Expressions and statements

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In JavaScript, the distinction between expressions and statements is fundamental to understanding how code executes.

Expressions:

An expression is any piece of code that evaluates to a single value. Expressions can be simple or complex, and they always produce a result. 

Examples of Expressions:

Literals: 5, "hello", true

Variables: x, myVariable

Arithmetic operations: 

2 + 3, x * y

String concatenation: 

"Hello" + " World"

Function calls: 

myFunction(), console.log("message")

Logical operations: 

a && b, !c

Assignment expressions: x = 10 (the expression itself evaluates to the assigned value, 10)

Key characteristics of expressions:

Always produce a value: This is the defining feature of an expression.

Can be simple or complex: They can be a single literal or a complex combination of operators, variables, and function calls.

Can be nested: Expressions can contain other expressions, allowing for complex computations.

Statements:

A statement is a complete instruction that performs an action or controls the flow of execution in a program. Statements do not necessarily evaluate to a value, although they often contain expressions.

Examples of Statements:

Variable declarations: 

let x;, const PI = 3.14;

Conditional statements: 

if (condition) { ... } else { ... }

Loop statements: 

for (let i = 0; i < 5; i++) { ... }, while (condition) { ... }

Function declarations: 

function greet() { ... }

Block statements: 

{ console.log("inside block"); }

Expression statements: 

An expression followed by a semicolon, like x = 5; or myFunction();

Key Differences:

Value Production: Expressions always produce a value, while statements perform an action.

Usage: Expressions can be used wherever a value is expected (e.g., as arguments to functions, in assignments, within other expressions). Statements form the core instructions of a program.

Context in Frameworks: In contexts like React's JSX, only expressions are permitted within curly braces {} for dynamic content, not statements. (What is JSX? JSX stands for JavaScript XML. JSX allows us to write HTML in React. JSX makes it easier to write and add HTML in React, React lets you build user interfaces out of individual pieces called components. Create your own React components like Thumbnail, LikeButton, and Video.)

Relationship:

Expressions are often components of statements. For instance, in if (x > 5) { ... }, x > 5 is an expression that evaluates to a boolean value, and the entire if construct is a statement. Similarly, let result = a + b; involves the expression a + b within a variable declaration statement.

JavaScript Statements

Example

let x, y, z;    // Statement 1
x = 5;          // Statement 2
y = 6;          // Statement 3
z = x + y;      // Statement 4
JavaScript statements are composed of:

Values, Operators, Expressions, Keywords, and Comments.

This statement tells the browser to write "Hello Dolly." inside an HTML element with id="demo":

Example

document.getElementById("demo").innerHTML = "Hello Dolly.";

 

Semicolons ;

Semicolons separate JavaScript statements.

Add a semicolon at the end of each executable statement:

Examples

let a, b, c;  // Declare 3 variables
a = 5;        // Assign the value 5 to a
b = 6;        // Assign the value 6 to b
c = a + b;    // Assign the sum of a and b to c

When separated by semicolons, multiple statements on one line are allowed:

a = 5; b = 6; c = a + b;

JavaScript Keywords

JavaScript statements often start with a keyword to identify the JavaScript action to be performed.

Keyword	Description
var	Declares a variable
let	Declares a block variable
const	Declares a block constant
if	Marks a block of statements to be executed on a condition
switch	Marks a block of statements to be executed in different cases
for	Marks a block of statements to be executed in a loop
function	Declares a function
return	Exits a function
try	Implements error handling to a block of statements

Note

JavaScript keywords are reserved words. Reserved words cannot be used as names for variables.