How can I get the value of a bootstrap4 checkbox?

I have the following checkbox:

<label class="custom-control custom-checkbox">
     <input type="checkbox" class="custom-control-input" id="modal_checkbox">
     <span class="custom-control-indicator"></span>
     <span class="custom-control-description">Do not show me this again.</span>
</label>

But I don't know how to get the value of the checkbox using jquery. I tried to get the attr or the prop of the above classes and ID but I get an "undefined"