Functions

Example

This example is just like the previous, except that we are passing a function as a message argument rather than the simple value.

In this example "fct" is also called a callback function (a function passed as an argument).

Sender

let holder, fct;
holder = new NumberHolder(10);
fct = n => n + 2;
holder.changeUsing(fct);

Receiver

Examples

Here are four options for different ways to pass a function as a message argument.

	passArrowFunction1() {
		let holder, fct;
		holder = new NumberHolder(10);
		fct = n => n + 2;
		holder.changeUsing(fct);
		console.log(holder);
	}
	
	passArrowFunction2() {
		let holder;
		holder = new NumberHolder(10);
		holder.changeUsing(n => n + 2);
		console.log(holder);
	}

	passTradDeclaredFunction() {
		//declared function
		let holder, fct;
		holder = new NumberHolder(10);
		function increaseByTwo(n) {
			return n + 2;
		};
		holder.changeUsing(increaseByTwo);
		console.log(holder);
	}

	passTradFunctionExpression() {
		//function expression
		let holder, increaseByTwoFct;
		holder = new NumberHolder(10);
		increaseByTwoFct = function(n) {
			return n + 2;
		};
		holder.changeUsing(increaseByTwoFct);
		console.log(holder);
	}

Simple Scope Example

Here we have a very simple JavaScript class.

When we run it, we get this output:

Middleware
wow!!

class Middleware {
	static wow() {
		console.log("wow!!");
    }
	static home(req, res) {
		console.log(this.name);
		this.wow();
    }
}
Middleware.home();

Express Framework Breaks Scope

When we use the Express framework (for Node.js) and route the home path "/" to the home method our code breaks.

The reason is that Express does not preserve the simple scope within the class and "this" is undefined.

Some of the Mongoose does similar. We should expect other frameworks may do similar.

app.get('/', Middleware.home);

Solution #1

This is probably the easiest solution.

We simply add an arrow function that then calls the class static method in the normal way.

The usage of "this" is then as expected.

//app.get('/', Middleware.home);
app.get('/', (req, res) => Middleware.home(req, res));

Solution #2

We could also change the original code. This is generally not desireable being that we coded our objects a certain way for a reason.

	static home(req, res) {
		console.log(this.name);
		Middleware.wow();
}

Solution #3

This solution is just a reminder to keep out "heads up" regarding function types.

In some cases we may run into (in Mongoose, for example) the suggestion is to use traditional functions over arrow functions.

Traditional Methods