JavaScript Introductin to Functions


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JavaScript Introductin to Functions

  1. 1. JavaScript Introduction to Functions Charles Russell Bennu Bird Media
  2. 2. What is a function. ● As Defined by W3Schools A function is a block of code that will be executed when "someone" calls it. ● Saved for later and use on demand
  3. 3. Function invocations ● functionName( parameterList); ● Parameters, or arguments, are the values to be used by the function – Parameters may be expressions, variables, literals, or objects ● Ex: alert('Hello World'); ● Functions may return values – name = fullName('Charles', 'Russell');
  4. 4. Making Your Own Functions ● Functions can be constructed two ways ● Statement form ● Expression form
  5. 5. Statement form function nameOfFunction(parameter1, parameter2, ...) { //statments and expressions return result; //Optional if no value to be returned } ● The return statement is optional and may or may not return a value ● Immediate invocation optional with () following end of code block ● If a parameter is declared and not passed no error is generated until an attempt is made to use the missing parameter. ● Some functions are selfcontained and need no arguments
  6. 6. Functions as Values ● Remember in the discussion on data types ● Anything not a primitive is an object ● In JavaScript functions can be used as values, be passed as parameters, and returned as values from functions
  7. 7. Expression form var nameOfFunction = function (parameter1, parameter2, ...) { //statments and expressions return result; //optional if no value to be returned } ● Later we will see that in general this is more useful than the statement form.
  8. 8. Scope ● In this Language variables default to global ● This means that any function anywhere can use and overwrite your values at any time ● The var keyword is what is used to limit scope to the function in which the variable was delcare ● There is no block scope only function scope ● This means that any local variable declared in a function can be accessed from anywhere in the function even sub functions ● Variables in parameters have local scope
  9. 9. this Keyword ● The keyword this is a special object that refers to different important objects based upon context. ● When a function is created with the statement form and is not part of some other Object this refers to the global Object if ES3 this is undefined if ES5 – For browsers this is the Window Object ● When function used as a constructor this refers to current instance of current function ● When as a method this refers to calling object (normally the parent)
  10. 10. Summary ● Function are code ment to be used later ● A function is called by using its name and opening and closing parents surround 0 or more parameters ● Functions may or may not return a value ● Functions are objects and can be used as values (not just the result) ● If a declared parameter is missing or out of order no error is generated ● Functions may be declared in statement or
  11. 11. Introduction to Objects