JavaScript Fundamentals

Core JavaScript values and declarations used to store and work with data.

Term / Syntax	Name	Description	Example
let	Block-Scoped Variable	Declares a variable whose value can be reassigned.	let count = 1;
const	Block-Scoped Binding	Declares a binding that cannot be reassigned.	const siteName = "Web Dev Reference";
var	Function-Scoped Variable	Declares a variable scoped to its containing function rather than its block.	var count = 1;
string	Text Value	Represents textual data.	const name = "Jennifer";
number	Numeric Value	Represents numeric values, including integers and decimals.	const score = 98.5;
bigint	Large Integer Value	Represents integer values of arbitrary size without losing integer precision.	const largeNumber = 9007199254740993n;
boolean	True/False Value	Represents one of two logical states.	const isVisible = true;
null	Intentional Empty Value	Represents the intentional absence of a value.	const selectedItem = null;
undefined	Missing Value	Represents a value that has not been assigned or is otherwise missing.	let result;
typeof	Type Operator	Returns a string describing a value's JavaScript type.	typeof 42; // "number"
Array.isArray()	Array Type Check	Checks whether a value is an array.	Array.isArray(items); // true
instanceof	Instance Check	Checks whether an object is an instance of a specified constructor or its inheritance chain.	user instanceof User; // true
Number.isNaN()	NaN Check	Checks whether a value is the numeric value NaN without converting its type.	Number.isNaN(result); // true
parseInt()	Integer Parsing	Parses a string and returns an integer.	parseInt("42", 10); // 42
parseFloat()	Decimal Parsing	Parses a string and returns a number that can include a decimal.	parseFloat("42.5"); // 42.5
Number()	Number Conversion	Converts a value to a number.	Number("42"); // 42
String()	String Conversion	Converts a value to a string.	String(42); // "42"
Boolean()	Boolean Conversion	Converts a value to a boolean based on its truthiness.	Boolean(1); // true
truthy / falsy	Boolean Context	Describes how values are treated as true or false in a boolean context.	Boolean(""); // false

Operators and statements used to calculate and assign values, compare conditions, make decisions, and repeat work.

Term / Syntax	Name	Description	Example
+ - * / % **	Arithmetic Operators	Performs addition, subtraction, multiplication, division, remainder, and exponentiation.	total = price * quantity;
=	Assignment	Assigns a value to a variable.	count = 2;
+= -= *= /= %=	Compound Assignment	Performs an arithmetic operation and assigns the result back to the variable.	total += price;
++ --	Increment / Decrement	Increases or decreases a numeric variable by one.	count++;
===	Strict Equality	Compares value and type without coercion.	status === "ready"
!==	Strict Inequality	Returns true when value or type differs.	count !== 0
< > <= >=	Relational Comparison	Compares whether one value is less than, greater than, or equal to another value.	score >= 70
!	Logical NOT	Converts a value to a boolean and returns the opposite boolean value.	!isLoggedIn
&&	Logical AND	Returns the first falsy operand, or the last operand if all are truthy.	isLoggedIn && isAdmin
||	Logical OR	Returns the first truthy operand, or the last operand if all are falsy.	displayName || "Guest"
??	Nullish Coalescing	Uses a fallback only when the left value is null or undefined.	username ?? "Guest"
?:	Conditional Operator	Returns one of two expressions based on a condition.	const label = isActive ? "Active" : "Inactive";
if / else	Conditional Statement	Runs different blocks based on a condition.	if (score >= 70) { pass(); } else { retry(); }
switch	Multi-Branch Conditional	Matches one expression against multiple cases.	switch (theme) { case "dark": applyDark(); break; }
break	Exit Statement	Exits the nearest loop or switch statement.	if (item === target) { break; }
continue	Skip Iteration	Skips the rest of the current loop iteration and continues with the next one.	if (!item.active) { continue; }
for	Counted Loop	Repeats a block while a condition remains true, with optional setup and update expressions.	for (let i = 0; i < items.length; i++) { console.log(items[i]); }
while	Conditional Loop	Repeats a block while a condition remains true.	while (count < 5) { count++; }
do...while	Post-Test Loop	Runs a block at least once, then repeats it while a condition remains true.	do { count++; } while (count < 5);
for...in	Object Property Loop	Loops through the enumerable property names of an object.	for (const key in user) { console.log(key); }
for...of	Iterable Loop	Loops through the values of an iterable such as an array, Map, or Set.	for (const item of items) { console.log(item); }

Reusable blocks of behavior for organizing logic and transforming values.

Term / Syntax	Name	Description	Example
function	Function Declaration	Creates a named reusable function.	function greet(name) { return `Hi ${name}`; }
function expression	Function Expression	Creates a function as a value that can be assigned to a variable.	const greet = function (name) { return `Hi ${name}`; };
=>	Arrow Function	Provides concise function syntax and inherits this from the surrounding scope.	const double = value => value * 2;
return	Return Value	Ends a function and sends a value back to the caller.	return total;
scope	Variable Accessibility	Determines where variables and functions can be accessed in the code.	function greet() { const message = "Hi"; }
this	Function Context	References a value determined by how a regular function is called.	const user = { name: "Jennifer", greet() { return this.name; } };
default parameter	Fallback Argument	Provides a fallback value when an argument is omitted or undefined.	function greet(name = "Guest") { }
rest parameter	Collect Arguments	Collects remaining arguments into an array.	function sum(...numbers) { }
callback	Function Argument	A function passed to another function so that function can call it.	items.forEach(item => console.log(item));
recursion	Self-Calling Function	A function calling itself until a stopping condition is reached.	function countdown(n) { if (n <= 0) return; countdown(n - 1); }