Example
let x, y, z; // Statement 1
x = 5; // Statement 2
y = 6; // Statement 3
z = x + y; // Statement 4
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JavaScript Programs
A computer program is a list of "instructions" to be "executed" by a computer.
In a programming language, these programming instructions are called statements.
A JavaScript program is a list of programming statements.
In HTML, JavaScript programs are executed by the web browser.
JavaScript Statements
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.";
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Most JavaScript programs contain many JavaScript statements.
The statements are executed, one by one, in the same order as they are written.
JavaScript programs [and JavaScript statements] are often called JavaScript code.
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
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When separated by semicolons, multiple statements on one line are allowed:
On the web, you might see examples without semicolons.
Ending statements with semicolon is not required, but highly recommended.
JavaScript White Space
JavaScript ignores multiple spaces. You can add white space to your script to make it more readable.
The following lines are equivalent:
let person = "Hege";
let person="Hege";
A good practice is to put spaces around operators [ = + - * / ]:
JavaScript Line Length and Line Breaks
For best readability, programmers often like to avoid code lines longer than 80 characters.
If a JavaScript statement does not fit on one line, the best place to break it is after an operator:
JavaScript Code Blocks
JavaScript statements can be grouped together in code blocks, inside curly brackets {...}.
The purpose of code blocks is to define statements to be executed together.
One place you will find statements grouped together in blocks, is in JavaScript functions:
Example
function myFunction[] {
document.getElementById["demo1"].innerHTML = "Hello Dolly!";
document.getElementById["demo2"].innerHTML = "How are you?";
}
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In this tutorial we use 2 spaces of indentation for code blocks.
You will learn more about functions later in this tutorial.
JavaScript Keywords
JavaScript statements often start with a keyword to identify the JavaScript action to be performed.
Our Reserved Words Reference lists all JavaScript keywords.
Here is a list of some of the keywords you will learn about in this tutorial:
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 |
JavaScript keywords are reserved words. Reserved words cannot be used as names for variables.
JavaScript applications consist of statements with an appropriate syntax. A single statement may span multiple lines. Multiple statements may occur on a single line if each statement is separated by a semicolon. This isn't a keyword, but a group of keywords. For an alphabetical listing see the sidebar on the left. Specifies the value to be returned by a function. Terminates the current loop, switch, or label statement and transfers program
control to the statement following the terminated statement.Statements and declarations by category
Control flow
return
break
continue
Terminates execution of the statements in the current iteration of the current or labeled loop, and continues execution of the loop with the next iteration.
throw
Throws a user-defined exception.
if...else
Executes a statement if a specified condition is true. If the condition is false, another statement can be executed.
switch
Evaluates an expression, matching the expression's value to a case clause, and executes statements associated with that case.
try...catch
Marks a block of statements to try, and specifies a response, should an exception be thrown.
Declaring variables
var
Declares a variable, optionally initializing it to a value.
let
Declares a block scope local variable, optionally initializing it to a value.
const
Declares a read-only named constant.
Functions and classes
function
Declares a function with the specified parameters.
function*
Generator Functions enable writing iterators more easily.
async function
Declares an async function with the specified parameters.
async function*
Asynchronous Generator Functions enable writing async iterators more easily.
class
Declares a class.
Iterations
do...while
Creates a loop that executes a specified statement until the test condition evaluates to false. The condition is evaluated after executing the statement, resulting in the specified statement executing at least once.
for
Creates a loop that consists of three optional expressions, enclosed in parentheses and separated by semicolons, followed by a statement executed in the loop.
for...in
Iterates over the enumerable properties of an object, in arbitrary order. For each distinct property, statements can be executed.
for...of
Iterates over iterable objects [including arrays, array-like objects, iterators and generators], invoking a custom iteration hook with statements to be executed for the value of each distinct property.
for await...of
Iterates over async iterable objects, array-like objects, iterators and generators, invoking a custom iteration hook with statements to be executed for the value of each distinct property.
while
Creates a loop that executes a specified statement as long as the test condition evaluates to true. The condition is evaluated before executing the statement.
Others
Empty
An empty statement is used to provide no statement, although the JavaScript syntax would expect one.
BlockA block statement is used to group zero or more statements. The block is delimited by a pair of curly brackets.
debugger
Invokes any available debugging functionality. If no debugging functionality is available, this statement has no effect.
export
Used to export functions to make them available for imports in external modules, and other scripts.
import
Used to import functions exported from an external module, another script.
labelProvides a statement with an identifier that you can refer to using a break
or continue
statement.
with
Extends the scope chain for a statement.
Difference between statements and declarations
In this section, we will be mixing two kinds of constructs: statements and declarations. They are two disjoint sets of grammars. The following are declarations:
let
const
function
function*
async function
async function*
class
export
[Note: it can only appear at the top-level of a module]import
[Note: it can only appear at the top-level of a module]
Everything else in the list above is a statement.
The terms "statement" and "declaration" have a precise meaning in the formal syntax of JavaScript that affects where they may be placed in code. For example, in most
control-flow structures, the body only accepts statements — such as the two arms of an if...else
:
if [condition]
statement1;
else
statement2;
If you use a declaration instead of a statement, it would be a SyntaxError
. For example, a
let
declaration is not a statement, so you can't use it in its bare form as the body of an if
statement.
if [condition]
let i = 0; // SyntaxError: Lexical declaration cannot appear in a single-statement context
On the other hand, var
is a statement, so you can use it on its own as the if
body.
if [condition]
var i = 0;
You can see
declarations as "binding identifiers to values", and statements as "carrying out actions". The fact that var
is a statement instead of a declaration is a special case, because it doesn't follow normal lexical scoping rules and may create side effects — in the form of creating global variables, mutating existing var
-defined variables, and defining variables that are visible outside of its block [because var
-defined variables aren't block-scoped].
As another example, labels can only be attached to statements.
label: const a = 1; // SyntaxError: Lexical declaration cannot appear in a single-statement context
To get around this, you can wrap the declaration in braces — this makes it part of a block statement.
label: {
const a = 1;
}
if [condition] {
let i = 0;
}
Browser compatibility
BCD tables only load in the browser