Conditions: if / else

Making decisions in code

So far every line runs, from top to bottom. Real programs also need to decide. For example: if it is raining, take an umbrella. Otherwise, do not. A condition is a question whose answer is true or false. Those two values are called booleans. The if statement runs a block (a group of lines inside curly braces) only when the condition is true.

Almost every DSA solution has decisions inside it. Examples are “is this number bigger than my best so far?”, “have I found the target?” and “is this character a vowel?”. This lesson teaches how to write them.

Comparisons give true or false

A comparison operator compares two values and gives true or false. The operators are > (greater than), < (less than), >=, <=, === (equal) and !== (not equal).

console.log(5 > 3);      // trueconsole.log(5 < 3);      // falseconsole.log(5 >= 5);     // true    (greater than OR equal)console.log(4 <= 3);     // falseconsole.log(7 === 7);    // true    (equal: same value AND same type)console.log(7 === "7");  // false   (number vs string)console.log(7 !== 8);    // true    (not equal)

if and else

const temperature = 31;if (temperature > 30) {  console.log("It's hot — drink water");} else {  console.log("Nice weather");}console.log("Have a good day");/* Output:It's hot — drink waterHave a good day*/

• The condition goes in round brackets ( ). The code to run goes in curly braces { }.
• else is optional. Its block runs when the condition is false.
• When there is an else, exactly one of the two blocks runs. Never both, and never neither.
• Indent the code inside the braces (move it to the right). The computer does not need this, but people who read your code, including interviewers, do.

else if — choosing one of many

When there are more than two possibilities, chain them with else if. The computer checks the conditions from the top. It runs only the first block whose condition is true, then it skips the rest of the chain.

const marks = 82;if (marks >= 90) {  console.log("A");} else if (marks >= 75) {  console.log("B");          // ← this one runs} else if (marks >= 50) {  console.log("C");          // skipped, even though 82 >= 50 is also true} else {  console.log("F");}/* Output:B*/

&&, || and ! — combining conditions

Logical operators join or change conditions. && means AND, || means OR, and ! means NOT.

const age = 20;const hasTicket = true;console.log(age >= 18 && hasTicket);  // true   both must be trueconsole.log(age < 13 || age > 60);    // false  at least one must be trueconsole.log(!hasTicket);              // false  ! flips true ↔ falseconst n = 15;if (n % 3 === 0 && n % 5 === 0) {  console.log("divisible by both");   // divisible by both}

The one-line if: ? :

The ternary operator is an operator with three parts. It chooses one of two values: condition ? valueIfTrue : valueIfFalse. If the condition is true, you get the first value. If not, you get the second. Use it for simple choices. For anything with more than one step, a normal if is clearer.

const n = 7;const kind = n % 2 === 0 ? "even" : "odd";console.log(kind); // odd

Truthy and falsy values

A condition does not have to be a comparison. JavaScript can treat any value as true or false inside a condition. A value that counts as false is called falsy. A value that counts as true is called truthy. These six common values are falsy. Every other value is truthy. (There are two rare extra falsy values, -0 and 0n, which you will not need now.)

// These six values count as false inside a condition ("falsy")://   false, 0, "" (empty text), null, undefined, NaN// Every other value counts as true ("truthy") — including "0", "false" and [].const name = "";if (name) {  console.log("Hello, " + name);} else {  console.log("No name given"); // No name given}const count = 0;console.log(count ? "has items" : "empty"); // empty

switch — matching one value against many

A switch statement compares one value against a list of exact options (a day number, a command, a menu choice). Each option is a case. It is easier to read than a long else if chain.

const day = 3;let name;switch (day) {  case 1:    name = "Monday";    break;  case 2:    name = "Tuesday";    break;  case 3:    name = "Wednesday";   // ← day is 3, so this case runs    break;                // ← break leaves the switch  default:    name = "Unknown";     // runs when no case matches}console.log(name); // Wednesday

• JavaScript compares the value with each case using ===.
• break ends the switch. Without it, the code continues into the next case. This is called falling through.
• default runs when no case matches. It works like a final else.

Falling through is useful when several values share the same result:

const month = 4;let days;switch (month) {  case 4:  case 6:  case 9:  case 11:    days = 30;            // several cases can share one block    break;  case 2:    days = 28;    break;  default:    days = 31;}console.log(days); // 30

Traced: a grading program

Step through this program. It turns marks into a grade.

▶const marks = 82;2if (marks >= 90) {3  console.log("A");4} else if (marks >= 75) {5  console.log("B");6} else {7  console.log("C or below");8}9console.log("done");

Now try it in your head with marks = 95 and marks = 40. Which lines run each time?

