1
2
3
4
5
6
7
8
9

// Store the student's score
const score = 72;

// Ask five questions about the score
console.log(score > 50);   // is it greater than 50?
console.log(score < 50);   // is it less than 50?
console.log(score >= 72);  // is it 72 or more?
console.log(score === 72); // is it exactly 72?
console.log(score !== 72); // is it anything other than 72?

1
2
3
4
5

// && is true only when BOTH sides are true
console.log(true && true);   // both true
console.log(true && false);  // right side false
console.log(false && true);  // left side false
console.log(false && false); // both false

1
2
3
4
5
6

// The two facts we know about this applicant
const score = 78;
const feePaid = false;

// Admitted only if the score passes AND the fee is paid
console.log(score >= 50 && feePaid);

1
2
3
4

// || is true when AT LEAST ONE side is true
console.log(true || false);  // left side true
console.log(false || true);  // right side true
console.log(false || false); // neither is true

1
2
3
4
5
6

// The two facts we know about this passenger
const isStudent = false;
const age = 64;

// Discount if they are a student OR aged 60 and above
console.log(isStudent || age >= 60);

1
2
3
4
5
6
7
8

// The user has not logged in
const isLoggedIn = false;

// ! flips the answer: read it as "not logged in"
console.log(!isLoggedIn);

// Flipping true gives the opposite
console.log(!true);

1
2
3
4
5
6
7
8
9
10
11
12

// The raw facts
const score = 78;
const attendance = 0.6;

// Give each question a name that says what it means
const passedTest = score >= 50;
const attendedEnough = attendance >= 0.75;

// Print each answer, then combine them
console.log('Passed test:', passedTest);
console.log('Attended enough:', attendedEnough);
console.log('Eligible:', passedTest && attendedEnough);

1
2
3
4
5
6
7
8
9
10
11
12
13
14

// The facts about this student
const average = 74;
const attendance = 0.8;
const wonNationalQuiz = false;

// Name each part of the scholarship rule
const strongResult = average >= 70;
const goodAttendance = attendance >= 0.75;

// Both good results AND attendance, OR a national quiz win
const getsScholarship = (strongResult && goodAttendance) || wonNationalQuiz;

// Show the final decision
console.log('Scholarship:', getsScholarship);

➜ webbo3 node app.js
Can enter: false

1
2
3
4
5
6
7
8
9
10
11

// How much money is in the wallet, and what the item costs
const walletBalance = 5000;
const price = 1500;

// Only runs when there is enough money
if (walletBalance >= price) {
  console.log('Purchase successful');
}

// This line always runs, after the if
console.log('Done');

1
2
3
4
5
6
7
8
9
10
11
12

// The PIN the user typed, and the real PIN
const enteredPin = '1234';
const correctPin = '4321';

// Compare them: exactly one of the two blocks will run
if (enteredPin === correctPin) {
  // Runs when the PINs match
  console.log('Welcome back');
} else {
  // Runs when they do not
  console.log('Wrong PIN, try again');
}

1
2
3
4
5
6
7
8
9
10
11
12
13
14

// The score to grade
const score = 64;

// Checked from top to bottom; the first true branch wins
if (score >= 70) {
  console.log('A');
} else if (score >= 60) {
  console.log('B');
} else if (score >= 50) {
  console.log('C');
} else {
  // Runs only when nothing above matched
  console.log('Fail');
}

1
2
3
4
5
6
7
8
9
10
11

// A very high score
const score = 95;

// The broadest condition is checked FIRST here
if (score >= 50) {
  console.log('C');
} else if (score >= 60) {
  console.log('B');
} else if (score >= 70) {
  console.log('A');
}

1
2
3
4
5
6
7
8
9
10
11
12
13
14

// How much data is left, and whether the bundle renews itself
const dataLeftMB = 300;
const autoRenew = false;

// Out of data completely?
if (dataLeftMB === 0) {
  console.log('You are out of data');
// Low on data AND not set to renew?
} else if (dataLeftMB < 500 && !autoRenew) {
  console.log('Running low: buy a bundle soon');
// Everything else
} else {
  console.log('You have enough data');
}

