Self-Taught Developer Journal, Day 27: TOP JavaScript Fundamentals Part 2 - Data Types Overview cont. and Comparisons

Today I learned...

Boolean

• true or false values
• values can also be a result of comparisons, ex. let isGreater = 5 > 2; \\true

null value

• a special value that represents "nothing", "empty", or "value unknown"

undefined value

• represents a value not assigned
• can explicitly assign undefined value to a variable but this is not recommended. Instead, use the null to assigning a "nothing" or an "empty" value.

Objects

• object type is special
• not a primitive value
• represents more complex data structures

Symbols

• used to create unique identifiers for objects

typeof operator

• returns the type of argument
• typeof x is equivalent to typeof(x)
• typeof null returns object which is incorrect and an error of JS
• typeof alert will return function but functions are technically an object data type

Conditionals

String Comparison

• strings are compared letter by letter
• longer strings are greater
• uses Unicode

Comparison of Different Types

• converts values to numbers

Comparison with null and undefined

• null === undefined is false (strict equality)
• null == undefined is true (loose equality)

For math and comparisons (<,>,>=,<=)

• null becomes 0
• undefined becomes NaN.

Strange Result: null vs 0

1. null > 0 is false, null becomes 0 with the comparison operator, 0 is not greater than 0.
2. null == 0 is false, the == equality does not convert null and null does not equal 0.
3. null >= 0 is true, null is converted to 0 and fulfills one of the operator condition (0 equals 0)

An incomparable undefined

• shouldn't be compared to other values
• undefined > 0 is false
• undefined < 0 is false, examples 1 and 2 show undefined converted to NaN by the comparison operators and NaN is a numeric value and returns false for all comparisons.
• undefined == 0 is false, returns false because the undefined only equals undefined or null.

Treat comparisons with undefined\null with care.

Resources

The Odin Project
https://javascript.info/types
https://javascript.info/comparison