Selection Using if statements

CS133JS Beginning Programming: JavaScript

Table of Contents

Review

Scope

• What is scope?

• What is meant by the terms: local variable, and global variable?

• Why are global variables dangerous¹?

• Why shouldn't we use a variable without declaring it with var or let²? Why is let better?

• Identify the scope of each of the variables below:

let extraVariable = 0;
function total(n1, n2, n3)  {
    var t = n1 + n2 + n3; 
    extraVariable = t;
    n1 = 0;
    return t;
}

let n1 = 2, n2 = 3, n3 = 6;
let sum = total(n1, n2, n3);
console.log(sum);
console.log(n1);
console.log(extraVariable);

Comparison Operators

Logical Operators

Logical operators and expressions

These are also called Boolean³ operators. They are used to form Boolean expressions that evaluate to a value of either true or false.

Truth tables

Control Structures

What are the three control structures? These are things in the programming language that control the flow of execution in a program.

This is the end of the review. This was a long review!

Selection Expressions

There are three ways to do selection in JavaScript:

1. ternary expressions using ? and :

2. if statements

3. switch statements

In this session we will learn to use the ternary oprator and if statements. Next time we will learn to use switch statements.

Ternary Operator

The ternary operator is the question mark, ?

• Syntax

  Example: let result = a === b ? "Strictly equal" : "Not equal";

  a === b	?	"Strictly equal"	:	"Not equal"
  Logical condition	The ternary operator	Expression returned if the condition is true	Colon separates the two expressions	Expression that is returned if the condition is false

  The ternary operator is used to form a conditional expression. An expression evaluates to a value. For example:

  let age = prompt("How old are you?");
  let legalStatus = age >= 18 ? "adult" : "child";
  

• Another example:

  let temperature = prompt("What is the temperature?");
  alert(temperature < 50 ? "Wear a coat" : "Enjoy the warm day");
  

if Statements

An if statement is used to conditionally determine whether or not one or more other statements are executed. Note that it does not form an expression—it doesn't evaluate to a value.

Types of if statements

In the examples below, assume this line of code was executed first:

let degrees = 0; // Temperature in Fahrenheit

• Single branch

  if(degrees < 45) 
    alert("Wear a coat");
  

  Parts of this statement:

  • keyword: if

  • condition: (degrees < 45)

  • conditionally executed statement: alert("wear a coat");

• Two branches

  if(degrees < 45) 
     alert("Wear a coat");             // executed if the condition is true
  else         
     alert("Enjoy the warm weather! "); // executed if the condition is false
  

• Multiple branches

  if(degrees < 32)   
     alert("Wear a warm coat"); // executed only if the first condition is true
  else if (degrees < 45)
     alert("Wear a jacket");    // executed only if the second condition is true   
  else
    alert("Enjoy the warm weather! ");  // executed only if neither condition is true
  

Designing a Selection Algorithm

Let's take a break from talking about coding and talk about designing an algorithm so that we know what it is we need to code.

• Algorithms: Earlier this term we talked about algorithms and we wrote some algorithms in English.

• IPO: We also talked about how every function has Input, Processing, and Output. We even put that into a table.

• Selection IPO table: We can make a table that shows each combination of inputs that will produce a particular output. For the Multiple branches code above, we could make a table like this:

  Input	Processing	Output
  31	temp less than 32 degrees	Wear a warm coat
  32		Wear a jacket
  44		Wear a jacket
  45	temp 45 degrees or higher	Enjoy the nice weather

  Q: Would be helpful to add "temp less than 45 degrees" to the 2nd row, processing column?⁴

  Try this:

  Write an expression using a ternary operator that determines whether a person is old enough to get a driver's license in Oregon⁴.

