Original subtitles

Instructor: Now with the changes we made

over the last lectures,

we are able to gather our expense data,

our user input and combine it into this object

and clear the form thereafter.

The only problem is that having this data is nice

but we technically don't need it

in the expense form component, do we?

in the expense form component, do we?

Instead we needed in the new expense

or to be precise in the app JS component even

or to be precise in the app JS component even

because there we have our expenses array

and ultimately our goal will be to add this new expense

which the user entered to this list of existing expenses

which the user entered to this list of existing expenses

and we probably also wanna enrich it by adding an ID.

So, we need to pass the data which we're collecting

and generating in expense form to the app component.

Now I put as point, we only learned

how we can pass data down.

So how we can, for example, in the expenses folder

pass title, amount and date from the expenses component

pass title, amount and date from the expenses component

to the expense items component.

So from parent to child, that's something we learned.

We can do that with props,

but how can we do it in the other direction

but how can we do it in the other direction

if data is generated in the expense form

and we then wanna pass it up to the app component?

Well, actually we already saw how it works

but it's easy to miss it.

So let me show it to you again.

So let me show it to you again.

In expense form, we are listening to user input

to change this to the title input, for example

and whenever the user types here,

this function title change handler executes

this function title change handler executes

and there we get this default event object,

that's something the browser gives us.

that's something the browser gives us.

Now, actually we can think about this input element here

as a component as well.

It's not one of our components

but it is simply a pre-built component,

we could say, provided to us by react

and of course translate it to the input Dom element

but it has this component character in the end

but it has this component character in the end

and we do also set some props on that component

including this special onchange prop.

Now, actually this onchange prop isn't that special,

it's just a prop named onchange

which wants a function as a value

which wants a function as a value

and then internally the input element

adds this event listener.

So react basically sees that we set a value

on this onchange prop and adds that listener

on the rendered input element.

But that is a pattern we can replicate

for our own components as well.

for our own components as well.

We can create our own event props,

if we wanna call it like, and we can expect functions

as values and that would allow us

to pass a function from a parent component

to a child component and then call that function

to a child component and then call that function

inside of the child component.

And when we then call a function,

we can of course pass data to that function as a parameter

and that's how we can communicate up from child to parent.

and that's how we can communicate up from child to parent.

Now, in case this was too quick,

let me simply show it to you.

Let's say we wanna pass the expense data

which we gather in the expense form component

which we gather in the expense form component

to the new expense component as a first step

because if we ultimately wanna reach the app component,

because if we ultimately wanna reach the app component,

we first of all have to reach the new expense component

because it's the new expense component

which uses the expense form

which uses the expense form

and then in a second step later, it's the app component

which use the new expense component

which use the new expense component

but we can't skip components in between,

that's also something you learned

in the previous course section.

Props can only be passed from parent to child,

we can't skip intermediate components.

So therefore as a first step,

let's make sure we can pass the expense data to new expense

let's make sure we can pass the expense data to new expense

and we can do it as by adding a new prop to expense form.

It's our components, so we can name it however we want,

but I will name it on save expense data,

but the name is totally up to you,

I'm just naming it on something

because I wanna make it clear

that the value for this prop should be a function,

that the value for this prop should be a function,

a function which will eventually be triggered

when something happens inside of this component.

In this case, when the user saves the entered expense data,

so when the form is submitted in the end.

so that's again a convention I'm following,

you don't have to name it like this,

you can name it however you want,

It's totally up to you.

It doesn't have to start with on.

It doesn't have to start with on.

I just do start with on to follow this convention

and to make it clear that the value for this prop

should be a function which can then be called

inside of the expense form component.

Hence, it's a function we should define here in new expense

Hence, it's a function we should define here in new expense

just as we did it for the input elements

with the functions we bound to the onchange prop,

we define those in the expense form

before we returned the JS Xcode as well.

before we returned the JS Xcode as well.

Now we're doing the same here one level above that

in the new expense component

for our own custom expense form component.

So here I'll add a const save expense data handler

and important as a parameter here,

I will expect the entered expense data.

The name of the parameter is up to you, it's your function

but now I make it clear that this function expects

to get this parameter.

And then here we can add our expense data

another object, copy in the entered expense data

another object, copy in the entered expense data

which I expect to be that object

which we generate here into submit handler,

so this object, I pull out all the key value pairs

and add them to this new object

and then I add in a new key, the ID key

and simply set that to math random.

Math random to string to convert it to a string.

Math random to string to convert it to a string.

It's not a perfect unique ID,

theoretically we could generate the same value twice

but it's good enough for this demo.

And then we can see what we do with that thereafter,

for the moment I'll just log does enriched expense data.

