A Note of JavaScript Learning

1. Introduction

2. Where to

The script Tag

  • In HTML, JavaScript code must be inserted between <script> and </scipt> tags.
  • Old JavaScript examples may use a type attribute: <script type="text/javascript">.
    The type attribute is not required. JavaScript is the default scripting language in HTML.

head or body ?

  • You can place any number of scripts in an HTML document.
  • Placing scripts at the bottom of the <body> element improves the display speed, because script interpretation slows down the display.

External JavaScript

  • JavaScript files have the file extension .js.

  • To use an external script, put the name of the script file in the src (source) attribute of a <script> tag:

  • Ex:

    <script src = "myscript.js"></script>

  • External scripts cannot contain <script> tags.

3. Output

Here are several ways to display:

  • Writing into an HTML element, using innerHTML.
  • Writing into the HTML output using document.write().
  • Writing into an alert box, using window.alert().
  • Writing into the browser console, using console.log().

Using innerHTML

  • By using document.getElementById(id) to access an element.

Using document.write()

  • Using document.write() after an HTML document is loaded, will delete all existing HTML.
  • The document.write() method should only be used for testing.

Using window.alert()

Using console.log()

  • Use this method to display data for debugging purpose.

Statements

Statements

  • JavaScript statements are composed of:

    Values, Operators, Expressions, Keywords, and Comments.

Semicolons

  • Use semicolons to separate.
  • On the web, you might see examples without semicolons.
    Ending statements with semicolon is not required, but highly recommended.

White Space

  • Unnecessary.
  • A good practice is to put spaces around operators ( = + - * / ).

Line Length and Line Breaks

  • For best readability, programmers often like to avoid code lines longer than 80 characters.

  • If a JavaScript statement does not fit on one line, the best place to break it is after an operator.

    • Ex:

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

Code Blocks

  • JavaScript statements can be grouped together in code blocks, inside curly brackets {...}.

    The purpose of code blocks is to define statements to be executed together(like a function).

Keywords

Reserved Words

04. Syntax

  • JavaScript syntax is the set of rules, how JavaScript programs are constructed.

Values

  • The JavaScript syntax defines two types of values: Fixed values and variable values.

    Fixed values are called literals. Variable values are called variables.

Literals

  • Numbers
  • String
    • Double or single quotes are allowed.

Variables

  • Use var keyword to declare variables.

Operators

  • JavaScript uses arithmetic operators ( + - * / ) to compute values.
  • JavaScript uses an assignment operator ( = ) to assign values to variables.

Expressions

  • An expression is a combination of values, variables, and operators, which computes to a value.
  • The computation is called an evaluation.
  • For example, 5 * 10 evaluates to 50.

Comments

  • Code after double slashes // or between /* and */ is treated as a comment.
  • Comments are ignored, and will not be executed.

Identifiers

  • In JavaScript, the first character must be a letter, or an underscore (_), or a dollar sign ($).
  • Subsequent characters may be letters, digits, underscores, or dollar signs.

Case Sensitive

05. Comments

  • This part can be ignored, which is the same as many languages.

06. Variables

One Statement, Many Variablves

  • Start the statement with var and separate the variables by comma.

  • A declaration can span multiple lines:

    Ex:

    var person = "John Doe",
carName = "Volvo",
price = 200;

Value = undefined

  • A variable declared without a value will have the value undefined.

Re-Declaring JavaScript Variables

  • If you re-declare a JavaScript variable, it will not lose its value.

    The variable carName will still have the value "Volvo" after the execution of these statements:

Arithmetic

  • If you put a number in quotes, the rest of the numbers will be treated as strings, and concatenated.

    **Ex1: **

    var x = "5"+2+3

    the result is 523.

    Ex2:

    var x = 2+3+"5"

    the result is 55.

06. Operators

  • It's like C++ and Python.
  • The result will be a string if adding string in it.

Comparison Operators

Operator Description
== equal to
=== equal value and equal type
!= not equal
!== not equal value or not equal type
> greater than
< less than
>= greater than or equal to
<= less than or equal to
? ternary operator

Logical Operators

Operator Description
&& logical and
|| logical or
! logical not

Type operators

Operator Description
typeof Returns the type of a variable
instanceof Returns true if an object is an instance of an object type

Bitwise Operators

  • Addition:

    >>> : Zero fill right shift.

07. Data Types

Dynamic

  • JavaScript has dynamic types. This means that the same variable can be used to hold different data types.

Null

  • In JavaScript, the data of null is an object.

  • Empty an object by setting it to null/undefined.

  • undefined and null are equal in value but different in type.

    null === undefined //false
null == undefined //true

Complex Data

  • The typeof operator can return one of two complex types:

    • `function``
    • ``object`

  • The typeof operator returns "object" for objects, arrays, and null.

  • The typeof operator does not return "object" for functions.

08. Functions

Syntax

  • A JavaScript function is defined with the function keyword, followed by a name, followed by parentheses ().
  • Function names can contain letters, digits, underscores, and dollar signs (same rules as variables).
  • The parentheses may include parameter names separated by commas:
    (*parameter1, parameter2, ...*)
  • The code to be executed, by the function, is placed inside curly brackets: {}

Invocation

  • When an event occurs (when a user clicks a button)
  • When it is invoked (called) from JavaScript code
  • Automatically (self invoked)

Return

  • The function will stop when reaches a return.

The () Operator Invokes the Function

  • Using the example above, toCelsius refers to the function object, and toCelsius() refers to the function result.
  • Accessing a function without () will return the **function definition **instead of the function result.

Local Variables

  • Functions use local variables which will be deleted when the function is completed.

09. Objects

  • JavaScript objects are containers for named values called properties or methods.

Definition

Ex:

var person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};

Accessing Object Properties

  • There are two ways to access properties.

    1st.

    objectName.propertyName

    2nd.

    objectName["propertyName"]

Method

  • A method is a function stored as a property.

"this"

  • Like C++ but not a pointer.

new?

  • When a JavaScript variable is declared with the keyword "new", the variable is created as an object.
  • Avoid String, Number, and Boolean objects. They complicate your code and slow down execution speed.

10. Events

<element event="some JavaScript">

  • Using this:

    <button onclick="this.innerHTML = Data()">
    The time is?
</button>

  • Calling a function:

    <button onclick="displayData()">
    The time is?
</button>

Common HTML Events

Event Description
onchange An HTML element has been changed
onclick The user clicks an HTML element
onmouseover The user moves the mouse over an HTML element
onmouseout The user moves the mouse away from an HTML element
onkeydown The user pushes a keyboard key
onload The browser has finished loading the page

11. Strings

Length

  • Use length to get the length of the string.

backslash

Code Result Description
' ' Single quote
" " Double quote
\ \ Backslash
  • You can also break up a code line within a text string with a single backslash. But this is not good, and use + is acceptable.

new?

  • Note the difference between (x==y) and (x===y).
    Comparing two JavaScript objects will always return false.

11. String Methods

Find Sub String

  • The indexOf() method returns the index of (the position of) the first occurrence of a specified text in a string.

  • The lastIndexOf() method returns the index of the last occurrence of a specified text in a string.

  • Both indexOf(), and lastIndexOf() return -1 if the text is not found.

  • Both methods accept a second parameter as the starting position for the search:

    var str = "Please locate where 'locate' occurs!";
var pos = str.indexOf("locate", 15);

  • The lastIndexOf() methods searches backwards (from the end to the beginning), meaning: if the second parameter is 15, the search starts at position 15, and searches to the beginning of the string.

  • indexOf() and search() are different.

posted @ 2020-03-04 16:39  iNx  阅读(201)  评论(0)    收藏  举报