FAQ: The State Hook - Objects in State

This community-built FAQ covers the “Objects in State” exercise from the lesson “The State Hook”.

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FAQs on the exercise Objects in State

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“Did you notice the square brackets around the name ? This Computed Property Name allows us to use the string value stored by the name variable as a property key!”

I don’t really understand this. What kind of datatype is name?

1 Like

The datatype of name is string. If you look at the input elements, they have an attribute called name (i.e. name="firstName", name="password"). The “firstName” and “password” are strings. The Computed Property Name notation allows us to use these strings as keys in our state object.
In the snippet

{ ...prev, 
[name]: value }

the string name “firstName” or “password” will be converted to property keys in our state object because of the square brackets around name. So, our object will have property keys called firstName and password. The value of the property will be whatever we have entered in the respective input field.
For example, if we enter Jack in the input field for First Name, then the [name]: value notation will mean that the object has a property whose key is firstName and value is “Jack”. If we did something like console.log(formState.firstName), it will output the string value Jack to the console.

1 Like
setFormState((prev) => ({ ...prev }))

Why do we use parentheses to surround the object? Is it to indicate that the object between it is to be returned? I’ve tried the same method in the previous exercise on return [item, ...prev], changing it into ([item, ...prev]), and it worked. But I just want to be sure if that is the case.

3 Likes

I didn’t think the parentheses in this snippet were significant until you posted your comment. Researching and thinking about the topic, the parentheses are indeed necessary in the snippet you posted. It has to do with the implicit return of an object in an arrow function.
If we wanted to explicitly return an object in an arrow function, we would have to use the return keyword and use curly braces:

setFormState((prev) => {
  return {...prev};
})

However, we want to do an implicit return of the object. If we tried to do something like:

setFormState((prev) => {...prev})

this wouldn’t capture our intent. We intend to implicitly return the {...prev} object, but JavaScript looks at the curly braces and decides that this is actually a block of code and not an object.
To implicitly return the object, we need the parentheses as you posted above:

setFormState((prev) => ({...prev}))

Have a look at this article. It is a short read, but it explains the issue very nicely with examples.

As regards the previous exercise, we are returning a list explicitly. Furthermore, the return keyword and the opening bracket of the list are on the same line. So, we don’t need parentheses.
return [item, ...prev]; does the job just fine.

3 Likes

Hi,

I can’t understand why do we need this braces (marked in arrows)? Is there an other way? Thanks!

setProfile((prevProfile) => → ( <–{
…prevProfile,
[name]: value
} → ) <–);

 const handleChange = ({ target }) => {
    const {name, value} = target
    setProfile((prevProfile) => ({
      ...prevProfile,
      [name]: value
      }));
  };

Have you looked at the previous post? It is the same reason about the implicit return of an object in an arrow function.
In the arrow function in setProfile, we want to return the object { ...prevProfile, [name]: value }

If you want to do an explicit return, you could do something like:

    setProfile((prevProfile) => {
        return { ...prevProfile, [name]: value };
    });

However, if we want to do an implicit return, then

    setProfile((prevProfile) => { ...prevProfile, [name]: value } );

will NOT work. We want to implicitly return the object, but the compiler will think that the curly braces { } aren’t meant to denote an object but a block of code constituting the body of the arrow function.
If we want to do the implicit return of the object, we need the parentheses:

    setProfile((prevProfile) => ( { ...prevProfile, [name]: value } ) );
6 Likes
    value={profile.password ||  ' '}   

can anyone tell me why we use or operator with single ’ ’ ?

I guess it might mean that this input field could also be left blank (as a string)?

I am confused as to how the state is stored.
Are there 4 different profile objects, each with a key:value pair? Or is profile just one large object holding 4 key:value pairs?

The latter case.

profile is one object and is initially an empty object. As we type into one of the fields, the handleChange event handler comes into play and the setProfile state setter updates the state.

In the state setter, the spread syntax ...prevProfile copies all the key value pairs in the existing profile.

[name]: value then creates a new entry in the object. If a key doesn’t exist in the profile, then this will create a new key value pair in the object. If the key-value already exists in the object, then it will overwrite that value. This is why the syntax is

{
  ...prevProfile,
  [name]: value
}

If we reversed the order,

{
  [name]: value,
  ...prevProfile
}

then it wouldn’t work as intended, because this snippet will first create our key-value pair and then copy all the existing key-value pairs from the previous profile object. If the previous profile object already has the same key as the new key-value pair, then the previous key-value pair will overwrite our new key-value pair.

Perhaps, this example will make things more clear.

const oldObj = {
  firstNum: 43,
  secondNum: 66
};

console.log(oldObj);

const newObj = {
  ...oldObj,
  firstNum: 60
}

console.log(newObj);

// Output:
// { firstNum: 43, secondNum: 66 }
//  { firstNum: 60, secondNum: 66 }

versus

const oldObj = {
  firstNum: 43,
  secondNum: 66
};

console.log(oldObj);

const newObj = {
  firstNum: 60,
  ...oldObj
}

console.log(newObj);

// Output:
// { firstNum: 43, secondNum: 66 }
//  { firstNum: 43, secondNum: 66 }
2 Likes

I used google developer tools to investigate the event handler and confused as to what exactly is going on in that function:

As per the screenshot, when you go into the setProfile function, isn’t there supposed to be a prevProfile value as a variable? (there’s nothing regarding prevProfile in the scope)

Also, as shown, the values for firstname and lastname is already determined even before running the entirety of the code (including the return function at the bottom):

profile = {firstName: ‘f’, lastName: ‘h’}
already assigned

confused.

In the lessons, to update the state for an object we create a new object and insert the data from the previous one into it. Would it also be fine to just append/overwrite the changed data in the previous object and return that? Or does the lesson’s method have a specific advantage? For example:

// In the lesson:
setFormState((prev) => ({
  ...prev,
  [name]: value
}));

// Also acceptable?:
setFormState((prev) => {
  prev[name] = value;
  return prev;
});

Wow, I needed to take an hour or so to decode the example in the lesson. Thought I’d share my insights here, hopefully if I misunderstood something, someone will tell me!

We initialise formState as an object to avoid getting errors as we will try to access properties of an object in this code, so an object needs to be defined to start with.

Inside handleChange(), we unpack the name and value properties from the event.target object using destructuring.

Inside the state setter callback function, we use curly brackets to indicate that we’ll return a new object (and parentheses because it’s an implicit return in an arrow function, thank you @mtrtmk for the in-depth explanations!).

We first use the spread operator to copy over the values of the previous object so that we don’t lose the values stored for inputs with any other name.

Then we create a new key-value pair or overwrite the appropriate key with its updated value, depending on which input handleChange() was called.

With value={formState.firstName} we assign the value of the property firstName on the formState object, to the input’s value. This property is created with the line [name]: value inside the state setter callback function.

The value of the input’s tag name attribute is contained in the event.target.name (unpacked as name in the example code), so if the value of the name tag is firstName, a property of firstName will be created on the object.

When calling handleChange() at each keyboard stroke, we add character by character what the value of formState.firstName will be.

1 Like

I was wondering why we use onSubmit for the form element as opposed to onClick for the button element.

This helped explain if anyone else was curious:
StackoverFlow link

A button with type “button” won’t submit a form but one with no type or type=submit (the default) will. Buttons with type=submit are nearly the same as inputs with type=submit but buttons are able to contain HTML content.

Hey guys,
This video helped me a lot to set down this entire topic.

enjoy :slight_smile: