Values, variables and types

Variables

A program works with values: a piece of text, a number, a yes or no. A variable is a name you give to a value so you can use it later. Think of it as a label stuck on the value.

You create a variable with const or let, a name, an equals sign, and the value:

const title = "Buy milk";
let doneCount = 0;

console.log(title);
console.log(doneCount);

doneCount = doneCount + 1;   // give doneCount a new value
doneCount = doneCount + 1;
console.log(doneCount);

Buy milk
0
2

Anything after // is a comment. JavaScript ignores it; it is a note for people.

The = sign does not mean "equals" like in maths. It means "store the value on the right under the name on the left". doneCount = doneCount + 1 reads the old value, adds 1, and stores the result. Changing the value of an existing variable is called reassigning it.

const or let

A const variable can never be reassigned. A let variable can. If you try to reassign a const, JavaScript stops with an error:

const maxTasks = 100;

try {
  maxTasks = 200;
} catch (error) {
  console.log(error.message);
}

console.log(maxTasks);

Assignment to constant variable.
100

try and catch let the program print the error and keep going. You will learn them properly in the lesson on errors; for now, read them as "try this, and if it fails, show why".

Why not var

Old code uses a third keyword, var. It works, but it has two surprises: you can declare the same name twice by accident, and a var made inside a block (the code between { and }) leaks out of it.

var count = 1;
var count = 2;          // no error, the first count is silently replaced
console.log(count);

{
  var leaked = "I escaped the block";
  let kept = "I stay inside";
}
console.log(leaked);
console.log(typeof kept);

2
I escaped the block
undefined

let and const fix both problems: declaring the same name twice is an error, and they stay inside their block. Never write var in new code. The lesson on scope explains the details.

Naming variables

• A name can use letters, digits, _ and $, but cannot start with a digit. task1 is fine; 1task is not.
• Names are case-sensitive: title and Title are two different variables.
• You cannot use words that JavaScript reserves, such as let, if or return.
• JavaScript uses camelCase: the first word in lowercase, each next word starts with a capital: doneCount, maxTitleLength.
• For a true constant, a fixed setting that never changes while the program runs, many teams use UPPER_SNAKE_CASE: const MAX_TASKS = 100;.

Pick names that say what the value means. doneCount tells the reader far more than x or n2.

The primitive types

Every value has a type, the kind of value it is. JavaScript has seven basic types, called primitive types. The operator typeof tells you the type of any value:

console.log(typeof "Buy milk");       // text
console.log(typeof 42);               // a number, whole or decimal
console.log(typeof 3.5);
console.log(typeof 9007199254740993n); // a very big whole number
console.log(typeof true);             // yes or no
console.log(typeof undefined);        // "no value yet"
console.log(typeof null);             // "no value, on purpose"
console.log(typeof Symbol("id"));     // a unique label

string
number
number
bigint
boolean
undefined
object
symbol

• string: text, in quotes.
• number: any number. JavaScript does not have a separate type for whole numbers and decimals.
• bigint: whole numbers of any size, written with an n at the end. You need it only for numbers too large for number; you will see why below.
• boolean: exactly two values, true and false.
• undefined and null: two ways to say "nothing here". The last section of this lesson explains the difference.
• symbol: a value that is guaranteed to be unique. Symbol("id") === Symbol("id") is false. Libraries use symbols as hidden keys; you will rarely create one yourself.

Everything that is not a primitive is an object: lists, functions, dates, and the objects you will build in the lesson on objects.

Converting between types

When an operation gets a type it did not expect, JavaScript quietly converts the value. This is called type coercion. It is the source of many beginner bugs:

console.log("5" + 1);     // + with a string joins text
console.log("5" - 1);     // - only works on numbers, so "5" becomes 5
console.log("5" * "2");
console.log(true + 1);    // true becomes 1
console.log("abc" - 1);   // "abc" is not a number

51
4
10
2
NaN

The first line is the classic trap. Text from a form, a URL or a file is always a string, so "5" + 1 gives "51", not 6. NaN means "not a number"; you will meet it again below.

Do not rely on coercion. Convert values yourself, on purpose, with Number(), String() and Boolean(). This is explicit conversion:

const fromForm = "5";

console.log(Number(fromForm) + 1);
console.log(String(42) + "!");
console.log(Number(""));        // careful: empty text becomes 0
console.log(Number("12px"));    // not a clean number
console.log(Boolean("hello"));
console.log(Boolean(""));

6
42!
0
NaN
true
false

Number() is strict: text that is not entirely a number gives NaN. The one surprise is Number(""), which is 0. When you validate input, check for empty text first.

