{
userInput += userInput;
counter++;
if(userInput > 10000)
{
break List;
}
}
3- In Switch Statement
switch (new Date().getDay()) {
case 6:
text = "Today is Saturday";
break;
case 0:
text = "Today is Sunday";
break;
default:
text = "Looking forward to the Weekend";
}
4- In Ternary Operator
document.getElementById("demo").innerHTML = age>18? "True" : "False";

Let's not forget the switch statement, where colon is used after each "case".

That's JSON, or JavaScript Object Notation. It's a quick way of describing an object, or a hash map. The thing before the colon is the property name, and the thing after the colon is its value. So in this example, there's a property "r", whose value is whatever's in the variable r. Same for t.

Another usage of colon in JavaScript is to rename a variable, that is:
const person = {
nickNameThatIUseOnStackOverflow: "schlingel",
age: 30,
firstName: "John"
};
let { nickNameThatIUseOnStackOverflow: nick } = person; // I take nickNameThatIUseOnStackOverflow but want to refer it as "nick" from now on.
nick = "schling";
This is useful if you use a third party library that returns values having awkward / long variable names that you want to rename in your code.

One stupid mistake I did awhile ago that might help some people.
Keep in mind that if you are using ":" in an event like this, the value will not change
var ondrag = (function(event, ui) {
...
nub0x: event.target.offsetLeft + event.target.clientWidth/2;
nub0y = event.target.offsetTop + event.target.clientHeight/2;
...
});
So "nub0x" will initialize with the first event that happens and will never change its value. But "nub0y" will change during the next events.

Hidden Features of JavaScript? [closed]

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Closed 11 years ago.
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What "Hidden Features" of JavaScript do you think every programmer should know?
After having seen the excellent quality of the answers to the following questions I thought it was time to ask it for JavaScript.
Hidden Features of HTML
Hidden Features of CSS
Hidden Features of PHP
Hidden Features of ASP.NET
Hidden Features of C#
Hidden Features of Java
Hidden Features of Python
Even though JavaScript is arguably the most important Client Side language right now (just ask Google) it's surprising how little most web developers appreciate how powerful it really is.

You don't need to define any parameters for a function. You can just use the function's arguments array-like object.
function sum() {
var retval = 0;
for (var i = 0, len = arguments.length; i < len; ++i) {
retval += arguments[i];
}
return retval;
}
sum(1, 2, 3) // returns 6

I could quote most of Douglas Crockford's excellent book
JavaScript: The Good Parts.
But I'll take just one for you, always use === and !== instead of == and !=
alert('' == '0'); //false
alert(0 == ''); // true
alert(0 =='0'); // true
== is not transitive. If you use === it would give false for
all of these statements as expected.

Functions are first class citizens in JavaScript:
var passFunAndApply = function (fn,x,y,z) { return fn(x,y,z); };
var sum = function(x,y,z) {
return x+y+z;
};
alert( passFunAndApply(sum,3,4,5) ); // 12
Functional programming techniques can be used to write elegant javascript.
Particularly, functions can be passed as parameters, e.g. Array.filter() accepts a callback:
[1, 2, -1].filter(function(element, index, array) { return element > 0 });
// -> [1,2]
You can also declare a "private" function that only exists within the scope of a specific function:
function PrintName() {
var privateFunction = function() { return "Steve"; };
return privateFunction();
}

You can use the in operator to check if a key exists in an object:
var x = 1;
var y = 3;
var list = {0:0, 1:0, 2:0};
x in list; //true
y in list; //false
1 in list; //true
y in {3:0, 4:0, 5:0}; //true
If you find the object literals too ugly you can combine it with the parameterless function tip:
function list()
{ var x = {};
for(var i=0; i < arguments.length; ++i) x[arguments[i]] = 0;
return x
}
5 in list(1,2,3,4,5) //true

Assigning default values to variables
You can use the logical or operator || in an assignment expression to provide a default value:
var a = b || c;
The a variable will get the value of c only if b is falsy (if is null, false, undefined, 0, empty string, or NaN), otherwise a will get the value of b.
This is often useful in functions, when you want to give a default value to an argument in case isn't supplied:
function example(arg1) {
arg1 || (arg1 = 'default value');
}
Example IE fallback in event handlers:
function onClick(e) {
e || (e = window.event);
}
The following language features have been with us for a long time, all JavaScript implementations support them, but they weren't part of the specification until ECMAScript 5th Edition:
The debugger statement
Described in: § 12.15 The debugger statement
This statement allows you to put breakpoints programmatically in your code just by:
// ...
debugger;
// ...
If a debugger is present or active, it will cause it to break immediately, right on that line.
Otherwise, if the debugger is not present or active this statement has no observable effect.
Multiline String literals
Described in: § 7.8.4 String Literals
var str = "This is a \
really, really \
long line!";
You have to be careful because the character next to the \ must be a line terminator, if you have a space after the \ for example, the code will look exactly the same, but it will raise a SyntaxError.

JavaScript does not have block scope (but it has closure so let's call it even?).
var x = 1;
{
var x = 2;
}
alert(x); // outputs 2

You can access object properties with [] instead of .
This allows you look up a property matching a variable.
obj = {a:"test"};
var propname = "a";
var b = obj[propname]; // "test"
You can also use this to get/set object properties whose name is not a legal identifier.
obj["class"] = "test"; // class is a reserved word; obj.class would be illegal.
obj["two words"] = "test2"; // using dot operator not possible with the space.
Some people don't know this and end up using eval() like this, which is a really bad idea:
var propname = "a";
var a = eval("obj." + propname);
This is harder to read, harder to find errors in (can't use jslint), slower to execute, and can lead to XSS exploits.

If you're Googling for a decent JavaScript reference on a given topic, include the "mdc" keyword in your query and your first results will be from the Mozilla Developer Center. I don't carry any offline references or books with me. I always use the "mdc" keyword trick to directly get to what I'm looking for. For example:
Google: javascript array sort mdc
(in most cases you may omit "javascript")
Update: Mozilla Developer Center has been renamed to Mozilla Developer Network. The "mdc" keyword trick still works, but soon enough we may have to start using "mdn" instead.

Maybe a little obvious to some...
Install Firebug and use console.log("hello"). So much better than using random alert();'s which I remember doing a lot a few years ago.

Private Methods
An object can have private methods.
function Person(firstName, lastName) {
this.firstName = firstName;
this.lastName = lastName;
// A private method only visible from within this constructor
function calcFullName() {
return firstName + " " + lastName;
}
// A public method available to everyone
this.sayHello = function () {
alert(calcFullName());
}
}
//Usage:
var person1 = new Person("Bob", "Loblaw");
person1.sayHello();
// This fails since the method is not visible from this scope
alert(person1.calcFullName());

Also mentioned in Crockford's "Javascript: The Good Parts":
parseInt() is dangerous. If you pass it a string without informing it of the proper base it may return unexpected numbers. For example parseInt('010') returns 8, not 10. Passing a base to parseInt makes it work correctly:
parseInt('010') // returns 8! (in FF3)
parseInt('010', 10); // returns 10 because we've informed it which base to work with.

Functions are objects and therefore can have properties.
fn = function(x) {
// ...
}
fn.foo = 1;
fn.next = function(y) {
//
}

I'd have to say self-executing functions.
(function() { alert("hi there");})();
Because Javascript doesn't have block scope, you can use a self-executing function if you want to define local variables:
(function() {
var myvar = 2;
alert(myvar);
})();
Here, myvar is does not interfere with or pollute the global scope, and disappears when the function terminates.

Know how many parameters are expected by a function
function add_nums(num1, num2, num3 ){
return num1 + num2 + num3;
}
add_nums.length // 3 is the number of parameters expected.
Know how many parameters are received by the function
function add_many_nums(){
return arguments.length;
}
add_many_nums(2,1,122,12,21,89); //returns 6

Here are some interesting things:
Comparing NaN with anything (even NaN) is always false, that includes ==, < and >.
NaN Stands for Not a Number but if you ask for the type it actually returns a number.
Array.sort can take a comparator function and is called by a quicksort-like driver (depends on implementation).
Regular expression "constants" can maintain state, like the last thing they matched.
Some versions of JavaScript allow you to access $0, $1, $2 members on a regex.
null is unlike anything else. It is neither an object, a boolean, a number, a string, nor undefined. It's a bit like an "alternate" undefined. (Note: typeof null == "object")
In the outermost context, this yields the otherwise unnameable [Global] object.
Declaring a variable with var, instead of just relying on automatic declaration of the variable gives the runtime a real chance of optimizing access to that variable
The with construct will destroy such optimzations
Variable names can contain Unicode characters.
JavaScript regular expressions are not actually regular. They are based on Perl's regexs, and it is possible to construct expressions with lookaheads that take a very, very long time to evaluate.
Blocks can be labeled and used as the targets of break. Loops can be labeled and used as the target of continue.
Arrays are not sparse. Setting the 1000th element of an otherwise empty array should fill it with undefined. (depends on implementation)
if (new Boolean(false)) {...} will execute the {...} block
Javascript's regular expression engine's are implementation specific: e.g. it is possible to write "non-portable" regular expressions.
[updated a little in response to good comments; please see comments]

