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

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.
Block
- : A block statement is used to group zero or more statements. The block is delimited by a pair of curly braces.
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 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 " 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

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.

label: {
  const a = 1;
}

if (condition) {
  let i = 0;
}