JavaScript Syntax(語法)&Statements(語句)

 

 JavaScript Programs(程序)

• A computer program is a list of "instructions" to be "executed" by the computer.
• In a programming language, these program instructions are called statements.(語句)
• JavaScript is a programming language.
• JavaScript statements are separated by semicolon.(分號)

Tips: In HTML, JavaScript programs can be executed by the web browser.

 

JavaScript Statements(語句)

JavaScript statements are composed of(語句組成):
Values, Operators, Expressions, Keywords, and Comments.
值，運算元，表達式，關鍵字和註釋

 

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JavaScript Values

The JavaScript syntax defines two types of values: Fixed(固定的) values and variable values.
Fixed values are called literals(文字). Variable values are called variables(變量/變數).

 

JavaScript Literals

The most important rules for writing fixed values are:

• Numbers(數值) are written with or without decimals:

10.50
1001

• Strings(字符串) are text(文本), written within double or single quotes:

"John Doe"
'John Doe'

• Expressions(表達式/表現) can also represent fixed values:

5 + 6
5 * 10

JavaScript Variables

In a programming language, variables are used to store(儲存) data values.
JavaScript uses the var keyword to define variables.(Var來定義變量/數)
An equal sign is used to assign values to variables.(等號用來配置值)
In this example, x is defined as a variable. Then, x is assigned (given) the value 6:

var x;
x = 6;

 

JavaScript Operators(運算符號)

JavaScript uses an assignment operator ( = ) to assign values to variables:

var x = 5;
var y = 6;

JavaScript uses arithmetic(算術/四則運算) operators ( + - *  / ) to compute values:

(5 + 6) * 10    //己是完整的運算式，只是沒有把結果賦予(=)放置到對象/變數。

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Tips: In HTML, JavaScript programs can be executed by the web browser.

JavaScript Statements

This statement tells the browser to write "Hello Dolly." inside an HTML element with id="demo":

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

JavaScript Programs

Most JavaScript programs contain many JavaScript statements.
The statements are executed, one by one, in the same order as they are written.
In this example, x, y, and z is given values, and finally z is displayed:

var x = 5;
var y = 6;
var z = x + y;
document.getElementById("demo").innerHTML = z;

tips:JavaScript programs (and JavaScript statements) are often called JavaScript code.

Semicolons( ;)

Semicolons separate JavaScript statements.
Add a semicolon at the end of each executable statement:

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

When separated by semicolons, multiple statements on one line are allowed:

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

On the web, you might see examples without semicolons.
Ending statements with semicolon is not required, but highly recommended.
語句的結束分號是不必要的，但避免程式解析時的岐異，最好不要省略，自找麻煩！

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JavaScript White Space

JavaScript ignores multiple spaces. You can add white space to your script to make it more readable.(忽略多個連續空白間隔)

The following lines are equivalent:

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

A good practice is to put spaces around operators ( = + - * / ):
好習慣：運算符號之間放上空白間隔

var x = y + z;

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JavaScript Line Length and Line Breaks

For best readability, programmers often like to avoid code lines longer than 80 characters.
//最佳的可讀性，通常喜歡避免代碼行超過80個字元
If a JavaScript statement does not fit on one line, the best place to break it, is after an operator:
//最好的斷句是在操作符號之後

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

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JavaScript Code Blocks(代碼塊/分段)

JavaScript statements can be grouped together in code blocks, inside curly brackets {...}.
The purpose of code blocks this is to define statements to be executed together.
One place you will find statements grouped together in blocks, are in JavaScript functions:

function myFunction() {
    document.getElementById("demo").innerHTML = "Hello Dolly.";
    document.getElementById("myDIV").innerHTML = "How are you?";
}

In this tutorial we use 4 spaces of indentation for code blocks.

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JavaScript Keywords

JavaScript statements often start with a keyword to identify the JavaScript action to be performed.

Here is a list of some of the keywords you will learn about in this tutorial:

Keyword	Description
break	Terminates a switch or a loop
continue	Jumps out of a loop and starts at the top
debugger	Stops the execution of JavaScript, and calls (if available) the debugging function
do ... while	Executes a block of statements, and repeats the block, while a condition is true
for	Marks a block of statements to be executed, as long as a condition is true
function	Declares a function
if ... else	Marks a block of statements to be executed, depending on a condition
return	Exits a function
switch	Marks a block of statements to be executed, depending on different cases
try ... catch	Implements error handling to a block of statements
var	Declares a variable

JavaScript keywords are reserved words. Reserved words cannot be used as names for variables.

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詳細內容可以參考：

• w3school.com (http://www.w3schools.com/)

• 程式語言教學誌(http://pydoing.blogspot.tw/)