Conditional expressions using logical operators

For these examples, assume this additional variable has been declared and initialized :

let wind = 0; // Wind speed in MPH

• AND, &&

  Both comparisons must evaluate to true for the entire conditional expression to be true.

  xxxxxxxxxx
  degrees = prompt("Enter the temperature.");
  wind = prompt("Enter the wind speed.");
  if(degrees < 60 && wind > 10)
     alert("Wear a jacket");
  

   

• OR, ||

  Either comparison can evaluate to true for the entire conditional expression to be true.

  xxxxxxxxxx
  degrees = prompt("Enter the temperature.");
  wind = prompt("Enter the wind speed.");
  if(degrees < 45 || wind > 20)
   alert("Wear a warm coat");
  

   

• NOT, !

  Not inverts the Boolean value resulting from the comparison.

  xxxxxxxxxx
  let name = "";  // Person wishing to fly the Millennium Falcon
  name = prompt("Enter your name");
  if( !(name == "Han Solo"))   // Only Han Solo can fly the Millennium Falcon!
     alert("This is not your spacecraft! ");
  

   

Nested if statements

Another way to combine multiple conditional expressions is to nest the if statements.

xxxxxxxxxx
if(degrees < 60) 
  if(wind > 10)
     alert("Wear a jacket");

Nesting that is equivalent to ANDing two conditions:

xxxxxxxxxx
if(degrees < 60 && wind > 10)
     alert("Wear a jacket");

Conditionally Executing Multiple Statements

So far, we've only executed one statement based on the condition in our if statement. We can execute multiple statements by enclosing them in curly braces, { }.

When to use curly braces

• By default, an if statement will execute only a single statement following the condition.

  • This is true even if the statement is another if statement (nesting).

• To execute more than one statement, put them in curly braces.

• Best practice: Use curly braces all the time so that you don't forget and so your code will be clearer. Example:

This example checks an user's input and prompts for input again if it the input wasn't valid:

xxxxxxxxxx
degrees = prompt("Enter the temperature");
if(degrees < -100 || degrees > 150)  // check for temperatures unlikely to be valid
{
   alert("Please enter a temperature between -100 and 150")
   degrees = prompt("Enter the temperature");
}

if(degrees < 45)
{
   alert("Wear a coat");
}

Scope of variables inside curly braces

When we declare variables inside curly braces, they have local scope... well, var is an exception.

• Variables declared with var are in a separate scope only if they are declared inside a function.

• Variables declared with let are in a separate scope any time they are declared inside of curly braces—another reason let is better than var.

Example using let:

xxxxxxxxxx
let calculation = 0;
let radius = 0;
calculation = prompt("Do you want to find 1) the circumference, or 2) the area of a circle? (Enter 1 or 2)");
radius = prompt("Enter the radius of the circle.");
if (calculation == 1)
{
  let circumference = 2 * 3.141 * radius;  // a local variable
  console.log(circumference);
}
else if (calculation == 2)
{
  let area = 3.141 * radius * radius;       // another local variable
  console.log(area);
}
else
{
  console.log("Not a valid choice.")
}

Lab Assignment

Take a look at Part 1 of the lab assignment now. You know enough to do the first 7 problems which use if statements. The last two problems use switch statements which we will cover in the next class session.

We'll do a few of these in class and we'll start by writing algorithms and/or making IPO tables.

Examples

An if statement can be used to check the validity of an argument passed to a function and return a status value:

xxxxxxxxxx
function inchesToCentemeters(inches) {
  centemeters = -1; // This value indicates inches was invalid
  if(inches >= 0) {
    centemeters = inches * 2.54;
  }
  return centemeters;
}

Note that it is best practice to not put a return statement inside an if...else statement.

More Examples/Exercises

There are three examples posted on Moodle. The first two use if else statements. Lets do one of them together.

Best Practices

• Only one return from a function
  When you use an if statement inside a funciton, you should only have one return statement in the function, don't return from inside an if statement.

• Use curly braces around the code in each branch of an if, else if, else statement whether they are requried or not.

Reference

W3Schools

JavaScript Comparison and Logical Operators

JavaScript if else and else if

JavaScript Switch Statement

Beginning JavaScript Lecture Notes by Brian Bird, 2018, revised 2025 are licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.