- Data is stored as bits – any kind of two-valued things
- Bits – usually described as zeros and ones
- Inside the computer, bits – high or low electrical charge, a strong or weak signal, a shiny or dull spot on a CD
- “Any piece of discrete information can be reduced to a sequence of zeros and ones and thus represented in bits” {?}
NB: Discrete data can only take distinct values; can be numeric or categorical (male or female, green or yellow) –stackexchange
Example: Show the number 13 in bits
-Have only 2 digits (0 and 1)
-The weight of each digit increases by a factor of 2 from right to left
-The binary number 00001101, or 8+4+1, is the decimal number 13
NB: a binary number is expressed in the binary numeral system or base-2 numeral system which represents numeric values using two different symbols: usually 0 and 1 –Wikipedia
Values
- modern computer has > 30 billion bits in its volatile data storage; more in nonvolatile data storage
NB: Volatile memory is computer memory that requires power to maintain the stored info; stored data is lost if the power is off or interrupted —Wikipedia
- hard to work with this large quantity of bits
- Solution: separate bits into chunks (called values) that represent pieces of information
- Every value has a type that determines its role
- The 6 basic types of values in JS:
-numbers
-strings
-Booleans
-objects
-functions
-undefined values
- To create a value — invoke its name
- Every value has to be stored somewhere
- Might run out of bits if you use many, many values simultaneously
- “When you no longer use a value, it will dissipate {?}, leaving behind its bits to be recycled as building material for the next values”
Numbers
[make more concise]
- The value of the number type is a numeric value
- JS uses 64 bits to store a single number value
- The amount of different numbers that can be represented is limited
- With 64 binary digits, 264 different numbers can be represented
- Computer memory used to be smaller; 8 bits or 16 bits were used to represent numbers
- Easy to overflow such small numbers – end up with a number that did not fit into the given amount of bits
- Today, computers have plenty of memory, so overflow occurs only when dealing with astronomical numbers
- Bits can store negative numbers (one bit indicates the sign of the number)
- Nonwhole numbers must also be represented (some of the bits store the position of the decimal point)
- Calculations with integers (whole numbers) are precise
- Treat fractional digital numbers as ap and proximations, not as precise values (because fractions cannot be expressed by a finite number of decimal digits — and there is only 64 bits available to store them)
Arithmetic
- 100 + 4*11
- + and * are operators
- + represents addition
- * represents multiplication
- <<value>> <<operator>> <<value>>
- An operator between two values applies itself to those values to produce a new value
- Order of operator application (precedence) is determined by BEDMAS
- Apply Left to Right if there are multiple operators with the same precedence
- % : represents the remainder operation — known as modulo or remainder
- X % Y : the remainder of dividing X by Y
- Example: 314 % 100 produces 14, 144 % 12 produces 0
Special Numbers
3 JS values considered numbers but don’t behave like normal numbers:
- Infinity : positive infinity
- -Infinity : negative infinity
- NaN : “Not a Number” – a value of the number type; will get NaN when i.e. 0/0 (zero divided by zero), Infinity – Infinity, any numeric operation that does not yield a precise result
Strings
- string – a basic data type
- represent text
- content enclosed in quotes (single, double – but start and end quote must match)
- i.e. “Hello there!”, ‘On the way to the moon, hold up.’
- Characters that are difficult to place within quotes: quotes, newlines (characters you get when you press Enter – basically spaces on a new line)
- Solution: escaping the character — using: backslash (\) — when this is inside a quoted text, it indicates the the character after it has a special meaning
- i.e. ‘She\’s my sister\’s dog’ means the quotes are a part of the string
- i.e. ‘ \n ‘ means a newline
- i.e. ‘ \t ‘ means a tab character
- i.e. “This is the first line\nAnd this is the second”
Actual text:
This is the first line
And this is the second
- When you want a backslash in a string to be just a backslash:
use \\ (two backslashes)
Example:
console.log(“A newline character is written like \”\\n\”.”)
