Variables and Data Types

A variable is a labelled box

Programs need to remember things: a score, a name, a running total. A variable is a named place in the computer's memory that holds one value. Think of a box with a label on it. You put a value in the box (a value is a piece of data, such as 21 or "Asha"). Later you use the label to get the value back, or to replace it with a new one.

In every DSA problem you will keep a few variables: the answer so far, a counter, the position you are looking at. A dry run means tracking what is in each box, line by line.

let and const

let age = 21;          // make a box called age, put 21 in itconsole.log(age);      // 21age = 22;              // replace what is in the box (no "let" the second time)console.log(age);      // 22const pi = 3.14159;    // a box that can never be given a new valueconsole.log(pi);       // 3.14159

• let is a keyword (a special word in JavaScript) that makes a box whose value you can change later. Putting a new value in a box is called assignment. Write let only the first time. After that, just use the name.
• const (short for “constant”) is a keyword that makes a box that cannot be given a new value. Trying to put something new in it is an error:

const pi = 3.14159;pi = 3;   // TypeError: Assignment to constant variable.

Naming rules

• Use letters, digits, _ and $ only. A name cannot start with a digit (2nd is not allowed, second is fine).
• Names are case-sensitive: total and Total are different boxes.
• You cannot use reserved words. These are words that already have a meaning in JavaScript, such as let, if and for.
• The usual JavaScript style is camelCase: the first word is small and each new word starts with a capital letter, like maxValue or studentCount. Short names like i, n and sum are normal in DSA code.

The five basic data types

A data type (or just type) is the kind of a value. It decides what you can do with the value. For example, you can add two numbers, but you cannot add two pieces of text in the same way. The typeof operator tells you the type of a value. These five types are enough for now (JavaScript has a few more, which you do not need yet):

let count = 7;              // number   (whole or decimal, one type for both)let price = 99.5;           // numberlet name = "Asha";          // string   (text, in quotes)let isOpen = true;          // boolean  (only true or false)let score;                  // undefined (made, but nothing put in yet)let winner = null;          // null     ("empty on purpose")console.log(typeof count);  // numberconsole.log(typeof name);   // stringconsole.log(typeof isOpen); // booleanconsole.log(typeof score);  // undefined

Difference in one line: undefined means “nothing was put here yet” (JavaScript sets it), and null means “I chose to put nothing here” (you set it). A small surprise: typeof null gives "object". This is an old mistake in JavaScript that was never fixed.

Arithmetic operators

An operator is a symbol that does something with values, such as + or *. The values it works on are called operands. An expression is any piece of code that produces a value, like 7 + 2. The arithmetic operators do maths:

console.log(7 + 2);    // 9console.log(7 - 2);    // 5console.log(7 * 2);    // 14console.log(7 / 2);    // 3.5   (normal division keeps the decimal)console.log(7 % 2);    // 1     (remainder)console.log(2 ** 5);   // 32    (power: 2 × 2 × 2 × 2 × 2)console.log(2 + 3 * 4);   // 14  (* before +, like school maths)console.log((2 + 3) * 4); // 20  (brackets first)

The order is the same as in school maths. First brackets, then **, then * / %, then + -. Operators at the same level go from left to right. When in doubt, add brackets. They cost nothing and make your meaning clear.

% and Math.floor — the two most useful operators

% is the modulo operator. It gives the remainder of a division (the part that is left over). For example, 17 % 5 is 2, because 5 fits into 17 three times (15) and 2 is left. Math.floor(x) is a function that rounds down to a whole number. So Math.floor(a / b) means “how many whole times b fits in a”. Together they answer three questions you will meet over and over:

let n = 4729;console.log(n % 10);             // 9     the LAST digitconsole.log(Math.floor(n / 10)); // 472   n WITHOUT its last digitconsole.log(n % 100);            // 29    the last two digitsconsole.log(10 % 2);             // 0     remainder 0 → 10 is evenconsole.log(7 % 2);              // 1     remainder 1 → 7 is oddconsole.log(15 % 5);             // 0     remainder 0 → 15 is divisible by 5console.log(Math.floor(17 / 5)); // 3     how many whole 5s fit in 17console.log(17 % 5);             // 2     what is left over

These examples use positive whole numbers. With a negative number, the result of % takes the sign of the left number. For example, -7 % 2 is -1, not 1.