Strings

A string is text. You can write it in double quotes, single quotes or backticks. Inside quotes, a backslash starts an escape sequence: \n is a new line and \" is a quote character.

const a = "Buy milk";
const b = 'Write report';
const c = "She said \"hi\"";
const d = "line one\nline two";

console.log(a, b);
console.log(c);
console.log(d);
console.log("Task: " + a + "!");   // + joins strings: concatenation

Buy milk Write report
She said "hi"
line one
line two
Task: Buy milk!

Joining strings with + is called concatenation. It gets hard to read once you mix in several values. A template literal, a string in backticks (`), is easier: ${…} inserts any value, and it can span several lines.

const title = "Buy milk";
const priority = 2;
const done = false;

console.log(`Task "${title}" has priority ${priority}`);
console.log(`Next priority: ${priority + 1}, done: ${done}`);
console.log(`Line 1
Line 2`);

Task "Buy milk" has priority 2
Next priority: 3, done: false
Line 1
Line 2

String methods

Every string has a .length and many methods: built-in functions you call with a dot. Each character has a position, its index, counted from 0.

const title = "Write the report";

console.log(title.length);
console.log(title[0], title.at(-1));    // first and last character
console.log(title.toUpperCase());
console.log(title.toLowerCase());
console.log(title.slice(0, 5));         // from index 0 up to (not including) 5
console.log(title.slice(-6));           // the last 6 characters
console.log("7".padStart(3, "0"));
console.log("-".repeat(10));

16
W t
WRITE THE REPORT
write the report
Write
report
007
----------

Strings never change. toUpperCase() does not change title; it gives you back a new string. To keep the result, store it in a variable.

Search, replace, split and trim

These are the methods you will use on almost every piece of text a user sends to your backend:

const input = "  buy milk, buy bread  ";
const clean = input.trim();              // remove spaces at both ends

console.log(`[${clean}]`);
console.log(clean.includes("milk"));
console.log(clean.startsWith("buy"), clean.endsWith("tea"));
console.log(clean.indexOf("bread"));     // where it starts, or -1
console.log(clean.indexOf("tea"));
console.log(clean.replace("buy", "get"));      // first match only
console.log(clean.replaceAll("buy", "get"));   // every match
console.log(clean.split(", "));                // cut into a list

[buy milk, buy bread]
true
true false
14
-1
get milk, buy bread
get milk, get bread
[ 'buy milk', 'buy bread' ]

split gives back an array, a list of values in square brackets. Arrays get a lesson of their own.

Numbers

Numbers support the usual arithmetic: +, -, * and /. The next lesson covers all the operators. Here is what makes JavaScript numbers special:

console.log(7 / 2);
console.log(0.1 + 0.2);
console.log(0.1 + 0.2 === 0.3);
console.log((0.1 + 0.2).toFixed(2));

3.5
0.30000000000000004
false
0.30

Why is 0.1 + 0.2 not 0.3? Computers store numbers in binary (only 0s and 1s). In binary, 0.1 has no exact form, just like 1/3 has no exact form in decimal (0.3333…). The tiny error shows up after adding. This is called floating point error, and almost every language has it. toFixed(2) rounds for display and gives back a string.

Math and rounding

Math is a built-in object full of number functions:

console.log(Math.round(2.5), Math.round(2.4));  // nearest whole number
console.log(Math.floor(2.9), Math.ceil(2.1));   // always down, always up
console.log(Math.trunc(-2.9));                  // cut off the decimals
console.log(Math.max(3, 9, 4), Math.min(3, 9, 4));
console.log(Math.abs(-7));                      // distance from zero
console.log(Math.sqrt(16), 2 ** 10);            // square root, power
console.log(Math.round(4.567 * 100) / 100);     // round to 2 decimals

3 2
2 3
-2
9 3
7
4 1024
4.57

Math.random() gives a different decimal between 0 and 1 every time, for example 0.7361…. Math.floor(Math.random() * 6) + 1 rolls a die. You will use it in the guessing game in the lesson on loops.

Parsing, NaN and Infinity

parseInt and parseFloat read a number from the start of a string and stop at the first character that does not fit. They are more forgiving than Number():

console.log(parseInt("42px", 10));
console.log(parseFloat("3.75 kg"));
console.log(parseInt("abc", 10));

const result = Number("twelve");
console.log(result);
console.log(result === NaN);        // never true!
console.log(Number.isNaN(result));  // the right check

console.log(1 / 0, -1 / 0);
console.log(Number.MAX_SAFE_INTEGER);
console.log(9007199254740992 + 1);  // too big to be exact
console.log(9007199254740992n + 1n);

42
3.75
NaN
NaN
false
true
Infinity -Infinity
9007199254740991
9007199254740992
9007199254740993n

• Always pass 10 as the second argument of parseInt. It says "read this as a normal base-10 number".
• NaN is the only value that is not equal to itself, so === NaN is always false. Use Number.isNaN(value).
• Dividing by zero does not crash; it gives Infinity.
• Above Number.MAX_SAFE_INTEGER, numbers lose precision: adding 1 did nothing. A bigint stays exact. You cannot mix the two in one sum; convert with BigInt(n) or Number(big) first.