➜ webbo3 node app.js
Nice weather

1
2
3
4
5
6
7

// Boolean() shows how an if would judge each value
console.log(Boolean(false));     // the boolean false
console.log(Boolean(0));         // the number zero
console.log(Boolean(''));        // an empty string
console.log(Boolean(null));      // deliberately empty
console.log(Boolean(undefined)); // never given a value
console.log(Boolean(NaN));       // a failed number conversion

1
2
3
4
5
6

// Values that are not on the falsy list
console.log(Boolean('hello')); // text with letters
console.log(Boolean(42));      // a number that is not zero
console.log(Boolean('0'));     // the TEXT "0", not the number
console.log(Boolean('false')); // the TEXT "false"
console.log(Boolean([]));      // an empty array

1
2
3
4
5
6
7
8
9
10

// The user left the name box empty
const username = '';

// An empty string counts as false in an if
if (username) {
  console.log(`Hello, ${username}`);
} else {
  // Fallback when no name was typed
  console.log('Hello, guest');
}

1
2
3
4
5
6
7
8
9
10
11
12

// What the user typed into the quantity box
const typedQuantity = '0';

// Convert the text into a number
const quantity = Number(typedQuantity);

// Reject the order when quantity is falsy
if (!quantity) {
  console.log('Please enter a quantity');
} else {
  console.log(`Ordering ${quantity}`);
}

➜ webbo3 node app.js
Ordering 0

1
2
3
4
5
6
7
8
9
10
11
12
13
14

// This profile has no nickname, but it has a first name
const nickname = null;
const firstName = 'Aisha';

// Use the nickname if there is one
if (nickname) {
  console.log(`Welcome, ${nickname}`);
// Otherwise use the first name
} else if (firstName) {
  console.log(`Welcome, ${firstName}`);
// Otherwise use a general greeting
} else {
  console.log('Welcome, student');
}

➜ webbo3 node app.js
true
true
false

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18

// The menu option the user pressed
const option = 2;

// Compare option against each case, one by one
switch (option) {
  case 1:
    console.log('Your balance is N2,500');
    break; // stop here
  case 2:
    console.log('Buy airtime');
    break; // stop here
  case 3:
    console.log('Buy data');
    break; // stop here
  default:
    // Runs when no case matched
    console.log('Invalid option');
}

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16

// The day being priced
const day = 'sunday';

switch (day) {
  // Two cases stacked together share one result
  case 'saturday':
  case 'sunday':
    console.log('Weekend rate');
    break;
  case 'friday':
    console.log('Friday rate');
    break;
  default:
    // Every other day
    console.log('Weekday rate');
}

1
2
3
4
5
6
7
8
9
10
11
12

// The menu option the user pressed
const option = 1;

// Notice: there is no break in any case below
switch (option) {
  case 1:
    console.log('Check balance');
  case 2:
    console.log('Buy airtime');
  case 3:
    console.log('Buy data');
}

1
2
3
4
5
6
7
8
9
10
11

// A form gives you text, not a number
const typed = '1';

// switch compares with ===, so the type must match too
switch (typed) {
  case 1: // this case is the NUMBER 1
    console.log('Check balance');
    break;
  default:
    console.log('Invalid option');
}

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21

// The option arrives from the phone as text
const typedOption = '3';

// Convert it to a number so it can match the cases
const option = Number(typedOption);

// Jump to the matching menu item
switch (option) {
  case 1:
    console.log('Your balance is N2,500');
    break;
  case 2:
    console.log('Enter airtime amount');
    break;
  case 3:
    console.log('Choose a data bundle');
    break;
  default:
    // Any number that is not on the menu
    console.log('Invalid option, try again');
}

➜ webbo3 node app.js
Very good

1
2
3
4
5
6
7
8

// Define the function: this prints nothing yet
function welcome() {
  console.log('Welcome to Webbo3');
}

// Call it twice: each call runs the body once
welcome();
welcome();

1
2
3
4
5
6
7
8
9

// Define a function that takes two numbers
function add(a, b) {
  // Hand the sum back to whoever called it
  return a + b;
}

// Call it with different arguments and print the results
console.log(add(2, 3));
console.log(add(10, 20));

