Vue split components from a single .js file in multiple files

I have created a component inside my index.js (where my main Vue code is). Now I would like to modularize the project and put the component into a separate file. I don’t know how to do that because if I create e.g. optionalapm.js the main index.js returns an error that it can’t locate the vue component optionalapm which I included.

How can I import the component into the index.js?

Here’s my code of the component:

var optionalapm=Vue.component('optionalapm', {
props:['value'],
template:   `<div class="col-l-12 col-m-12 col-s-12 col-xs-emphased" style="background-color: #f9f9f9"">
                <p class="hidden-xl hidden-l hidden-m"></p>
                <p class="hidden-xl hidden-l hidden-m"></p> 
                <h3 style="text-align: center">APM</h3>
                <p>&nbsp;</p>
                <div class="col-l-4">
                    <p style="text-align: center">Number of nodes</p>
                    <div class="form-input-set">
                        <select v-bind:value='value' v-on:input="$emit('input', $event.target.value)" v-on:change="$emit('calcapmprice')" class="form-select" style="background: white">
                            <option value="apmnodes0">
                                <p style="text-align: center">0</p>
                            </option>                                            
                            <option value="apmnodes1">
                                <p style="text-align: center">1</p>
                            </option>
                            <option value="apmnodes2">
                                <p style="text-align: center">2</p>
                            </option>
                            <option value="apmnodes3">
                                <p style="text-align: center">3</p>
                            </option>  
                        </select>
                    </div> 
                </div>
              </div>`,       
})                               

And that’s the relevant code in the index.js:

var ececontent = new Vue({
    el:'#ece-content',
    data: {
        ...
    },
    methods: {
        ...
    },
    components: {
        optionalapm: optionalapm,
    }

});

Answer

You should use Single File Components, as described in the documentation. Next to it, read this useful blog post about using SFC’s.