JavaScript for ABAP Programmers - 5/7 Functions

JavaScript for ABAP Programmers
Functions as 1st Class Citizens
Chris Whealy / The RIG

ABAP
Strongly typed
Syntax similar to COBOL
Block Scope
No equivalent concept
OO using class based inheritance
Imperative programming

JavaScript
Weakly typed
Syntax derived from Java
Lexical Scope
Functions are 1st class citizens
OO using referential inheritance
Imperative or Functional programming

1st Class Functions
Any programming language in which functions are treated as “1st class citizens” is said to implement
“1st class functions”.
So, what does that mean – functions have voting rights?

© 2013 SAP AG. All rights reserved.

3

1st Class Functions
No, a 1st class citizen is any object that can be:
 Created or destroyed dynamically


4

1st Class Functions
 Treated as data, meaning that it can be both:
 Passed as an argument to a function and
 Returned as the result of running a function


5

1st Class Functions
 Treated as data, meaning that it can be both:
 Passed as an argument to a function and
 Returned as the result of running a function
In JavaScript, all the above statements are applicable to functions because a function is simply an
object “with an executable part”. This means a JavaScript function can be treated either as:
 An object like any other data object, or as
 A executable unit of code


6

1st Class Functions: Dynamic Creation
A JavaScript function can be created dynamically based on the data available at runtime.
// List of animals and the noises they make
var animalNoises = ["dog","woof","cat","meow","pig","oink","cow","moo"];
// A function that returns a text string describing an animal noise
function noiseMaker(name) {
var n
= animalNoises.indexOf(name);
var noise = (n == -1) ? "a sound I don't know" : animalNoises[n + 1];
return "A " + name + " says " + noise;
}


7

var n
}
// Dynamically create some function objects specific to certain animals
var pigSays
= new Function("alert("" + noiseMaker("pig") + "");");
var sheepSays = new Function("alert("" + noiseMaker("sheep") + "");");


8

var n
}
var pigSays
// Call these dynamically created function objects using the invocation operator
pigSays();


9

var n
}
var pigSays
// Call these dynamically created function objects using the invocation operator
pigSays();
sheepSays();

10

1st Class Functions: Dynamic Destruction
Setting a function object to null destroys that object.
// Destroy the dynamically created function objects
pigSays
= null;
sheepSays = null;

Notice here that reference is made to the function object itself, not the result of invoking that function.
In other words, the invocation operator () must not be used.


11

1st Class Functions: Functions as Arguments
Since JavaScript treats a function is an object, the functions created in the previous example can be
passed as parameters in the same way you would pass an object containing only data.
// A function that will execute any number of functions it is passed
function noisyFarmyard() {
// Did we receive any parameters?
if (arguments.length > 0) {
// Loop around those parameters checking to see if any of them are of type ‘function’
for (var i=0; i<arguments.length; i++) {
if (typeof arguments[i] === 'function') {
arguments[i]();
}
}
}
}
noisyFarmyard(pigSays, sheepSays);


Function objects can be passed as
arguments just like any other object

12

1st Class Functions: Functions as Arguments
Since JavaScript treats a function is an object, the functions created in the previous example can be
passed as parameters in the same way you would pass an object containing only data.
// A function that will execute any number of functions it is passed
function noisyFarmyard() {
// Did we receive any parameters?
if (arguments.length > 0) {
// Loop around those parameters checking to see if any of them are of type ‘function’
for (var i=0; i<arguments.length; i++) {
if (typeof arguments[i] === 'function') {
arguments[i]();
}
Because we test the data type of each
}
argument, we can determine whether the value
}
we’ve been passed is executable or not
}
noisyFarmyard(pigSays, sheepSays);


13

1st Class Functions: Functions as Return Values 1/2
Having a function that returns a function is powerful tool for abstraction. Instead of a function returning
the required value, it returns a function that must be executed to obtain the required value.
// A function that works out how many animal functions have been created
function animalList() {
var listOfAnimals = [];
// Check whether a function has been created for each animal
for (var i=0; i<animalNoises.length; i=i+2) {
var fName = animalNoises[i]+"Says";

if (typeof window[fName] === 'function') {
listOfAnimals.push(fName);
}
}
return function() {
return listOfAnimals;
}

Using the array syntax for accessing an object
property, we can check whether the required
function not only exists within the Global Object,
but is also of type 'function'

}

14

Having a function that returns a function is powerful tool for abstraction. Instead of a function returning
the required value, it returns a function that must be executed to obtain the required value.
var listOfAnimals = [];
// Check whether a function has been created for each animal
for (var i=0; i<animalNoises.length; i=i+2) {
var fName = animalNoises[i]+"Says";

if (typeof window[fName] === 'function') {
listOfAnimals.push(fName);
}
}
return function() {
return listOfAnimals;
}

Here, we return a function which when invoked,
will return the array called listOfAnimals

}

15

Once this function is called, it doesn't return the data we require; instead, it returns a function that
when called, will return the required data.
... snip ...
}
// The call to function animalList() returns a function object. So 'animalListFunction' is
// not the data we want, but a function that when executed, will generate the data we want.
var animalListFunction = animalList();


16

Once this function is called, it doesn't return the data we require; instead, it returns a function that
when called, will return the required data.
... snip ...
}
// The call to function animalList() returns a function object. So 'animalListFunction' is
// not the data we want, but a function that when executed, will generate the data we want.
var animalListFunction = animalList();

// 'animalListFunction' must now be executed using the invocation operator.
animalListFunction(); //  ["pigSays","sheepSays"]


17

JavaScript for ABAP Programmers - 5/7 Functions

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