1
2
3
4
5
6
7
8
9
10

// A function that returns twice the number it is given
function double(n) {
  return n * 2;
}

// The returned value is stored in a variable
const result = double(5);

// ...and can be used in more calculations
console.log(result + 1);

1
2
3
4
5
6
7

// Store a function in a variable
const triple = function (n) {
  return n * 3;
};

// Call it through the variable's name
console.log(triple(4));

1
2
3
4
5
6
7

// Arrow functions: no braces means the value is returned automatically
const square = (n) => n * n;
const area = (width, height) => width * height;

// Call them like any other function
console.log(square(6));
console.log(area(3, 5));

1
2
3
4
5
6
7
8

// This function calculates a total...
function addVat(amount) {
  const total = amount * 1.075;
  // ...but never returns it
}

// So the call hands back nothing
console.log(addVat(1000));

1
2
3
4
5
6
7
8
9

// With braces: the arrow needs an explicit return, and has none
const withBraces = (n) => { n * 2; };

// Without braces: the value is returned automatically
const withoutBraces = (n) => n * 2;

// Compare what each one gives back
console.log(withBraces(4));
console.log(withoutBraces(4));

1
2
3
4
5
6
7
8
9
10
11
12

// Each return leaves the function immediately
function gradeFor(score) {
  if (score >= 70) return 'A'; // stop here for 70 and above
  if (score >= 60) return 'B'; // only reached below 70
  if (score >= 50) return 'C'; // only reached below 60
  return 'F';                  // everything else
}

// Grade three different scores
console.log(gradeFor(95));
console.log(gradeFor(64));
console.log(gradeFor(31));

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15

// Job 1: calculate the cost of one line
function lineTotal(price, quantity) {
  return price * quantity;
}

// Job 2: format that line for a receipt
function receiptLine(item, price, quantity) {
  // Reuse the first function to do the maths
  const total = lineTotal(price, quantity);
  return `${item} x${quantity}: N${total}`;
}

// Print two receipt lines
console.log(receiptLine('Data bundle', 1500, 3));
console.log(receiptLine('Airtime', 500, 2));

➜ webbo3 node app.js
Aisha Bello
Tunde Okafor

1
2
3
4
5
6
7
8

// name and course are the PARAMETERS (the placeholders)
function enrol(name, course) {
  return `${name} is enrolled in ${course}`;
}

// The values in the calls are the ARGUMENTS, filled in order
console.log(enrol('Aisha', 'JavaScript'));
console.log(enrol('Musa', 'Data Analysis'));

1
2
3
4
5
6
7

// A function that expects two numbers
function add(a, b) {
  return a + b;
}

// Called with only ONE argument, so b gets nothing
console.log(add(5));

1
2
3
4
5
6
7
8
9

// vat falls back to 7.5 when the caller does not pass it
function totalWithVat(amount, vat = 7.5) {
  // Add the VAT percentage on top of the amount
  return amount + (amount * vat / 100);
}

console.log(totalWithVat(1000));     // no vat passed: the default is used
console.log(totalWithVat(1000, 10)); // vat of 10%
console.log(totalWithVat(1000, 0));  // vat of 0%, meaning no VAT

1
2
3
4
5
6
7
8
9

// The older way of writing a default
function totalWithVat(amount, vat) {
  // Use 7.5 whenever vat is falsy
  vat = vat || 7.5;
  return amount + (amount * vat / 100);
}

// The caller asks for 0% VAT
console.log(totalWithVat(1000, 0));

1
2
3
4
5
6
7
8
9
10

// bundles defaults to 1, and discount defaults to 0
function bundleCost(pricePerBundle, bundles = 1, discount = 0) {
  // Multiply first, then take the discount off
  const total = pricePerBundle * bundles;
  return total - discount;
}

console.log(bundleCost(1500));         // both defaults used
console.log(bundleCost(1500, 3));      // three bundles, no discount
console.log(bundleCost(1500, 3, 500)); // three bundles, N500 off

➜ webbo3 node app.js
Hello, Chidi!
Good morning, Chidi!

