objects

Author: James Halliday

javascript.markdown

@@ -458,6 +458,25 @@ second: 4
 third: 9
 ```
 ---
+# arrays: indexing with variables
+
+You can use variables for indexing too, not just constant
+integers:
+
+``` js
+var arr = [ 1, 4, 9 ];
+var n = 2;
+console.log(arr[n]);
+```
+
+prints:
+
+```
+9
+```
+
+Any expression inside the square brackets will work.
+---
 # arrays: length
 
 To get the length of an array, just use `.length`:
@@ -621,6 +640,183 @@ We've already been using some objects:
 * `console` is an object with a property
 `log` that is a function.
 ---
+# objects
+
+Objects map keys to values.
+
+Like how a phone book maps names to telephone numbers:
+
+```
+key                     value
+----------------------|---------
+Benjamin Franklin     | 123-4142
+Alexander Graham Bell | 214-6821
+Marie Curie           | 615-2904
+```
+---
+# objects
+
+To create the same structure in javascript we could do:
+
+``` js
+var phonebook = {
+    'Benjamin Franklin': '123-4142',
+    'Alexander Graham Bell': '214-6821',
+    'Marie Curie': '615-2904'
+};
+```
+---
+# objects
+
+If a key is only letters, numbers, underscores, and dollar
+signs (but doesn't start with a number), you can leave off
+the quotes:
+
+``` js
+var obj = {
+    abc: 555,
+    def: 123,
+    x1: 1000,
+    y1: 5000,
+    $x: 4444,
+    "x y z": 12 // otherwise you'll need to use quotes
+}
+```
+
+This way of building objects is sometimes called
+"object literal" syntax.
+---
+# objects: pick out a single record
+
+Once an object exists, you can reference an individual value
+by its key using a `.` followed by the key name. Here we use
+`obj.y` to get at the `y` key:
+
+``` js
+var obj = { x: 7, y: 8, z: 9 };
+console.log(obj.y);
+```
+
+will print:
+
+```
+8
+```
+---
+# objects: create a new record
+
+Reference a key with a `.` followed by a key then use an
+equal sign to create a new value under that key:
+
+``` js
+var obj = { a: 3, b: 4 };
+obj.c = 5;
+console.log(obj);
+```
+
+prints:
+
+```
+{ a: 3, b: 4, c: 5 }
+```
+---
+# objects: update an existing record
+
+The same `obj.c = 5` syntax works even if a key already
+exists:
+
+``` js
+var obj = { x: 500, y: 200 };
+obj.y = 300;
+console.log(obj);
+```
+
+prints:
+
+```
+{ x: 500, y: 300 }
+```
+---
+# objects: assignment operators
+
+All of the assignment operators work for object keys just
+like for variables!
+
+``` js
+var obj = { x: 500, y: 200 };
+obj.y += 1000;
+obj.x *= 3;
+console.log(obj);
+```
+
+prints:
+
+```
+{ x: 1500, y: 1200 }
+```
+---
+# objects: square brackets
+
+Using the `.` operator to access keys is handy, but what
+about special characters in keys?
+
+We can use square brackets with a string to reference
+keys with special characters:
+
+```
+var obj = { 'a b c': 123, 'x#y*z': 456 };
+console.log(obj['a b c']);
+```
+
+prints:
+
+```
+123
+```
+---
+# objects: square brackets
+
+We can also use square brackets to reference dynamic keys.
+For example, if we want to load a key from a variable:
+
+``` js
+var key = 'def';
+var obj = {
+    abc: 555,
+    def: 333,
+    xyz: 222
+};
+console.log(obj[key]);
+```
+
+prints:
+
+```
+333
+```
+
+Everything we can do with dot (updates, assignment
+operators, etc) works with square bracket notation.
+---
+# objects: delete
+
+To delete items from an object, use the `delete` keyword:
+
+``` js
+var obj = { a: 4, b: 5, d: 700, x: 6 };
+delete obj.d;
+console.log(obj);
+```
+
+prints:
+
+```
+{ a: 4, b: 5, x: 6 }
+```
+
+If you try to delete a key that doesn't exist, nothing
+happens.
+---
 # Object.keys
 
 Use `Object.keys()` to get an array of all the keys that an