I know I'm late to the party, but I just can't believe the + operator's usefulness hasn't been mentioned beyond "convert anything to a number". Maybe that's how well hidden a feature it is?
// Quick hex to dec conversion:
+"0xFF"; // -> 255
// Get a timestamp for now, the equivalent of `new Date().getTime()`:
+new Date();
// Safer parsing than parseFloat()/parseInt()
parseInt("1,000"); // -> 1, not 1000
+"1,000"; // -> NaN, much better for testing user input
parseInt("010"); // -> 8, because of the octal literal prefix
+"010"; // -> 10, `Number()` doesn't parse octal literals
// A use case for this would be rare, but still useful in cases
// for shortening something like if (someVar === null) someVar = 0;
+null; // -> 0;
// Boolean to integer
+true; // -> 1;
+false; // -> 0;
// Other useful tidbits:
+"1e10"; // -> 10000000000
+"1e-4"; // -> 0.0001
+"-12"; // -> -12
Of course, you can do all this using Number() instead, but the + operator is so much prettier!
You can also define a numeric return value for an object by overriding the prototype's valueOf() method. Any number conversion performed on that object will not result in NaN, but the return value of the valueOf() method:
var rnd = {
"valueOf": function () { return Math.floor(Math.random()*1000); }
};
+rnd; // -> 442;
+rnd; // -> 727;
+rnd; // -> 718;

"Extension methods in JavaScript" via the prototype property.
Array.prototype.contains = function(value) {
for (var i = 0; i < this.length; i++) {
if (this[i] == value) return true;
}
return false;
}
This will add a contains method to all Array objects. You can call this method using this syntax
var stringArray = ["foo", "bar", "foobar"];
stringArray.contains("foobar");

To properly remove a property from an object, you should delete the property instead of just setting it to undefined:
var obj = { prop1: 42, prop2: 43 };
obj.prop2 = undefined;
for (var key in obj) {
...
The property prop2 will still be part of the iteration. If you want to completely get rid of prop2, you should instead do:
delete obj.prop2;
The property prop2 will no longer will make an appearance when you're iterating through the properties.

with.
It's rarely used, and frankly, rarely useful... But, in limited circumstances, it does have its uses.
For instance: object literals are quite handy for quickly setting up properties on a new object. But what if you need to change half of the properties on an existing object?
var user =
{
fname: 'Rocket',
mname: 'Aloysus',
lname: 'Squirrel',
city: 'Fresno',
state: 'California'
};
// ...
with (user)
{
mname = 'J';
city = 'Frostbite Falls';
state = 'Minnesota';
}
Alan Storm points out that this can be somewhat dangerous: if the object used as context doesn't have one of the properties being assigned to, it will be resolved in the outer scope, possibly creating or overwriting a global variable. This is especially dangerous if you're used to writing code to work with objects where properties with default or empty values are left undefined:
var user =
{
fname: "John",
// mname definition skipped - no middle name
lname: "Doe"
};
with (user)
{
mname = "Q"; // creates / modifies global variable "mname"
}
Therefore, it is probably a good idea to avoid the use of the with statement for such assignment.
See also: Are there legitimate uses for JavaScript’s “with” statement?

Methods (or functions) can be called on object that are not of the type they were designed to work with. This is great to call native (fast) methods on custom objects.
var listNodes = document.getElementsByTagName('a');
listNodes.sort(function(a, b){ ... });
This code crashes because listNodes is not an Array
Array.prototype.sort.apply(listNodes, [function(a, b){ ... }]);
This code works because listNodes defines enough array-like properties (length, [] operator) to be used by sort().

