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.
• using
  • : Declares local variables that are synchronously disposed.
• await using
  • : Declares local variables that are asynchronously disposed.

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 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:

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

[!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;
}

Browser compatibility

See also

• Expressions and operators