Statements and declarations
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.
Statements and declarations by category
For an alphabetical listing see the sidebar on the left.
Control flow
return
-
Specifies the value to be returned by a function.
break
-
Terminates the current loop, switch, or label statement and transfers program control to the statement following the terminated statement.
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
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.
- Block
-
A block statement is used to group zero or more statements. The block is delimited by a pair of curly brackets.
- Expression statement
-
An expression statement evaluates an expression and discards its result. It allows the expression to perform side effects, such as executing a function or updating a variable.
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.
- label
-
Provides a statement with an identifier that you can refer to using a
break
orcontinue
statement. with
Deprecated-
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
:
js
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.
js
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.
js
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.
js
label: const a = 1; // SyntaxError: Lexical declaration cannot appear in a single-statement context
Note: there's a legacy grammar that allows function declarations to have labels, but it's only standardized for compatibility with web reality.
To get around this, you can wrap the declaration in braces — this makes it part of a block statement.
js
label: {
const a = 1;
}
if (condition) {
let i = 0;
}
Browser compatibility
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