Prototypal inheritance (popularized by Douglas Crockford) completely revolutionizes the way you think about loads of things in Javascript.
Object.beget = (function(Function){
return function(Object){
Function.prototype = Object;
return new Function;
}
})(function(){});
It's a killer! Pity how almost no one uses it.
It allows you to "beget" new instances of any object, extend them, while maintaining a (live) prototypical inheritance link to their other properties. Example:
var A = {
foo : 'greetings'
};
var B = Object.beget(A);
alert(B.foo); // 'greetings'
// changes and additionns to A are reflected in B
A.foo = 'hello';
alert(B.foo); // 'hello'
A.bar = 'world';
alert(B.bar); // 'world'
// ...but not the other way around
B.foo = 'wazzap';
alert(A.foo); // 'hello'
B.bar = 'universe';
alert(A.bar); // 'world'

Some would call this a matter of taste, but:
aWizz = wizz || "default";
// same as: if (wizz) { aWizz = wizz; } else { aWizz = "default"; }
The trinary operator can be chained to act like Scheme's (cond ...):
(cond (predicate (action ...))
(predicate2 (action2 ...))
(#t default ))
can be written as...
predicate ? action( ... ) :
predicate2 ? action2( ... ) :
default;
This is very "functional", as it branches your code without side effects. So instead of:
if (predicate) {
foo = "one";
} else if (predicate2) {
foo = "two";
} else {
foo = "default";
}
You can write:
foo = predicate ? "one" :
predicate2 ? "two" :
"default";
Works nice with recursion, too :)

Numbers are also objects. So you can do cool stuff like:
// convert to base 2
(5).toString(2) // returns "101"
// provide built in iteration
Number.prototype.times = function(funct){
if(typeof funct === 'function') {
for(var i = 0;i < Math.floor(this);i++) {
funct(i);
}
}
return this;
}
(5).times(function(i){
string += i+" ";
});
// string now equals "0 1 2 3 4 "
var x = 1000;
x.times(function(i){
document.body.innerHTML += '<p>paragraph #'+i+'</p>';
});
// adds 1000 parapraphs to the document

How about closures in JavaScript (similar to anonymous methods in C# v2.0+). You can create a function that creates a function or "expression".
Example of closures:
//Takes a function that filters numbers and calls the function on
//it to build up a list of numbers that satisfy the function.
function filter(filterFunction, numbers)
{
var filteredNumbers = [];
for (var index = 0; index < numbers.length; index++)
{
if (filterFunction(numbers[index]) == true)
{
filteredNumbers.push(numbers[index]);
}
}
return filteredNumbers;
}
//Creates a function (closure) that will remember the value "lowerBound"
//that gets passed in and keep a copy of it.
function buildGreaterThanFunction(lowerBound)
{
return function (numberToCheck) {
return (numberToCheck > lowerBound) ? true : false;
};
}
var numbers = [1, 15, 20, 4, 11, 9, 77, 102, 6];
var greaterThan7 = buildGreaterThanFunction(7);
var greaterThan15 = buildGreaterThanFunction(15);
numbers = filter(greaterThan7, numbers);
alert('Greater Than 7: ' + numbers);
numbers = filter(greaterThan15, numbers);
alert('Greater Than 15: ' + numbers);

You can also extend (inherit) classes and override properties/methods using the prototype chain spoon16 alluded to.
In the following example we create a class Pet and define some properties. We also override the .toString() method inherited from Object.
After this we create a Dog class which extends Pet and overrides the .toString() method again changing it's behavior (polymorphism). In addition we add some other properties to the child class.
After this we check the inheritance chain to show off that Dog is still of type Dog, of type Pet, and of type Object.
// Defines a Pet class constructor
function Pet(name)
{
this.getName = function() { return name; };
this.setName = function(newName) { name = newName; };
}
// Adds the Pet.toString() function for all Pet objects
Pet.prototype.toString = function()
{
return 'This pets name is: ' + this.getName();
};
// end of class Pet
// Define Dog class constructor (Dog : Pet)
function Dog(name, breed)
{
// think Dog : base(name)
Pet.call(this, name);
this.getBreed = function() { return breed; };
}
// this makes Dog.prototype inherit from Pet.prototype
Dog.prototype = new Pet();
// Currently Pet.prototype.constructor
// points to Pet. We want our Dog instances'
// constructor to point to Dog.
Dog.prototype.constructor = Dog;
// Now we override Pet.prototype.toString
Dog.prototype.toString = function()
{
return 'This dogs name is: ' + this.getName() +
', and its breed is: ' + this.getBreed();
};
// end of class Dog
var parrotty = new Pet('Parrotty the Parrot');
var dog = new Dog('Buddy', 'Great Dane');
// test the new toString()
alert(parrotty);
alert(dog);
// Testing instanceof (similar to the `is` operator)
alert('Is dog instance of Dog? ' + (dog instanceof Dog)); //true
alert('Is dog instance of Pet? ' + (dog instanceof Pet)); //true
alert('Is dog instance of Object? ' + (dog instanceof Object)); //true
Both answers to this question were codes modified from a great MSDN article by Ray Djajadinata.

