Objects
Notes
The Object Type
The object
is a data structure that stores other data, similar to how an array stores elements.
The object differs in that each
value
stores in an obj is associated with akey
.
The Object of My Affections
In other programming languages, objects are referred to as, "dictionaries", "maps", or "associative arrays".
Objects are indexed with
keys
instead of numbers.Order is not guaranteed within an Object.
Objects are defined by using curly braces
{}
You can think of Objects as tables.
Fun Fact: Objects are affectionately known as POJO's (Plain Old Javascript Objects)
Setting Keys and Values
We assign values to an object by defining the name of the key in brackets and assigning it to a value.
If we try to access a key that has not yet been assigned within an object we will output undefined.
The preferred method for checking to see if an object exists at a key is to use the
in
operator.
Using Variables as Keys
It is useful to set a variable as a key, because variables can be re-assigned new values - this way we can quickly access different data and also create new key/value pairs.
Using Different Notations
We can also use dot notation "." to access key/value pairs in an object.
One thing to note is that when using dot notation, we do not have to use string quotes as the key.
Bracket Notation vs Dot Notation
Dot | Bracket |
Easier To Read | You can use variables as keys! |
Easier To Write b/c do not need Quotations. | Okay to use variables and Strings that start with numbers. |
Cannot access with Variables | |
Keys cannot contain numbers as their first character |
When accessing object keys: Bracket notation needs to refer to that key in quotations, dot notation doesn't.
When accessing object keys via a variable: Bracket notation can refer to that key w/o use of quotations, dot notation can't do this at all.
As illustrated above, the dot notation cannot access a varible key - since it takes a literal interpretation of the key.
Putting it All Together
You can put an object together in a single statement.
Operator Precedence Revisited
The concept of Operator Precedence also applies to objects.
There are two types of associativity:
Right Associativity
: When code is evaluted right to left.Since assignment of variables takes lowest precendence, we end up evaluating b = 1 first before a = b.
Left Associativity
: When code is evaluated left to right.We first resolve the document variable, then use dot notation to retrive the getElementById function, we eval it's arguments, access it's value, and then retrieve assignment (the lowest precedence).
Iterating Through Objects
Because objects store unordered key-value pairs, we do not rely on indices to access values; instead we rely on our keys.
A New Kind of For Loop
We use a special syntax to iterate through each key of an object called a
for-in loop
.
Methods vs Functions
A Method
is a function that belongs to an object. Every method is a function, but not every function is a method.
Methods are just a key-value pair where the key is the function name and the value is the function definition.
To invoke these methods we just need to specify which object is calling that method.
Useful Object Methods
Object.keys()
: A method that allows us to iterate through keys, it accepts an obj as the argument and returns an array of the keys.Object.values()
: Method that accepts an object as the argument and returns an array of the values.
Iterating through an Object's keys & values
Object.entries
: Method that accepts an object as the argument and returns an array of the [key,value] pairs within.
References vs Primitives
Primitives vs Objects
So far we have learned about 6 different data types:
Primitive : Boolean, Null, Undefined, Number, String.
Reference : Object (Arrays are a type of object)
Remember that primitive types are immutable!
Immutabiity
When we reassign primitives we simply have our variable point elsewhere in memory.
In a nutshell, immutability cannot change values in memory, but only reassign where our variables are pointing to.
Mutabulity
If we change either cat1 or cat2, our computer memory will change because they are both pointing at the same memory location.
Rest and Spread
Using the Spread Operator and Rest Parameter Syntax Accepting Arguments
Just keep in mind that function will still run even if it is not passed any arguments.
Parameters will take just as many arguments they need even if more than enough are offered.
We will encounter an error if there are not enough parameters ( > 0).
Utilizing Rest Parameters
Rest Parameter Syntax
: Allows us to capture all of a function's incoming arguments into an array.Only the last parameter can be a rest parameter.
Utilizing Spread Syntax
Spread Operator : Allows us to break down a data type into the elements that make it up.
Takes a data type (i.e. array, obj) and spreads the values of that type where elements are expected.
Takes iterable data and spreads the elements of that type where arguments are expected.
With Objects
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