Useful Math functions

Math is a built-in JavaScript object (a ready-made group of tools) that holds maths functions. A function is a named set of steps. You call it by writing its name, then brackets with the values to give it, like Math.max(4, 9, 2). It gives back a result. These functions appear constantly in DSA answers:

console.log(Math.abs(-7));       // 7     distance from zeroconsole.log(Math.min(4, 9, 2));  // 2     smallestconsole.log(Math.max(4, 9, 2));  // 9     largestconsole.log(Math.round(2.5));    // 3     nearest whole numberconsole.log(Math.floor(2.9));    // 2     round downconsole.log(Math.ceil(2.1));     // 3     round upconsole.log(Math.trunc(-2.9));   // -2    drop the decimal partconsole.log(Math.sqrt(49));      // 7     square rootconsole.log(Math.pow(2, 10));    // 1024  same as 2 ** 10

Joining text, and a common + mistake

A string is a value that is text. + between two strings joins them into one string. A newer and clearer way to build text is a template literal. It is a string written between backticks `…` that can hold ${…} placeholders. JavaScript replaces each placeholder with the value inside it.

let first = "Asha";let city = "Pune";console.log("Hi " + first);                    // Hi Ashaconsole.log(first + " lives in " + city);      // Asha lives in Puneconsole.log(`${first} lives in ${city}`);     // Asha lives in Pune  (template literal)console.log(`2 + 3 = ${2 + 3}`);              // 2 + 3 = 5

Be careful. If either side of + is a string, JavaScript joins instead of adding. It works from left to right, one + at a time:

console.log(5 + 3);      // 8      number + number → addconsole.log("5" + 3);    // 53     string + anything → JOINconsole.log(5 + 3 + "5");// 85     left to right: 5 + 3 = 8, then 8 + "5" = "85"console.log("5" + 3 + 5);// 535    "5" + 3 = "53", then "53" + 5 = "535"console.log("5" - 3);    // 2      - only means subtract, so "5" becomes 5console.log(Number("42") + 1); // 43  convert text to a number first

Converting between text and numbers

Input often arrives as text, and output often needs to be text. So you need to convert between the two (this is called type conversion). These functions do it:Number turns text into a number, parseInt reads a whole number from the start of some text, and String turns a value into text.

console.log(Number("42"));       // 42    text → numberconsole.log(Number("4x"));       // NaN   "Not a Number": the text is not a valid numberconsole.log(parseInt("42px"));   // 42    reads digits from the start, stops at the first non-digitconsole.log(String(42));         // 42    number → textconsole.log((42).toString(2));   // 101010  the number written in binaryconsole.log(typeof String(42));  // string

NaN means “Not a Number”. It is a special number value that appears when a conversion or calculation fails. Check for it with Number.isNaN(x). Do not use x === NaN, because NaN is not equal to anything, even to itself.

Shorthand operators: += and ++

Updating a variable using its own value is very common, so JavaScript has short forms. += adds a value to the variable. ++ adds 1 (this is called incrementing), and -- subtracts 1. You will use += and ++ in nearly every loop.

let total = 10;total += 5;    // same as total = total + 5   → 15total -= 3;    // total = total - 3           → 12total *= 2;    // total = total * 2           → 24total++;       // total = total + 1           → 25total--;       // total = total - 1           → 24console.log(total); // 24

Traced: swapping two variables

A common first question is to swap the values in a and b. The obvious attempt a = b; b = a; fails. After the first line both boxes hold 9, and the 5 is gone. You need a spare box. Step through it:

▶let a = 5;2let b = 9;3let temp = a;4a = b;5b = temp;6console.log(a, b);