// -> A newline character is written like “\n”.
- Strings cannot be divided, multiplied, or subtracted
- + operator used on strings to concatenate – combines two strings together
Example:
console.log(“con” + “cat” + “e” + “nate”)
// -> concatenate [not enclosed by single quotes like in Python]
Unary Operators
unary operator: operator that takes\operates on one value
binary operator: operator that uses two values
ternary operator: operator that operates on three values
typeof: an unary operator that produces a string value naming the type of the value given to it
Example 1:
console.log(typeof 4.5)
//-> number
Example 2:
console.log(typeof Infinity)
//-> number
Example 3:
console.log(typeof NaN)
//->number
Example 4:
console.log(typeof 3 > 2)
//-> false
Example 5:
console.log(typeof “x”)
//-> string
NB1: console.log() outputs a message to the Web Console (a way for us to see the result of evaluating something)
NB2: The minus operator (-) can be used as both a binary and an unary operator
Example 6:
console.log(-(10-2))
//-> -8
Boolean Values
Distinguish between two possibilities i.e. yes and no, on and off, …
Boolean type: has two values — true and false
Comparisons
A way to produce Boolean values
Example 1:
console.log(3 > 2)
//-> true Indicates 3 is greater than 2
Example 2:
console.log(3 < 2)
//-> false
Strings can be compared to each other :
- strings are ordered alphabetically i.e. “a” < “z”
- uppercase letters are “less” than lowercase letters i.e. “Z” < “a”
- non-alphabetic characters (!, -, ?, etc.) are included in the ordering i.e. “.” > “!”, “?” > “!”
- comparison is based on the Unicode standard : assigns a number to every character ever needed, including characters from other languages i.e. Japanese, Arabic, Greek, …
- comparing strings : Left to Right, compare numeric codes of characters one by one
- other comparison operators:
>= greater than or equal to
<= less than or equal to
== equal to
!= not equal to
- one value in JS is not equal to itself : NaN (“not a number”)
Example:
console.log(NaN == NaN)
//-> false
NB: NaN denotes the result of a nonsensical computation; therefore it isn’t equal to the result of other nonsensical computations.
Logical Operators
Operators on Boolean values
and, or, not
used to “reason” about Booleans
&& represents logical and
-a binary operator
-its result is true iff both values given to it are true
Example 1:
console.log(true && false)
//-> false
Example 2:
console.log(true && true)
//-> true
|| represents logical or
-a binary operator
– proudces true if either of the values given to it is true
Example 1:
console.log(false || true)
//-> true
Example 2:
console.log(false || false)
//-> false
! represents not
-an unary operator
-flips the value given to it
Example 1:
console.log(!true)
//-> false
Example 2:
console.log(!false)
//-> true
Order of Precedence
Arithmetic operators (highest)
Comparison operators
&&
|| (lowest)
NB: This order minimizes the use of parentheses.
Example:
console.log(1+1 == 2 && 10*10 > 50)
// 2 == 2 && 100 > 50
// true && true
//-> true
<<value1>> ? <<value2>> : <<value3>>
-represents the conditional/ternary operator
– a ternary operator
-written with a question mark and a colon
– The first value “picks” which of the other two values will come out :
-if true, the second value is chosen
-if false, the third value is chosen
NB: << … >> just represents that a value is written; <<…>> itself is not included in the value
Example 1:
console.log(true ? 1 : 2)
//-> 1
Example 2:
console.log(false ? 1 : 2)
//-> 2
Undefined Values
Two special values:
null
undefined
represent the absence of a meaningful value (carry no information)
NB1: operations that don’t produce a meaningful value yield undefined because they have to yield some value
NB2:
-there is a difference in meaning between undefined and null (but it doesn’t matter most of the time)
-this is an accident of JS’ design
-treat them as interchangeable (for now)
Automatic Type Conversion
Here we’ll see how JS accepts almost any program given to it, even if they are odd.
