Uploading multiple images and convert them to base64?

I'm trying to create a form where you can upload multiple images using two file inputs, see the images as base64 and later submit them using jQuery's POST. The problem is that when I select 3 different images, I see only the last one 3 times. I guess the problem is in the for in the addEventListener - maybe it's not in the right place?

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-html -->

<div class="form-group">
<label for="product_photo_1">Photos 1</label>
<div class="custom-file">
<input type="file" class="custom-file-input" id="product_photo_1" multiple />
<label class="custom-file-label" for="product_photo_1" data-browse="Select">Please, select a file...</label>
</div>
<div id="product_photo_1_result"></div>
</div>
<div class="form-group">
<label for="product_photo_2">Photos 2</label>
<div class="custom-file">
<input type="file" class="custom-file-input" id="product_photo_2" multiple />
<label class="custom-file-label" for="product_photo_2" data-browse="Select">Please, select a file...</label>
</div>
<div id="product_photo_2_result"></div>
</div>
<script>
const convertBase64 = (file) => {
return new Promise((resolve, reject) => {
const fileReader = new FileReader();
fileReader.readAsDataURL(file);
fileReader.onload = () => {
resolve(fileReader.result);
};
fileReader.onerror = (error) => {
reject(error);
};
});
};
const uploadImage = async (event, id) => {
const file = event.target.files[0];
const base64 = await convertBase64(file);
document.getElementById("product_photo_" + id + "_result").innerHTML += '<img src="' + base64 + '" class="img-thumbnail m-2" id="product_photo_' + id + '[]" style="height: 50px;" />';
};
document.getElementById("product_photo_1").addEventListener("change", (event) => {
for(var index = 1; index <= document.getElementById("product_photo_1").files.length; index++) {
uploadImage(event, 1);
}
});
document.getElementById("product_photo_2").addEventListener("change", (event) => {
for(var index = 1; index <= document.getElementById("product_photo_2").files.length; index++) {
uploadImage(event, 2);
}
});
</script>

<!-- end snippet -->

In uploadImage function you are passing event argument and select first file every time

const file = event.target.files[0];

Instead you can pass files one by one to the uploadImage.

Here is the example:

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-html -->

<div class="form-group">
<label for="product_photo_1">Photos 1</label>
<div class="custom-file">
<input type="file" class="custom-file-input" id="product_photo_1" multiple />
<label class="custom-file-label" for="product_photo_1" data-browse="Select">Please, select a file...</label>
</div>
<div id="product_photo_1_result"></div>
</div>
<div class="form-group">
<label for="product_photo_2">Photos 2</label>
<div class="custom-file">
<input type="file" class="custom-file-input" id="product_photo_2" multiple />
<label class="custom-file-label" for="product_photo_2" data-browse="Select">Please, select a file...</label>
</div>
<div id="product_photo_2_result"></div>
</div>
<script>
const convertBase64 = (file) => {
return new Promise((resolve, reject) => {
const fileReader = new FileReader();
fileReader.readAsDataURL(file);
fileReader.onload = () => {
resolve(fileReader.result);
};
fileReader.onerror = (error) => {
reject(error);
};
});
};
const uploadImage = async (file, id) => {
const base64 = await convertBase64(file);
document.getElementById("product_photo_" + id + "_result").innerHTML += '<img src="' + base64 + '" class="img-thumbnail m-2" id="product_photo_' + id + '[]" style="height: 50px;" />';
};
document.getElementById("product_photo_1").addEventListener("change", (event) => {
let files = document.getElementById("product_photo_1").files;
for (let index = 0; index <= files.length; index++) {
uploadImage(files[index], 1);
}
});
document.getElementById("product_photo_2").addEventListener("change", (event) => {
let files = document.getElementById("product_photo_2").files;
for (let index = 0; index <= document.getElementById("product_photo_2").files.length; index++) {
uploadImage(files[index], 2);
}
});
</script>

<!-- end snippet -->

In the event handlers, you are doing a loop, but you are not using the value of index per iteration:

    document.getElementById("product_photo_1").addEventListener("change", (event) => {
for(var index = 1; index <= document.getElementById("product_photo_1").files.length; index++) {
uploadImage(event, 1); //passing only the event and an index
}
});
document.getElementById("product_photo_2").addEventListener("change", (event) => {
for(var index = 1; index <= document.getElementById("product_photo_2").files.length; index++) {
uploadImage(event, 2); //passing only the event and the index
}
});
And now, let's use `index` for all calls as well as clearing up when it's 0:

const uploadImage = async (event, id, index) => {
let context = document.getElementById("product_photo_" + id + "_result");
const file = event.target.files[index]; //we get the correct file;
const base64 = await convertBase64(file);
let template = '<img src="' + base64 + '" class="img-thumbnail m-2" id="product_photo_' + id + '[]" style="height: 50px;" />';
if (index === 0) context.innerHTML = template; //clearing older values and adding the first element
else context.innerHTML += template; //appending the (not first) element
};

Instead, you will need to pass the index as well, like this:

    document.getElementById("product_photo_1").addEventListener("change", (event) => {
for(var index = 0; index < document.getElementById("product_photo_1").files.length; index++) {
uploadImage(event, 1, index);
}
});
document.getElementById("product_photo_2").addEventListener("change", (event) => {
for(var index = 0; index < document.getElementById("product_photo_2").files.length; index++) {
uploadImage(event, 2, index);
}
});