Intro to JavaScript Control Flow

Learning Objectives

• Know what is "truthy" and "falsey" in JavaScript

• Use the if...else statement to perform branching

• Use the for statement to perform looping

• Use the while statement to perform looping

Roadmap

• What is Control Flow?

• Conditional Expressions

• Branching Statements

• Looping Statements

• Closing Questions

• Practice Exercises

• Bonus Material

Lesson Setup

For this lesson, we're going to code along using an HTML, CSS & JSrepl.it

What is Control Flow?

"The execution sequence of instructions in a program determined at run time with the use of control structures".

Basic Types of Control Flow

• Sequence:

  • Statements execute one at a time in sequence.

• Branching:

  • Different code paths are executed based upon a conditional expression.

• Looping:

  • Code is repeatedly executed while a condition is truthy.

Conditional Expressions

• In JavaScript, what is considered to be True/Truthy & False/Falsey?

• Comparison Expressions

What is True/Truthy & What is False/Falsey?

• To test what is truthy and what is falsey, let's type the following code into script.js inside our repl.it:

  if (true) {
    console.log('truthy!');
  } else {
    console.log('falsey!');
  }

• We can "run" this code using repl.it's built in JavaScript Engine by pressing the [ run ] button

• Now we can easily test expressions by typing it in the place of true

• Why this truthy and falsey business? Why not just use true and false?

• Answer: Truthy and falsey are conceptual and an attempt to treat non-boolean expressions as booleans (true or false) during runtime. The concept of truthiness/falseyness will often allow us to write code that is more concise

• For example, the number 3, is considered to be truthy - test it out

What is True/Truthy & What is False/Falsey? (cont)

• Most things in JS are truthy, so it's easier to remember what's falsey...

• There are two data types that are always falsey: null and undefined

• There are four values that are falsey: false, 0 (zero), NaN (special value of number), and an empty string (FYI, a string with a value of a space is not empty)

• Everything else is truthy!

• Take a couple of minutes to test a few of the above

The Not Operator

• The not operator (!), also known as the "bang" operator, "flips" a true or truthy expression to the boolean value of false, and vice-versa.

• For example, test the following expressions:

  !false === true // true
  !null === true // true
  !3 === false // true
  !'' === true // true

• A double ! operator is a great way to force an expression into its actual boolean value of true or false:

  console.log(!!3); // outputs true

Boolean Logic Comparison Operators

• Let's review these Comparison Operators that you saw in the pre-work:

  • === strict equality - best practice
  • == performs type conversion (called coercion) if necessary
  • !== strict inequality
  • != inequality
  • < less than
  • > greater than
  • <= less than or equal
  • >= greater than or equal

• The logical operators || and && are more powerful than meets the eye

• The logical || (OR) operator always returns the first operand if it is truthy, otherwise the second operand is returned:

  'hello' || 'goodbye'  // evaluates to 'hello'
  0 || null  // evaluates to null

• The logical && (AND) operator always returns the first operand if it is falsey, otherwise the second operand is returned:

  'hello' && 'goodbye'  // evaluates to 'goodbye'
  0 && null  // evaluates to 0

Conditional Expressions

• The if, for and while statements all require a conditional expression. For example:

  let x = 1;
  while (x <= 10) {
    var msg = 'Item ' + x;
    console.log(msg);
    x++;
  }

  Where, x <= 10 is the conditional expression.

❓ If x <= 10 was replaced with just x, would it still be considered a valid conditional expression?

Review Questions

❓ Is the value of 0 (zero) truthy or falsey?

❓ Is an empty string truthy or falsey

❓ Is an "empty" object (an object with no properties) truthy or falsey?

❓ What value does !!0 evaluate to?

The if..else Branching Statement

• As you saw in the pre-work, the if and the optional else clause allows us to conditionally execute code

The if Branching Statement Single Path

Single path if:                

if (val === 1) {
	console.log('This code will run only if val equals 1');
}

• Conditional expression must be surrounded by parens

• If you have only a single statement that needs to execute, you can write that statement without using curly braces (used to define a block statement):

  if (val === 1) console.log('This code will run only if val equals 1');

  This code is the same as the example above.

