In C# 4.0 (or earlier if it can be done), how does a parent class pass a reference of itself to a child class. For example:
Self Regional Healthcare, Greenwood, SC Resource Pool / Varied shifts / Varied hours.
How can I get Page to recognize which book it's in? I'm trying to pass the Book class in the constructor, but don't know how.
John Saunders149k2222 gold badges207207 silver badges367367 bronze badges
EugeneEugene4,5441616 gold badges4949 silver badges7575 bronze badges
2 Answers
You're having trouble because you've made your book class the entry point for your application, which means you don't have an actual Book instance.
Try something like this instead:
Paul BatumPaul Batum5,83533 gold badges2929 silver badges3737 bronze badges
Just add a field for
Book
in the Page
class
Then create an overload that takes a Book argument.
Use
GabeGabethis
to reference the Book
object
when calling the constructor from the Book
class
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JavaScript functions are defined with the
function
keyword.You can use a function declaration or a function expression.
Function Declarations
Earlier in this tutorial, you learned that functions are declared with the following syntax:
function functionName(parameters) {
// code to be executed
}
// code to be executed
}
Declared functions are not executed immediately. They are 'saved for later use', and will be executed later, when they are invoked (called upon).
Example
Try it Yourself »Semicolons are used to separate executable JavaScript statements.
Since a function declaration is not an executable statement, it is not common to end it with a semicolon.
Since a function declaration is not an executable statement, it is not common to end it with a semicolon.
Function Expressions
A JavaScript function can also be defined using an expression.
A function expression can be stored in a variable:
Example
Try it Yourself »After a function expression has been stored in a variable, the variable can be used as a function:
Example
var x = function (a, b) {return a * b};
var z = x(4, 3);
Try it Yourself »var z = x(4, 3);
The function above is actually an anonymous function (a function without a name).
Functions stored in variables do not need function names. They are always invoked (called) using the variable name.
The function above ends with a semicolon because it is a part of an executable statement.
The Function() Constructor
As you have seen in the previous examples, JavaScript functions are defined with the
function
keyword.Functions can also be defined with a built-in JavaScript function constructor called
Function()
.Example
var myFunction = new Function('a', 'b', 'return a * b');
var x = myFunction(4, 3);
Try it Yourself »var x = myFunction(4, 3);
You actually don't have to use the function constructor. The example above is the same as writing:
Example
var myFunction = function (a, b) {return a * b};
var x = myFunction(4, 3);
Try it Yourself »var x = myFunction(4, 3);
Most of the time, you can avoid using the
new
keyword in JavaScript.Function Hoisting
![Self keyword php Self keyword php](/uploads/1/2/6/2/126248886/562899903.jpg)
Earlier in this tutorial, you learned about 'hoisting' (JavaScript Hoisting).
Hoisting is JavaScript's default behavior of moving declarations to the top of the current scope.
Hoisting applies to variable declarations and to function declarations.
Because of this, JavaScript functions can be called before they are declared:
myFunction(5);
function myFunction(y) {
return y * y;
}
function myFunction(y) {
return y * y;
}
Functions defined using an expression are not hoisted.
Self-Invoking Functions
Function expressions can be made 'self-invoking'.
A self-invoking expression is invoked (started) automatically, without being called.
Function expressions will execute automatically if the expression is followed by ().
You cannot self-invoke a function declaration.
You have to add parentheses around the function to indicate that it is a function expression:
Example
(function () {
var x = 'Hello!!'; // I will invoke myself
})();
Try it Yourself »var x = 'Hello!!'; // I will invoke myself
})();
The function above is actually an anonymous self-invoking function (function without name).
Functions Can Be Used as Values
JavaScript functions can be used as values:
Example
function myFunction(a, b) {
return a * b;
}
var x = myFunction(4, 3);
Try it Yourself »return a * b;
}
var x = myFunction(4, 3);
JavaScript functions can be used in expressions:
Example
function myFunction(a, b) {
return a * b;
}
var x = myFunction(4, 3) * 2;
Try it Yourself »return a * b;
}
var x = myFunction(4, 3) * 2;
Functions are Objects
The
typeof
operator in JavaScript returns 'function' for functions.But, JavaScript functions can best be described as objects.
JavaScript functions have both properties and methods.
The
arguments.length
property returns the number of arguments received when the function was invoked:Example
function myFunction(a, b) {
return arguments.length;
}
Try it Yourself »return arguments.length;
}
The
toString()
method returns the function as a string:Example
function myFunction(a, b) {
return a * b;
}
var txt = myFunction.toString();
Try it Yourself »return a * b;
}
var txt = myFunction.toString();
A function defined as the property of an object, is called a method to the object.
A function designed to create new objects, is called an object constructor.
A function designed to create new objects, is called an object constructor.
Arrow Functions
Arrow functions allows a short syntax for writing function expressions.
You don't need the
function
keyword, the return
keyword, and the curly brackets.Example
// ES5
var x = function(x, y) {
return x * y;
}
// ES6
const x = (x, y) => x * y;
Try it Yourself »var x = function(x, y) {
return x * y;
}
// ES6
const x = (x, y) => x * y;
Arrow functions do not have their own
this
. They are not well suited for defining object methods.Arrow functions are not hoisted. They must be defined before they are used.
Using
const
is safer than using var
, because a function expression is always constant value.You can only omit the
return
keyword and the curly brackets if the function is a single statement. Because of this, it might be a good habit to always keep them:Example
Try it Yourself »Arrow functions are not supported in IE11 or earlier.