type coercion:
– an operator is applied to the “wrong” type of value i.e. “three” + 3
-JS converts that value to the type it wants
-JS uses a set of unpredictable rules to do this
Example 1:
console.log(8*null)
//-> 0 null becomes 0
Example 2:
console.log(“5” – 1)
//-> 4 string -> number ; “5” becomes 5
Example 3:
console.log(“5” + 1)
//-> 51 + tries string concatenation before numeric addition; 1 converted to “1”
Example 4:
console.log(“five” * 2)
//-> NaN
Example 5:
console.log(false == 0)
//-> true
Regarding Ex. 4: When something that doesn’t map to a number i.e. “five”, “undefined”, is converted to a number, the value NaN is produced.
NB: Further arithmetic operations on NaN keep producing NaN.
Comparing values of different types using ==
- most cases: JS converts one of the values to the other value’s type
- For true to be produced —
if null is on one side, undefined must be on the other and vice versa
Example 1:
console.log(null == undefined)
//-> true
Example 2:
console.log(undefined == null)
//-> true
Example 3:
console.log(null == 0)
//-> false
Example 4:
console.log(undefined == 1)
//-> false
NB: to test whether a value has a real value instead of null or undefined, compare it to null with the == or != operator.
QUESTION: Can you compare it to undefined too?
Rules for converting strings and numbers to Boolean values:
- 0, NaN, the empty string (“”) count as false
- all other values count as true
Ex. 1:
console.log(0 == false)
//-> true
Ex. 2:
console.log(“” == false)
//-> true
For these cases, automatic type conversion is unwanted.
To test whether something refers to the precise value false:
===
!==
=== tests whether a value is precisely equal to the other
!== tests whether a value is not precisely equal to the other
Ex:
console.log(“” === false)
//-> false
In this example, “” counts as false, but it is not precisely equal to the value false.
NB: Use the three-character comparison operators to make certain that both sides are the same type.
QUESTION: Because automatic type conversion could change the type of a value in the comparison?
Short Circuiting of Logical Operators
- Logical operators && and || handle values of different types strangely
- They will convert the value on their left side to Boolean type (to decide what to do)
- Depending on the operator and the result of that conversion, either the original LH-value or RH-value is returned
|| operator
– returns the value to its left when that can be converted to true
-returns RH-value otherwise
Ex. 1:
console.log(null || “user”)
//-> user
Ex. 2
console.log(“Karl” || “user”)
Karl
NB: Therefore || operator can be used as a way to fall back on a default value i.e. give it an expression that might produce an empty value on the left, the value on the right will be used as a replacement
&& operator
– works the other way around
-when LH-value is something that converts to false, it returns that original LH-value
-otherwise, RH-value is returned
Property of && and || operators:
Short-circuit evaluation: expression on right evaluated only when necessary
Ex. 1: true || X
-LH-value is true so the result is true
-X is never evaluated, no what X is
Ex. 2: false && X
-result is false
-X is ignored
Conditional operator
-works similarly
-first expression is always evaluated
-second or third value (the one not picked) is ignored {?}
Summary
We made some values and applied operators to them to get new values.
- Went through four types of JS values: numbers, strings, Booleans, undefined values
- Values are created by typing in their name (true, false, null, undefined) or value (“abc”, 13)
- Can combine and transform values with operators
- Binary operators for
-arithmetic (+, -, *, /, %)
-string concatenation (+)
-comparison (==, !=, ===, !==, <, >, <=, >=)
-logic (&&, ||)
- Unary operators:
-(!) to negate logically
-(-) to negate a number
-(typeof) to find a value’s type
- Ternary operator:
( ? : ) – to pick one of two values based on a third value
- NaN (“not a number”) is the only JS value not equal to itself, because the result of a nonsensical computation cannot equal the result of another nonsensical computation
Progress: We can now use JS as a pocket calculator. LOL FML.
Ode to STEM 