javascript.md

Java Script Cheat Sheet

Here are my notes as I learn JS (heaven help me).

Data types

JS is a little wonky about how it types things:

• It's dynamically typed.
• There are some primative types.
• There's a generic Object type that gets used as a container.

Using typeof

The typeof operator lets you get the type of an object, but beware: the value you get might not be the one you want. Search the Web for information about how to predict what you'gg get, or just test it out when you need it.

if (typeof someVariable == 'string'){
    var isString = true;
}

Strings

Strings are objects. When you use a literal, you're essentially creating a new String object. These two statements are functionally identical:

var someString = 'This is a string.';

var someOtherString = new String('This is a string.');

More about string literals

Type of quotes

You can use single or double quotes to signify a string literal. These are equivalent:

var s1 = "this is a string";
var s2 = 'this is a string';

Escaping characters

Use a backslash (\) in front of a character to escape it, C-style. There are two common cases:

• Non-visible characters, as in C: (\n, \t, etc.). Note that many of these are of little use in many JS scripts, because they have no effect on HTML rendering of strings.

• Escaping the type of quote character used to define the string:

  var s1 = 'What's wrong with this string?';
  var s2 = 'What\'s wrong with this string? Nothing!';

Dealing with long string literals

You can use the backslash character break string literals across lines. JS continues defining the string on the next line:

var s1 = 'String literal broken \
          over two lines.';

When you're working in HTML, the extra spaces before the text of the second line is collapsed, so it works out fine. However, it can be confusing to have those extra spaces in the object. One way out is to start the new line unindented, but that's not very clean. Instead, it's almost always better to just concatenate multiple strings:

var s2 = 'Two strings ' +
         'concatenated.';

String methods

Using a generic Object as a dictionary

JS has no explicit type for a dictionary, but you can use a generic object.

Creating

As with many object-oriented languages, there's a formally correct way:

var myDict = new Object();

And there's a shorthand way that everybody uses:

var myDict = {};

You can add some key/value pairs at creation when using the shortcut:

var myDict = {
  aVal: 42,
  anotherVal: 333,
  // Note that both identifiers and string literals can be used for keys.
  'yetAnother': 'This one\'s a string!'
};

Getting values

You can get values using object notation:

var myCat = {'breed': 'oriental shorthair', 'age': 16, 'sex': 'neutered male'};

var breed = myCat.breed;

Or you can use dictionary notation:

var breed = myCat['breed'];

Note: JS handles requests for nonexistent properties gracefully. It doesn't throw an exception or otherwise register an error. Instead, it returns the keyword undefined. What you do with that information is entirely up to you.

Setting values

You can arbitrarily set values, again using either object or dictionary notation.

var myDict = {};

// This works.
myDict.term1 = 'first term';

// So does this.
myDict['term2'] = 'second term';

Terms and definitions can be of basically any type.

Iterating

The easiest way is to do a for loop:

for(var key in myDict) {
    var value = myDict[key];
}

Miscelany

This is a section to contain anything I learn that's not an obvious part of another section, or that would be hard to find is buried within a related topic.

Null coalescing

This is a term that's new to me. Funnily enough, there isn't a null coalescing operator in JavaScript, so I learned about it from the JavaScript workaround.

The basic idea is to define a specific operator syntax that covers a very common case: you assign a variable to the value that mightbe null and you want to assign some default value if it is.

a = someFunc(b);

if (a == null) {
    a = '';
}

In other languages

There's a big list of other languages and their special null coalescing operators on Wikipedia.

For example, in C#:

string pageTitle = suppliedTitle ?? "Default Title";

In JavaScript

Instead of a special operator as such, JS uses a special case of the logical OR:

var pageTitle = (suppliedTitle || 'Default Title');