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JavaScript – Function

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Function - Definition

  • A function is a reusable code-block that will be executed by an event, or when the function is called.

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  • A function is a JavaScript procedure—a set of statements that performs a specific task.

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Function - Definition

  • You may call a function from anywhere within the page (or even from other pages if the function is embedded in an external .js file).

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  • Functions can be defined both in the <head> and in the <body> section of a document. However, to assure that the function is read/loaded by the browser before it is called, it could be wise to put it in the <head> section.

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Function Definition - Syntax

  • function functionname(var1,var2,...,varX)

{

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//some code(Javascript statements)

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}

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Function Definition

  • A function definition consists of the function keyword
  • The name of the function.
  • A list of arguments to the function, enclosed in parentheses and separated

by commas.

  • The JavaScript statements that define the function, enclosed in curly braces

{ }.

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Function Definition - Syntax

  • Note:

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A function with no parameters must include the parentheses () after the function name.

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Do not forget about the importance of capitals in JavaScript! The word function must be written in lowercase letters, otherwise a JavaScript error occurs! Also note that you must call a function with the exact same capitals as in the function name.�

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Function Definition

The return Statement:

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  • This statement is used to specify the value that is returned by the function.

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  • Syntax:
    • return <value>

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Function calling

  • After defining the function it must be Called to perform the specified actions with the indicated parameters.

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  • After it is defined a function can be called any number of times.

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  • Syntax –

<function name>(value1,value2….valueN)

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Example - Function Definition and calling

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Example - Function Definition with arguments and calling

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Example - Function Definition with arguments and return statement

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JavaScript – Predefined function

  • eval
  • isFinite
  • isNaN
  • parseInt
  • parseFloat
  • escape
  • Unescape
  • String
  • Number

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eval function

  • This function evaluates a string of JavaScript code without reference to a particular object.

syntax:

eval(expr)

where expr is a string to be evaluated.

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  • If the string represents an expression, eval evaluates the expression.

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  • If the argument represents one or more JavaScript statements, eval performs the statements.

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isFinite function

  • The isFinite function evaluates an argument to determine whether it is a finite number.

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  • syntax:

isFinite(number)

where number is the number to evaluate.

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  • When the argument is NaN, positive infinity or negative infinity, this method returns false, otherwise it returns true.

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  • Example:

if(isFinite(ClientInput) == true)

{

/* take specific steps */ }

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isNaN function

  • The isNaN function evaluates an argument to determine if it is “NaN” (not a number).

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  • Syntax:

isNaN(testValue)

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  • isNaN returns true if passed “NaN,” and false otherwise.

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parseInt function

  • It parses the argument, the string and attempts to return an integer
  • Syntax:

parseInt(str)

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Note : If the first letter is a character in the given string it returns “NaN” value.

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parseFloat function

  • It parses argument, the string and attempts to return an floating point value.
  • Syntax:

parseFloat(str)

Note : If the first letter is a character in the given string it returns “NaN” value.

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escape function

    • This function encodes a string, so it can be read on all computers.

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    • Syntax
      • escape(string)�
    • Parameter
      • stringRequired. The string to be encoded. �

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escape function

    • This function encodes special characters, with the exception of: * @ - _ + . /

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    • Use the unescape() function to decode strings encoded with escape().

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unescape function

    • The unescape() function decodes a string encoded with escape().
    • Syntax
      • unescape(string)
    • Parameter –
      • string - The string to be decoded�
      • Note: These functions used by the server-side JavaScript to encode and decode name/value pairs in URLs.

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Number function

    • This function converts the value of an object to a number.
    • Syntax
      • Number(object)
    • Parameter
      • Object - A JavaScript object
    • Note:
    • If the parameter is a Date object, the Number() function returns the number of milliseconds since midnight January 1, 1970 UTC.
    • If the object's value cannot be converted to a number, the Number() function returns NaN.

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String function

    • This converts the value of an object to a string.
    • Syntax
      • String(object)
    • Parameter
      • Object - A JavaScript object�Tips and Notes
    • Note:
      • The String() function returns the same value as toString() of the individual objects.

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