Now it's a pointer at this function,

which I wanna pass as a value to all onSave expense data,

so that this onSave expense data prop

in my custom component receives this function as a value.

in my custom component receives this function as a value.

Again, I don't execute it here,

we don't add parenthesis,

I just point at the function,

so that the function itself a pointer edit

is passed to expense form.

Now that's the first step.

The second step now is to use this function

inside of our custom component.

That's a step we didn't have to do for the inputs

because these are built in components basically,

because these are built in components basically,

but they're also we pass a function to onchange

and internally react and we'll add a listener

and call this function which we pass in

whenever that event occurs, that change event.

Now, since we're doing this on our own custom component,

we also have to call the passed in function manually

and that's what I'll do next.

and that's what I'll do next.

So inside of expense form, we now can expect this

onSave expense data prop because I am setting it

when I used the expense form component,

when I used the expense form component,

hence instead of expense form,

we can extract the value passed

for this prop ie, this function.

So, inside of expense form,

I of course now expect to get some props

because we're setting a prop now

because we're setting a prop now

and now, inside of the submit handler,

instead of I logging my expense data,

I will access props onSave expense data and execute it here.

And that's now important,

now I execute it.

I can execute it because the value which we get on this

onSave expense data key will be a function.

We are passing in a function here after all.

So it's this function defined in the new expense component

which we will now execute in a different component,

inside of expense form to be precise.

inside of expense form to be precise.

And we can execute the function

even though it's not defined inside of expense form

because we are passing a pointer at it

through the onSave expense data prop.

through the onSave expense data prop.

And this is a super important pattern

which you will use a lot in react.

This is how you can communicate between components

and how you can communicate up,

and how you can communicate up,

how you can make sure that a child component,

the expense form component here for example,

can communicate up to the parent component,

the new expense component in this case.

the new expense component in this case.

We can call a function in the new expense component

and we can pass data as a parameter.

and we can pass data as a parameter.

So here, when we call onSave expense data

in the expense form, I can pass the expense data

which are generated here as our argument

and that's the value which we'll receive

as a parameter here in new expense.

as a parameter here in new expense.

I hope this flow is clear.

The trick really is that we pass around

a pointer at a function.

Now, if we go back and I reload,

if I again, enter something here

and pick a date if I click add expense,

and pick a date if I click add expense,

you see this is being locked to the console

and it's being locked from the new expense file line 12,

so it's does log here.

so it's does log here.

And we can tell that it's from there

because we also have an ID field there

which has this random number

which has generated and converted to a string.

which has generated and converted to a string.

And that's how we can communicate up

and this is a super important pattern.

Now, of course, we can continue in this chain now

and we can communicate up from inside new expense to app

and we can communicate up from inside new expense to app

because it's this app component which needs

the new expense in the end to add it to this expenses array.

So therefore again, in app JS,

we can add a function which we defined

before we return JSX code

before we return JSX code

and we can simply name this at expense handler

and be expect to get our expense here as a parameter

and be expect to get our expense here as a parameter

and then we do something with it.

and then we do something with it.

And for the moment I'll just console log in app JS

and console log the expense

because we haven't quite yet learned

how we can render lists of data dynamically

and therefore for the moment,

I will not update the expenses array,

that is something we'll do

and learn about in the next course section though.

For the moment, I just wanna confirm that the data arrived.

For the moment, I just wanna confirm that the data arrived.

So here's the function now again,

using the same pattern as before,

we can pass a pointer at this function to a new expense

we can pass a pointer at this function to a new expense

so that inside of new expense,

we can call this function and pass that expense data

up to the app component.

So we could name this prop here on add expense,

again, the name is up to you, it's your component

but again, I will follow this convention

starting with on to make it clear

that it's a function pointer

which has passed as our argument

and then I pass a pointer at add expense handler

to the on expense prop on new expense,

and therefore inside of new expense, we can now call it.

We can accept the props argument here as well,

this parameter and in safe expense data handler

this parameter and in safe expense data handler

which itself is called when something happens

in the expense form, we can of course

in the expense form, we can of course

now also called props on add expense

and of course here, we wanna use the name

which we chose here in the app component.

We're calling the function past as a value

for that on add expense prop.

I'm calling us here, and it will forward

my enriched expense data here.

And if we now save everything,

again if I reload, and I add a book here for 1299,

if I click add expense, this is being logged

from inside the app component as we can clearly tell

and I'm actually logging the expenses as I see,

I wanted to log the expense instead,

so let me fix this.

Let me give this another try, a book 1299,

pick a date, yeah, so now we're logging the single expense.

Again, we'll use that expense

and change the array and output DRA dynamically too,

we'll do that in the next course section

but for the moment I wanted to focus

on that state and event handling thing

and being able to pass data up is a crucial part of it

and here's how it works.