陳述式與宣告

JavaScript 應用程式由適當的陳述式組成。一個單一的陳述式可以跨用好幾行。 多個陳述式也可以藉由分號分隔來寫在同一行。 這不是一個關鍵字,而是一群關鍵字。

陳述式與宣告分類

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)陳述式,並將程式流程轉到被中斷陳述式後的陳述式。
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
當特定的條件為真時執行一段陳述式,若為假則另一段陳述式就會被執行。
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
用指定的變數宣告一個函數
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.

迭代(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

瀏覽器相容性

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參見