Practice questions

Area: 15
Perimeter: 16

Area: 100
Perimeter: 40

const length = 5;const width = 3;const area = length * width;const perimeter = 2 * (length + width);console.log("Area:", area);           // Area: 15console.log("Perimeter:", perimeter); // Perimeter: 16

const length = 5;const width = 3;console.log("Area:", length * width);             // Area: 15console.log("Perimeter:", 2 * (length + width));  // Perimeter: 16

const length = 5;const width = 3;console.log(`Area: ${length * width}, Perimeter: ${2 * (length + width)}`);// Area: 15, Perimeter: 16

let a = 5;let b = 9;let temp = a;  // temp = 5a = b;         // a = 9b = temp;      // b = 5console.log("a =", a, "b =", b); // a = 9 b = 5

let a = 5;let b = 9;[a, b] = [b, a];console.log("a =", a, "b =", b); // a = 9 b = 5

let a = 5;let b = 9;a = a + b;  // a = 14b = a - b;  // b = 14 - 9 = 5a = a - b;  // a = 14 - 5 = 9console.log("a =", a, "b =", b); // a = 9 b = 5

const total = 135;const hours = Math.floor(total / 60); // 2const minutes = total % 60;           // 15console.log(hours + " hours " + minutes + " minutes"); // 2 hours 15 minutes

const total = 135;const hours = Math.floor(total / 60);  // 2const minutes = total - hours * 60;    // 135 - 120 = 15console.log(hours + " hours " + minutes + " minutes"); // 2 hours 15 minutes

Last digit: 9
Rest: 472

Last digit: 0
Rest: 5

const n = 4729;const last = n % 10;const rest = Math.floor(n / 10);console.log("Last digit:", last); // Last digit: 9console.log("Rest:", rest);       // Rest: 472

const n = 4729;const last = n % 10;          // 9const rest = (n - last) / 10; // 4720 / 10 = 472console.log("Last digit:", last); // Last digit: 9console.log("Rest:", rest);       // Rest: 472

const n = 4729;const s = String(n);                        // "4729"const last = Number(s[s.length - 1]);       // 9const rest = Number(s.slice(0, s.length - 1)); // 472console.log("Last digit:", last); // Last digit: 9console.log("Rest:", rest);       // Rest: 472

let n = 472;let sum = 0;sum += n % 10;           // + 2n = Math.floor(n / 10);  // n = 47sum += n % 10;           // + 7n = Math.floor(n / 10);  // n = 4sum += n % 10;           // + 4console.log(sum); // 13

const n = 472;const hundreds = Math.floor(n / 100);   // 4const tens = Math.floor(n / 10) % 10;   // 47 % 10 = 7const ones = n % 10;                    // 2console.log(hundreds + tens + ones); // 13

const s = String(472);   // "472"const sum = Number(s[0]) + Number(s[1]) + Number(s[2]);console.log(sum); // 13

console.log("5" + 3);console.log("5" - 3);console.log(5 + 3 + "5");console.log("5" + 3 + 5);

console.log("5" + 3);     // 53console.log("5" - 3);     // 2console.log(5 + 3 + "5"); // 85console.log("5" + 3 + 5); // 535

console.log(Number("5") + 3);     // 8console.log(5 + 3 + Number("5")); // 13

const a = 4, b = 8, c = 9;const average = (a + b + c) / 3;console.log(average); // 7

const a = 1, b = 2, c = 2;const total = a + b + c;      // 5const average = total / 3;console.log(average);                 // 1.6666666666666667console.log(Math.round(average));     // 2     nearest whole numberconsole.log(average.toFixed(2));      // 1.67  two decimal places, as text

const total = 3725;const hours = Math.floor(total / 3600);       // 1const afterHours = total % 3600;               // 125 seconds leftconst minutes = Math.floor(afterHours / 60);  // 2const seconds = afterHours % 60;              // 5console.log(`${hours}h ${minutes}m ${seconds}s`); // 1h 2m 5s

const total = 3725;const totalMinutes = Math.floor(total / 60); // 62const seconds = total % 60;                  // 5const hours = Math.floor(totalMinutes / 60); // 1const minutes = totalMinutes % 60;           // 2console.log(`${hours}h ${minutes}m ${seconds}s`); // 1h 2m 5s

Make it stick

Next lesson: making decisions with if / else (running some code only when a condition is true). You will solve even or odd, largest of three, and other common interview questions.