JavaScript Conditionals

Source Material

See the first two chapters of the second edition of Eloquent JavaScript for more detailed information:

• Chapter 1 - Values
• Chapter 2 - Program Structure

Boolean Values

Besides numbers and strings, there is another type that we talked about briefly. They are booleans. What are the possible values that a boolean can be? →

true
false

Boolean Operators

What are some operators that return boolean values? →

Boolean Expressions

You can create boolean expressions with these operators. What is the value that results from the following expressions? What type are they? →

1 === "1"
12 !== "cat"

// boolean
false
true

Additional Comparison Operators

Can you think of other comparison operators that JavaScript may have? →

Additional Comparison Operators Continued

// false!
5 > 7

// true!
5 >= 5

Boolean Expressions Give Back Boolean Values!

Conditionals

See Conditional Execution in Eloquent JavaScript

A conditional is a way of allowing us to conditionally execute code based on a boolean expression.

If Statement

Conditionally execute a branch of code.

if (some_boolean_expression) {
	// do stuff here if expression is true
}

• Diagram for if statements
• Diagram for consecutive if statements

If / Else Statement

Execute one branch if condition is true, another branch if condition is false. An else must have a preceding if!

if (some_boolean_expression) {
	// do stuff here if expression is true
} else {
	// do stuff here if expression is false
}

• Diagram for if / else statements
• else statements are always attached to an if!

If / Else If

Chain multiple conditions. You can add else at the end as well. Conditions are evaluated until the first true condition, at which point the if statement finishes immediately.

if (boolean_expression_1) {
	// do stuff here if expression 1 is true
} else if (boolean_expression_2) {
	// do stuff here if expression 2 is true
} else if (boolean_expression_3) {
	// do stuff here if expression 3 is true
}

• Diagram for if / else if statements

Usage

• use if statements if there’s a single alternative path of execution
• use if/else statements if there’s a two possible alternative paths of execution
• use if/else if statements if there’s more than two possible alternative paths of execution

Boolean Expressions

Note that within the parentheses is some expression that produces a boolean values (true or false).

if (boolean_expression_1) {
	// do stuff here if expression 1 is true
}

Blocks of Code

Curly braces denote statements of code that are grouped together. Everything within the curly braces below is considered part of the if statement.

if (boolean_expression_1) {
	// do stuff here if expression 1 is true
}

Logical Operators

Logical operators allow you to combine boolean values!

• and (represented as &&) - takes two operands.
  
  • only gives back true when both operands are true.
  • gives back false otherwise
• or (represented as ||) - takes two operands.
  • returns true if either operand is true
  • it only gives back false when both operands are false.

Using Logical Operators

Examples

// variable num is between 1 and 10
if(num >= 1 && num <=10) {
	print("it's between 1 and 10")
}

// answer is yeah or yes
if(answer === 'yes' || answer === 'yeah' ) {
	print("YUP!")
}

Converting from Numbers to Strings

Use the built-in function parseInt to convert a number to a string.

• parseInt takes two arguments, the string to convert, and the radix (which will be 10 for us)
• we can use this when we get user input… why would we need to do this?…

var num = parseInt(answer, 10);