You may catch exceptions depending on their type. Quoted from MDC:
try {
myroutine(); // may throw three exceptions
} catch (e if e instanceof TypeError) {
// statements to handle TypeError exceptions
} catch (e if e instanceof RangeError) {
// statements to handle RangeError exceptions
} catch (e if e instanceof EvalError) {
// statements to handle EvalError exceptions
} catch (e) {
// statements to handle any unspecified exceptions
logMyErrors(e); // pass exception object to error handler
}
NOTE: Conditional catch clauses are a Netscape (and hence Mozilla/Firefox) extension that is not part of the ECMAScript specification and hence cannot be relied upon except on particular browsers.

Off the top of my head...
Functions
arguments.callee refers to the function that hosts the "arguments" variable, so it can be used to recurse anonymous functions:
var recurse = function() {
if (condition) arguments.callee(); //calls recurse() again
}
That's useful if you want to do something like this:
//do something to all array items within an array recursively
myArray.forEach(function(item) {
if (item instanceof Array) item.forEach(arguments.callee)
else {/*...*/}
})
Objects
An interesting thing about object members: they can have any string as their names:
//these are normal object members
var obj = {
a : function() {},
b : function() {}
}
//but we can do this too
var rules = {
".layout .widget" : function(element) {},
"a[href]" : function(element) {}
}
/*
this snippet searches the page for elements that
match the CSS selectors and applies the respective function to them:
*/
for (var item in rules) {
var elements = document.querySelectorAll(rules[item]);
for (var e, i = 0; e = elements[i++];) rules[item](e);
}
Strings
String.split can take regular expressions as parameters:
"hello world with spaces".split(/\s+/g);
//returns an array: ["hello", "world", "with", "spaces"]
String.replace can take a regular expression as a search parameter and a function as a replacement parameter:
var i = 1;
"foo bar baz ".replace(/\s+/g, function() {return i++});
//returns "foo1bar2baz3"

You can use objects instead of switches most of the time.
function getInnerText(o){
return o === null? null : {
string: o,
array: o.map(getInnerText).join(""),
object:getInnerText(o["childNodes"])
}[typeis(o)];
}
Update: if you're concerned about the cases evaluating in advance being inefficient (why are you worried about efficiency this early on in the design of the program??) then you can do something like this:
function getInnerText(o){
return o === null? null : {
string: function() { return o;},
array: function() { return o.map(getInnerText).join(""); },
object: function () { return getInnerText(o["childNodes"]; ) }
}[typeis(o)]();
}
This is more onerous to type (or read) than either a switch or an object, but it preserves the benefits of using an object instead of a switch, detailed in the comments section below. This style also makes it more straightforward to spin this out into a proper "class" once it grows up enough.
update2: with proposed syntax extensions for ES.next, this becomes
let getInnerText = o -> ({
string: o -> o,
array: o -> o.map(getInnerText).join(""),
object: o -> getInnerText(o["childNodes"])
}[ typeis o ] || (->null) )(o);

Be sure to use the hasOwnProperty method when iterating through an object's properties:
for (p in anObject) {
if (anObject.hasOwnProperty(p)) {
//Do stuff with p here
}
}
This is done so that you will only access the direct properties of anObject, and not use the properties that are down the prototype chain.

Private variables with a Public Interface
It uses a neat little trick with a self-calling function definition.
Everything inside the object which is returned is available in the public interface, while everything else is private.
var test = function () {
//private members
var x = 1;
var y = function () {
return x * 2;
};
//public interface
return {
setx : function (newx) {
x = newx;
},
gety : function () {
return y();
}
}
}();
assert(undefined == test.x);
assert(undefined == test.y);
assert(2 == test.gety());
test.setx(5);
assert(10 == test.gety());