Assigning default values for parameters

Question

I'm studying Javascript and I found a problem. I must define the constructor given some parameters. I insert them but I wanted to insert an if to check when the user inserts nothing. I'm working with the parasitic Inheritance model, but I don't know how to insert this case. Here is the part of my constructor:

function user(_id, userName, email, password, firstName, lastName, date_created, playlists) {

if (firstName === 'undefined') {
    firstName = userName;

}
if (lastName === 'undefined') {
    lastName = userName;
}
return {
    _id: _id,
    userName: userName,
    email: email,
    password: password,
    firstName: firstName,
    lastName: lastName,
    date_created: new Date(),
    playlists: []

};
}

Suppose that the user does not use firstName and LastName. It will return undefined. But I want to store a default value. How do I do that?

You can put the `if` logic between the declaration of the function and the `return` statement. In fact, you should not put your logic anyplace else (after the `return` it would not get executed) — SJuan76, Sep 30 '14 at 18:48
possible duplicate of [Set a default parameter value for a JavaScript function](http://stackoverflow.com/questions/894860/set-a-default-parameter-value-for-a-javascript-function) — Eric Alberson, Sep 30 '14 at 18:48
I would think you could use something like: `firstName: (firstName != null) ? firstName : 'default value'` — Joel Etherton, Sep 30 '14 at 18:48
ok so you are saying that I have to put it between function user(...) and return{...} right? — pp94, Sep 30 '14 at 18:49
@EricAlberson they are specific question. Basically I use the code just to ask the explanation of where I have to put my statement — pp94, Sep 30 '14 at 18:50

score 3 · Accepted Answer · answered Sep 30 '14 at 18:55

3

First of all, if this is a constructor you should write it like this:

function User(_id, userName, email, password, firstName, lastName, date_created, playlists) {
    this._id = _id;
    this.userName = userName;
    this.email = email;
    this.password = password;
    this.firstName =  firstName;
    this.lastName =  lastName;
    this.date_created = date_created,
    this.playlists = playlists 
}

This way, you can call it using the new keyword get a new object with the proper values set.

Next, in order to set default values you can do some shortfusing using || on each property. This way the default value is used only when the value is not provided.

function User(_id, userName, email, password, firstName, lastName, date_created, playlists) {
    this._id = _id;
    this.userName = userName;
    this.email = email;
    this.password = password;
    this.firstName =  firstName || "defaultValue";
    this.lastName =  lastName || "defaultValue";
    this.date_created = date_created || new Date(),
    this.playlists = playlists || [] 
}

answered Sep 30 '14 at 18:55

William Barbosa

4,936
2
19
37

Ok but if I have a default value I want to insert "William" and "Barbosa" how can I do it with your implementation? – pp94 Sep 30 '14 at 18:59
If the user provides the names when he calls the function, the value provided is used. If the user supplies no value, "DefaultValue" is used. `new User(1, "willkuns", "foo@bar.baz", "asd", "William")` will set the first name to "William" and the last name to "defaultValue", because only the first name was provided. – William Barbosa Sep 30 '14 at 19:01
Ok. I change my code because the one I inserted is the only that works but I don't understand why the if statements does not works!!! – pp94 Sep 30 '14 at 19:11
Not all falsy values are undefined. Check [this](http://www.sitepoint.com/javascript-truthy-falsy/) – William Barbosa Sep 30 '14 at 19:17

Assigning default values for parameters

1 Answers1