1
2
3
4
5
6
7
8
9
10
11

// Declared outside everything: GLOBAL scope
const schoolName = 'Webbo3 Academy';

// A function can read a global variable
function showSchool() {
  console.log(schoolName);
}

// Use it from inside the function, then from outside
showSchool();
console.log(schoolName);

1
2
3
4
5
6
7
8
9
10
11

function makeGreeting() {
  // Declared inside the function: FUNCTION scope
  const greeting = 'Hello from inside';
  console.log(greeting);
}

// Works: the function uses its own variable
makeGreeting();

// Outside the function, greeting does not exist
console.log(greeting);

1
2
3
4
5
6
7
8

if (true) {
  // Declared inside the braces: BLOCK scope
  const inside = 'only here';
  console.log(inside);
}

// Outside the braces, inside does not exist
console.log(inside);

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15

// The score to check
const score = 64;

// Declare result OUTSIDE the if, with no value yet
let result;

// Only assign it inside the branches
if (score >= 50) {
  result = 'Pass';
} else {
  result = 'Fail';
}

// It still exists here, after the if
console.log(result);

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20

// Global: every function can read the pass mark
const passMark = 50;

function checkResult(name, score) {
  // Function scope: exists anywhere inside checkResult
  const message = `${name} scored ${score}`;

  if (score >= passMark) {
    // Block scope: exists only inside this if
    const note = 'well done';
    return `${message}: Pass, ${note}`;
  }

  // note is not available here, but message is
  return `${message}: Fail`;
}

// Check two students
console.log(checkResult('Amaka', 72));
console.log(checkResult('Femi', 41));

➜ webbo3 node app.js
ReferenceError: greeting is not defined

➜ webbo3 node app.js
Good morning

1
2
3
4
5
6
7
8
9

// true for 18 and above, false otherwise
function isAdult(age) {
  // The comparison itself is the answer, so return it directly
  return age >= 18;
}

// Test one adult and one child
console.log(isAdult(21));
console.log(isAdult(15));

1
2
3
4
5
6
7
8
9
10
11
12
13

// Work out the delivery fee for an order total
function deliveryFee(orderTotal) {
  // Big orders deliver free
  if (orderTotal >= 20000) {
    return 0;
  }
  // Everything else pays the standard fee
  return 1500;
}

// Test a big order and a small one
console.log(deliveryFee(25000));
console.log(deliveryFee(8000));

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27

// isStudent is false unless the caller says otherwise
function ticketPrice(day, isStudent = false) {
  // The price is decided below, so start with no value
  let price;

  // Step 1: the base price depends on the day
  switch (day) {
    case 'saturday':
    case 'sunday':
      price = 3000;
      break;
    default:
      price = 2000;
  }

  // Step 2: students get N500 off
  if (isStudent) {
    price = price - 500;
  }

  // Step 3: hand the final price back
  return price;
}

console.log(ticketPrice('sunday'));        // weekend, not a student
console.log(ticketPrice('sunday', true));  // weekend, student
console.log(ticketPrice('tuesday', true)); // weekday, student

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34

// 1. Add the three scores and divide by three
function average(a, b, c) {
  return (a + b + c) / 3;
}

// 2. Turn an average into a grade letter, strictest first
function gradeFor(avg) {
  if (avg >= 70) return 'A';
  if (avg >= 60) return 'B';
  if (avg >= 50) return 'C';
  return 'F';
}

// 3. Apply the school's pass rule
function verdict(avg, attendance) {
  // Name the two conditions
  const passed = avg >= 50;
  const attended = attendance >= 0.75;

  if (passed && attended) return 'Pass';
  if (passed && !attended) return 'Withheld: attendance below 75%';
  return 'Fail';
}

// 4. Put everything together into one line
function report(name, a, b, c, attendance) {
  const avg = average(a, b, c);
  return `${name} | average ${avg} | grade ${gradeFor(avg)} | ${verdict(avg, attendance)}`;
}

// Run the analyser for three students
console.log(report('Chidinma', 72, 66, 90, 0.9));
console.log(report('Musa', 55, 61, 58, 0.5));
console.log(report('Tunde', 30, 44, 40, 0.95));

➜ webbo3 node app.js
Aisha: Admitted
Bayo: Passed, awaiting fee
Ngozi: Not admitted