In the previous chapters, we have already learned that functions in JavaScript are values. Also, each JavaScript value has a type.
Now, it’s time to learn the types of functions.
Functions are objects in JavaScript. It is possible not only to call objects, but to deal with them as objects (add or remove properties, pass by reference, and more).
The Property “name”
The function objects consist of usable properties.
The name-assigning logic gives the right name to a function, even if it’s made without it.
It is actual even when a default value implements an assignment:
This feature is known as “contextual name.”
The object methods also have names.
When there is no way to find out the correct name, the name property turns out empty.
However, as a rule most of the functions have names.
The “length” Property
Another significant property is the “length” property. It is used for returning the number of function parameters.
In the example above, the rest of the parameters are not included.
At times, you can use the length property to introspect in the functions operating on other functions.
Custom Properties
You can choose to add your own properties, as well.
You can use the counter property for tracking the total calls count.
It is visualized in the following example:
Take into account that a property is not a variable. Hence, a property counter and a let counterlet counter variable can not be related to each other.
A function can be treated as an object, store properties, but it won’t affect its execution.
Named Function Expression
The Named Function Expression (NFE) is for the Function Expressions having a name.
An ordinary Function Expression looks like this:
let welcomeSite = function (user) {
console.log(`Welcome to Docs, dear ${user}`);
};You can add a name to it:
let welcomeSite = function func(user) {
console.log(`Welcome to Docs, dear ${user}`);
};Note that adding a name "func" after the function can’t transform it into a Function Declaration, as it is a part of an assignment expression.
The name "func" allows the function referencing itself internally. Moreover, there is no possibility of seeing it outside of the function.
In this case, the welcomeSite function calls itself with “Guest”, if the user is not provided.
But there can be a problem with such codes: welcomeSite might transform into external code. In case, the function is assigned to another variable instead, errors will occur in the code:
Such a scenario happens because the function takes welcomeSite from the external Lexical Environment. No local welcomeSite exists. Hence, the external variable is used. So, during the call, the external welcomeSite may become null .
The optional name you may put into the Function Expression is capable of solving issues like that.
As you can see, it worked here. That’s because the name "func" is function-local. It’s not brought from outside. The external code has its variable welcomeSite or sayWelcome.
Summary
So, it can be stated that functions are objects.
In this chapter, we represented the following properties of the functions:
- name : it’s the name of the function, as a rule, taken from the function definition. In case there is not any, JavaScript may try to guess it from the context.
- length : the arguments’ number in the function definition. Rest parameters are not included.
In case the function is declared as a Function Expression, carrying the name, it is known as a Named Function Expression. It can be used to reference itself, for recursive calls, and more.
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