Four common mistakes

1. One equals sign in a condition. = assigns a value, === compares two values. This code has a serious bug:
   JavaScriptCopy

   let n = 5;if (n = 10) {            // ✗ ONE equals sign: this SETS n to 10  console.log("ten?");   // runs, because 10 counts as true}console.log(n);          // 10 — the bug changed n!/* Output:ten?10*/

2. Wrong order in an else-if chain. A broad condition placed first matches values that were meant for later branches (see above).
3. Chaining comparisons like maths. 1 < x < 5 does not mean “x is between 1 and 5”. JavaScript first works out 1 < x, which gives true or false, and then compares that result with 5. Write x > 1 && x < 5 instead.
4. Separate ifs instead of else if. With three separate if statements, all three are checked, and more than one can run. An else if chain runs at most one block.

Practice questions

const n = 7;if (n % 2 === 0) {  console.log("even");} else {  console.log("odd"); // odd}

const n = 10;console.log(n % 2 === 0 ? "even" : "odd"); // even

const n = 1234;const last = n % 10;   // 4if (last === 0 || last === 2 || last === 4 || last === 6 || last === 8) {  console.log("even"); // even} else {  console.log("odd");}

const n = -3;if (n > 0) {  console.log("positive");} else if (n < 0) {  console.log("negative"); // negative} else {  console.log("zero");}

const n = 12;const sign = Math.sign(n);   // 1if (sign === 1) console.log("positive"); // positiveelse if (sign === -1) console.log("negative");else console.log("zero");

const a = 4, b = 9, c = 2;if (a >= b && a >= c) {  console.log(a);} else if (b >= a && b >= c) {  console.log(b); // 9} else {  console.log(c);}

const a = 4, b = 9, c = 2;let best = a;if (b > best) best = b;if (c > best) best = c;console.log(best); // 9

console.log(Math.max(4, 9, 2)); // 9console.log(Math.max(7, 7, 3)); // 7

const marks = 75;if (marks >= 90) {  console.log("A");} else if (marks >= 75) {  console.log("B"); // B} else if (marks >= 50) {  console.log("C");} else {  console.log("F");}

const marks = 91;if (marks >= 90) console.log("A"); // Aif (marks >= 75 && marks < 90) console.log("B");if (marks >= 50 && marks < 75) console.log("C");if (marks < 50) console.log("F");

const year = 1900;const isLeap = year % 400 === 0 || (year % 4 === 0 && year % 100 !== 0);console.log(isLeap); // false

const year = 2024;let isLeap;if (year % 400 === 0) isLeap = true;else if (year % 100 === 0) isLeap = false;else if (year % 4 === 0) isLeap = true;else isLeap = false;console.log(isLeap); // true

const a = 1, b = 2, c = 3;const valid = a + b > c && a + c > b && b + c > a;console.log(valid); // false

const a = 3, b = 4, c = 5;const longest = Math.max(a, b, c);const total = a + b + c;const valid = total - longest > longest;   // the other two sides vs the longestconsole.log(valid); // true

const n = 15;if (n % 3 === 0 && n % 5 === 0) {  console.log("FizzBuzz"); // FizzBuzz} else if (n % 3 === 0) {  console.log("Fizz");} else if (n % 5 === 0) {  console.log("Buzz");} else {  console.log(n);}

const n = 10;let word = "";if (n % 3 === 0) word += "Fizz";if (n % 5 === 0) word += "Buzz";console.log(word === "" ? n : word); // Buzz

const units = 250;let bill;if (units <= 100) {  bill = units * 5;} else if (units <= 200) {  bill = 100 * 5 + (units - 100) * 7;} else {  bill = 100 * 5 + 100 * 7 + (units - 200) * 10;}console.log(bill); // 1700

const units = 250;const slab1 = Math.min(units, 100);                       // 100const slab2 = Math.min(Math.max(units - 100, 0), 100);    // 100const slab3 = Math.max(units - 200, 0);                   // 50console.log(slab1 * 5 + slab2 * 7 + slab3 * 10); // 1700

const day = 7;let name;switch (day) {  case 1: name = "Monday"; break;  case 2: name = "Tuesday"; break;  case 3: name = "Wednesday"; break;  case 4: name = "Thursday"; break;  case 5: name = "Friday"; break;  case 6: name = "Saturday"; break;  case 7: name = "Sunday"; break;  default: name = "Invalid day";}console.log(name); // Sunday

const day = 1;let name;if (day === 1) name = "Monday";else if (day === 2) name = "Tuesday";else if (day === 3) name = "Wednesday";else if (day === 4) name = "Thursday";else if (day === 5) name = "Friday";else if (day === 6) name = "Saturday";else if (day === 7) name = "Sunday";else name = "Invalid day";console.log(name); // Monday

const day = 9;const names = ["Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"];const name = day >= 1 && day <= 7 ? names[day - 1] : "Invalid day";console.log(name); // Invalid day

Make it stick

Next lesson is the most important one in Part 1: the for loop (code that repeats a block many times). We trace it step by step until the pattern is clear.