Handling a Form Submit

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Hey! Our repLogs live in state! And so, I think it's finally time to add some magic to our form and get it functional. Here's our next goal: when the user submits this form, we want to take its data and update the repLogs state so that a new row is rendered in the table.

The form itself lives in RepLogs, near the bottom. But, the state we need to modify lives in our parent: RepLogApp. To communicate back up the tree, we'll follow a familiar pattern: pass a callback from parent to child, just like we did with onRowClick.

Adding the onSubmit Behavior

Start in RepLogApp: add the handler function: handleNewItemSubmit() with an event object. To prevent the form from actually trying to submit, use event.preventDefault(), just like normal JavaScript.

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export default class RepLogApp extends Component {
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handleNewItemSubmit(event) {
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For now, log some stuff! I love when a good form submits! Oh, and also log event.target. Because this function will handle the form element's submit, event.target will be the form itself. We're going to need that so we can read the values from its fields.

... lines 1 - 24
handleNewItemSubmit(event) {
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console.log('I love when a good form submits!');
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Pass this callback as a new prop: onNewItemSubmit = {this.handleNewItemSubmit}. And, hey! We're starting to see a naming convention. This isn't anything official, but I like to name my methods "handleSomeEvent" and my props "onSomeEvent".

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render() {
return (
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In RepLogs, head straight down to propTypes to describe the prop: onNewItemSubmit is a required function.

... lines 1 - 90
RepLogs.propTypes = {
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onNewItemSubmit: PropTypes.func.isRequired,
... line 96

Love it! Back in render, destructure this into a variable. So: how can we attach a "submit" listener to the form? Ah... it's just onSubmit={onNewItemSubmit}.

... lines 1 - 15
export default function RepLogs(props) {
const { withHeart, highlightedRowId, onRowClick, repLogs, onNewItemSubmit } = props;
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return (
... lines 25 - 51
<form className="form-inline" onSubmit={onNewItemSubmit}>
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So simple! Go over to the browser and give it a nice refresh! Select an item... fill in a number and... we got it! Every time we submit by pressing enter or clicking the button, we see our insightful message. And as promised, the event.target that we're logging is literally the raw, form DOM element.

This is actually really nice. React always guarantees that event.target will be the element that you attached the listener to.

Reading the Form Data

Next question! How can we read the values from our fields? Look at the form in RepLogs: there's the select element and... the text area. Check it out: it has a name attribute: reps. We can use that and normal, boring JavaScript to find that field and get its value.

By the way... if you've read a little bit about forms and React, this might not be what you were expecting. Don't worry. I'm going to show you a few different ways to get the values from form fields, including the pros and cons of each, and which method I recommend and when.

But right now, forget about React, and remember that, under the hood, there is a boring HTML form sitting on the page that we can interact with.

In RepLogApp, it's time to flex our native JavaScript muscles! To read the reps textarea, use event.target - that's the form - .elements.namedItem('reps'). This will give us the text element. Reads its value with .value.

... lines 1 - 24
handleNewItemSubmit(event) {
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... lines 31 - 46

Let's go try it! Move over, refresh... select "My Laptop" and lift it 50 times. Yes! There's the 50! Victory!

Keep your Smart Component Unaware of Markup

But, before we go further, I need to ask an important philosophical question:

If your shirt isn't tucked into your pants, are your pants tucked into your shirt?

Hmm. Thought provoking. And also: if our smart component - RepLogApp - should not be responsible for rendering any HTML, should its handleNewItemSubmit() method be aware that there is an HTML form and a field with a name="reps" attribute inside?

Actually... no! It makes no sense for handleNewItemSubmit() to suddenly be aware of a specific HTML structure that's rendered by its child. In fact, all RepLogApp should care about is that, when - somehow - a new rep log is created in the app, its handleNewItemSubmit() function is called so that it can update the repLogs state. If it's created with a form, or with some random fields during a 10-step process or just with black magic... RepLogApp should not care!

So, check this out: copy the inside of the function: I'm going to move most of this callback into RepLogs as a new handler function. Inside render(), add a new function: handleFormSubmit() with our normal event argument. Then, paste the logic.

... lines 1 - 23
function handleFormSubmit(event) {
console.log('I love when a good form submits!');
... lines 30 - 105

Down in onSubmit, instead of calling the parent handler, call the new function: handleFormSubmit.

... lines 1 - 30
return (
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<form className="form-inline" onSubmit={handleFormSubmit}>
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Yep, this feels much better. handleFormSubmit() is responsible for calling event.preventDefault() and uses the form structure - which is created right inside this component - to read the names of the fields. Finally, at the bottom, call the parent handler: onNewItemSubmit().

... lines 1 - 23
function handleFormSubmit(event) {
... lines 25 - 29
onNewItemSubmit('Big Fat Cat', event.target.elements.namedItem('reps').value);
... lines 32 - 107

Actually, this is the reason why I put the new function inside of render() instead of above the function like I did with calculateTotalWeightFancier(): our callback needs access to the props.

Here's the last important part: instead of passing the event object or the form element to the parent onNewItemSubmit() callback, only pass it what it needs: the new rep log's raw data. For now, hardcode an item name - "Big fat cat" - but copy the number of true rep logs and paste.

... lines 1 - 24
function handleFormSubmit(event) {
... lines 26 - 30
onNewItemSubmit('Big Fat Cat', event.target.elements.namedItem('reps').value);
... lines 33 - 110

Back in RepLogApp, clear out handleNewItemSubmit and give it two fresh args: itemName and reps. Log a todo below: we will eventually use this to update the state. And log those values so we can check things!

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handleNewItemSubmit(itemName, reps) {
console.log('TODO - handle this new data');
console.log(itemName, reps);
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I love it! RepLogApp still has a callback, but it's now unaware of the form. It doesn't care how rep logs are created, it only cares that its callback is executed when that happens. All the form logic is right where it should be.

Try it out! Refresh the page, select an item, enter 45 and... submit! The Big fat cat is hardcoded, but the 45 is our real data.

As simple as it is to read the values of the fields by using the name attribute, you probably won't do this in practice. Instead, we'll learn two other ways: refs & state. We'll jump into refs next.

Leave a comment!

  • 2018-12-04 mouerr

    Hi Victor,
    Thanks for your support.

  • 2018-12-04 Victor Bocharsky

    Hey Mouerr,

    Good question! But the answer is simple: we just don't need it for now. You need to understand why we call that .bind(this). Actually, we'll do it later in https://symfonycasts.com/sc... , but for now that binding context is not important because we just call console.log() inside, i.e. don't use "this" at all.


  • 2018-12-04 mouerr

    why you didn't add handleNewItemSubmit to the constructor and bind it like
    this.handleRowClick = this.handleRowClick.bind(this);