The if..else (dual path)

Dual paths if with else:

if (val === 1) {
	console.log('val is one');
} else {
	console.log('val is not one');
}

The if..else..if (three or more paths)

If you have three or more code paths use if with as many else if statements as necessary and optionally a final else:

if (val === 1) {
	console.log('val is one');
} else if (val === 2) {
	console.log('val is two');
} else if (val === 3) {
	console.log('val is three');
} else {
	console.log('not one, two, or three');
}

• As always, the final else is optional

• Any questions regarding branching with if...else?

💪 Exercise - Branching Statements (5 mins)

• Write the if...else..if statement that console.logs the following based upon the value of a variable named color:

  • If the value is green, log Go
  • If the value is yellow, log Slow
  • If the value is red, log Stop
  • If the value is anything else, log Whatever

• Hint: Don't forget to declare and initialize a variable named color

• As always, be sure to ask your instructional team for help if you get stuck!

Looping Statements

• Looping statements provide us with the ability to execute a block of code multiple times while a conditional expression is truthy

• We'll take a look at these statements:

  • while

  • do while

  • for

Looping Statements while

The first looping statement we'll look at is while:

let word = '';
let words = [];
while (word !== 'end') {
	word = prompt('Enter a word ("end" to quit)');
 	if (word !== 'end') words.push(word);
	alert("You've entered: " + words.join(', '));
}

• Use while when you want to continue to execute a block of code while a condition is true

• Beware of infinite loops!

Looping Statements do...while

You may choose to use the do...while statement instead of while to force the code block to always execute at least once

let num = 0;
do {
	console.log(num);
	num += 2;
} while (num <= 10);

• Do you see why the code block will always run at least once?

• Again, beware of infinite loops!

Looping Statements for

The next looping statement we'll look at is the for statement:

let colors = ['red', 'white', 'blue'];
for (let idx = 0; idx < colors.length; idx++) {
	console.log(colors[idx]);
}

• Notice the for loop has three parts after the for keyword:
  1. The initializer which runs only once before looping begins. It is used to declare and initialize a looping variable.
  2. The condition which will be evaluated before each loop. If truthy, the code block will execute.
  3. The last part will execute after each loop and is typically used to increment or decrement the looping variable by one or more units.

Looping Statements break

Use the break statement within any while or for loop to immediately exit the loop:

let word = '';
let words = [];
while (true) {
word = prompt('Enter a word ("end" to quit)');
	if (word === 'end') break;
	words.push(word);
	alert("You've entered: " + words.join(', '));
}

Note how the if statement does not require braces in this case.

Question - Looping Statements

❓ When using a while or do...while loop, we must be careful not put the program's execution into an __________ loop.

❓ How can we avoid the above scenario?

Closing Questions

❓ In your own words, how would you describe Control Flow?

❓ The three primary types of control flow are: 1) Sequence 2) ___________ 3) ___________

❓ What does expression 'happy' || 'sad' return?

💪 Practice Exercises (15 mins)

Practice Exercises

Exercise 1 - Branching

• The following JavaScript code will accept string input from the user and store the string in a variable named choice:

  let choice = prompt('Enter a, b or c');

• Write an if statement that console.logs the following messages:

  • a entered - "a is for apple"

  • b entered - "b is for banana"

  • c entered - "c is for cantaloupe"

  • anything else - "you're a rebel"

Practice Exercises

Exercise 2 - Looping

• Use one of the looping statements to continue to execute the code you wrote in the previous exercise until the phrase no more fruit is entered by the user

Bonus Material

Ternary Operator

The ternary operator is ideal when you need to return one of two values depending upon a condition:

let message = score > 100 ? "You rock!" : "Keep trying!";

The above one line of code replaces this code:

let message;
if (score > 100) {
	message = "You rock!";
} else {
	message = "Keep trying!";
}

It can also be used to evaluate one of two expressions, so you can actually run a method if you'd like:

score > 100 ? gameWinner() : gameLoop();

Switch Statement

• Look into using the switch statement instead of if...else if you have more than three code paths and your conditionals always check the same variable

• Here's the documentation for the switch statement