【JavaScript Tutorial】JavaScript Statement-JS Tutorial-php.cn

JavaScript Statement

JavaScript statement is a command issued to the browser. The purpose of a statement is to tell the browser what to do.

JavaScript Statements

JavaScript statements are commands sent to the browser.

The purpose of these commands is to tell the browser what to do.

The following JavaScript statement outputs the text "Hello Dolly" to the HTML element with id="demo":

Example

document.getElementById("demo").innerHTML = "Hello Dolly.";

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Semicolon;

Semicolon is used to separate JavaScript statements.

Usually we add a semicolon at the end of each executable statement.

Another use of semicolons is to write multiple statements in one line.

Writing:

a = 5;
b = 6;
c = a + b;

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Is the same as writing:

a = 5; b = 6; c = a + b;

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You may also see cases without semicolons.
In JavaScript, terminating statements with a semicolon is optional.

JavaScript Code

JavaScript code is a sequence of JavaScript statements.

The browser executes each statement in sequence in the order in which it was written.

This example outputs a title and two paragraphs to the web page:

Example

document.getElementById("demo").innerHTML="你好 Dolly";
document.getElementById("myDIV").innerHTML="你最近怎么样?";

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JavaScript code blocks

JavaScript can be combined in batches.

A code block starts with a left curly brace and ends with a right curly brace.

The function of a code block is to execute a sequence of statements together.

This example outputs a title and two paragraphs to the web page:

Example

function myFunction()
{
document.getElementById("demo").innerHTML="你好Dolly";
document.getElementById("myDIV").innerHTML="你最近怎么样?";
}

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You will learn more about functions in later chapters.

JavaScript Statement Identifier

JavaScript statements usually start with a Statement Identifier and execute the statement.

Statement identifiers are reserved keywords and cannot be used as variable names.

The following table lists JavaScript statement identifiers (keywords):

statement

description

break is used to break out of the loop.

catch statement block, executed in the try statement block Execute the catch statement block when an error occurs.

continue Skips an iteration in the loop.

do... while Execute a statement block and continue executing the statement block when the conditional statement is true. , you can execute the code block a specified number of times.

for ... in is used to traverse the properties of the array or object (loop through the properties of the array or object).

function defines a function

if ... else is used to perform different actions based on different conditions.

return exit function

switch is used to perform different actions based on different conditions.

try implements error handling. Used with catch

var 声明一个变量。

while 当条件语句为 true 时，执行语句块。

JavaScript 对大小写敏感。

JavaScript 对大小写是敏感的。

当编写 JavaScript 语句时，请留意是否关闭大小写切换键。

函数 getElementById 与 getElementbyID 是不同的。

同样，变量 myVariable 与 MyVariable 也是不同的。

空格

JavaScript 会忽略多余的空格。您可以向脚本添加空格，来提高其可读性。下面的两行代码是等效的：

var person="Hege";
var person = "Hege";

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对代码行进行折行

您可以在文本字符串中使用反斜杠对代码行进行换行。下面的例子会正确地显示：

document.write("你好 \
世界!");

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不过，您不能像这样折行：

document.write \ 
("你好世界!");

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您知道吗？

提示：JavaScript 是脚本语言。浏览器会在读取代码时，逐行地执行脚本代码。而对于传统编程来说，会在执行前对所有代码进行编译。

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