Booleans, truthy and falsy

A boolean is true or false. Comparisons produce booleans:

const priority = 3;
const isUrgent = priority > 2;

console.log(isUrgent);
console.log(typeof isUrgent);
console.log(priority === 5);

true
boolean
false

When JavaScript needs a yes or no, for example in an if, it converts any value to a boolean. Only eight values become false. They are called falsy. Every other value is truthy:

const falsy = [false, 0, -0, 0n, "", null, undefined, NaN];
for (const value of falsy) {
  console.log(Boolean(value));
}

console.log(Boolean("0"), Boolean(" "), Boolean("false"), Boolean(-1));

false
false
false
false
false
false
false
false
true true true true

The loop runs the console.log once for each value in the list; loops come later in this part. The last line is the surprise: the strings "0", " " and "false" are all truthy, because they are not empty.

null and undefined

Both mean "no value", but they appear in different places:

• undefined is what JavaScript gives you when something was never set: a variable declared without a value, a function parameter nobody passed, a missing property, or a function that returns nothing.
• null is what a programmer writes to say "there is deliberately nothing here", for example "this task has no due date".

let dueDate;
console.log(dueDate);            // declared, never set

const task = { title: "Buy milk", assignee: null };
console.log(task.assignee);      // set to "nobody", on purpose
console.log(task.priority);      // this property does not exist

console.log(dueDate === undefined);
console.log(task.assignee === null);
console.log(task.assignee == null, dueDate == null);   // == null catches both

undefined
null
undefined
true
true
true true

{ title: "Buy milk", assignee: null } is an object: several named values kept together. Objects come a few lessons later; here you only need to read task.assignee as "the assignee of the task".

To check for "no value", use === undefined or === null. The one accepted use of the loose == is value == null, which is true for both null and undefined and nothing else. The next lesson shows the ?? operator, the modern way to give a missing value a default.

Put it together: a task summary

Here is everything from this lesson in one small program. It takes raw values, the way they arrive from a form, cleans and converts them, and prints a summary:

const rawTitle = "   write the REPORT  ";
const rawEstimate = "90";     // minutes, as text from a form
const rawPriority = "high";   // not a number at all
const assignee = null;

const title = rawTitle.trim().toLowerCase();
const estimate = Number(rawEstimate);
const priority = Number(rawPriority);

const hours = Math.floor(estimate / 60);
const minutes = estimate % 60;     // % gives the remainder of a division

console.log(`Title: ${title[0].toUpperCase()}${title.slice(1)}`);
console.log(`Estimate: ${hours}h ${minutes}min`);
console.log(`Priority valid: ${!Number.isNaN(priority)}`);
console.log(`Assigned: ${assignee !== null}`);

Title: Write the report
Estimate: 1h 30min
Priority valid: false
Assigned: false

! turns true into false and back. You will see it, and %, in the next lesson.

Practice

const a = "10";
const b = "20";
console.log(`Total: ${a + b}`);

const a = "10";
const b = "20";
console.log(`Total: ${Number(a) + Number(b)}`);

Total: 30

const title = "  Buy Milk and Bread ";
const slug = title.trim().toLowerCase().replaceAll(" ", "-");

console.log(slug);
console.log(slug.length);

buy-milk-and-bread
18

console.log(typeof "42");
console.log(typeof Number("42"));
console.log(typeof NaN);
console.log(typeof null);
console.log(Boolean("0"));
console.log(Number(" 7 "));

console.log(typeof "42");
console.log(typeof Number("42"));
console.log(typeof NaN);
console.log(typeof null);
console.log(Boolean("0"));
console.log(Number(" 7 "));

string
number
number
object
true
7

Recap

• A variable is a name for a value. Use const by default, let when you must reassign, and never var.
• The seven primitive types are string, number, bigint, boolean, undefined, null and symbol. typeof tells you which, except that typeof null is "object".
• JavaScript converts types on its own ("5" + 1 is "51"). Convert on purpose with Number(), String() and Boolean().
• Template literals build text; trim, includes, replaceAll and split clean and search it.
• 0.1 + 0.2 is not exactly 0.3. Check for NaN with Number.isNaN.
• Only eight values are falsy. undefined means "never set", null means "empty on purpose".