述句與宣告

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.

述句與宣告分類

For an alphabetical listing see the sidebar on the left.

流程控制

Block

A block statement is used to group zero or more statements. The block is delimited by a pair of curly brackets.

break

中斷當下的迴圈、條件判斷（switch）或是標籤（label）述句，並將程式流程轉到被中斷述句後的述句。

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.

Empty

An empty statement is used to provide no statement, although the JavaScript syntax would expect one.

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.

throw

Throws a user-defined exception.

try...catch

Marks a block of statements to try, and specifies a response, should an exception be thrown.

宣告

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.

函數與類別（Class）

function

Declares a function with the specified parameters.

function*

Generators functions enable writing iterators more easily.

async function

Declares an async function with the specified parameters.

return

Specifies the value to be returned by a function.

class

Declares a class.

迴圈（Iteration）

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 each...in

Iterates a specified variable over all values of object's properties. For each distinct property, a specified statement is executed.

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.

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.

其他

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, another 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.

規範

參見

• 運算式與運算子