Insert multi-select checkboxes

multiselect.js

let choics=['A','B','C']; // Your options

    let optns=[];
    let opts='';
    for (let i=0, len=choics.length; i<len;++i){
        let opi='opt'+i;
        let s=`<label style="display: inline-flex;">
        <input type="checkbox"></input>${choics[i]}</label>`;
        optns.push(s);
    }
    opts=optns.join('\n');

    document.body.insertAdjacentHTML('afterbegin',`<div style="width: max-content">
    <div style="position: relative;min-width: 17.7ch;">
      <select style="width: 100%;font-weight: bold;">
        <option style="color:black">Select types</option>
      </select>
      <div style="position: absolute;left: 0;right: 0; top: 0; bottom: 0;"></div>
    </div>
    <div style="border: buttonface 0.2ch outset;display: flex !important;flex-direction: column !important;">
      ${opts}
    </div>
    </div>`);

    let sels=document.body.firstElementChild;
    
    let sopt=sels.getElementsByTagName('OPTION')[0];
    let cOpts=[...sels.getElementsByTagName('INPUT')];
    
let el=…;//Place checkboxes around element
el.insertAdjacentElement('…',sels); // Place checkboxes
    let p=[...sels.getElementsByTagName('INPUT')];//All checkboxes
    
let toFilt=[...document.querySelectorAll('…')]; //To filter with checkboxes

let choiceInp=()=>{ // On check!
    let toDisplay=[];
    
    p.forEach( (opt)=>{ // Each checkbox option
        let ptx=opt.parentElement.innerText; // Checkbox label text
        if(opt.checked){
            toFilt.forEach((u,x)=>{
                if(!toDisplay.includes(x) && //Mandatory condition
                    … // Insert user condition(s) here!
                  ){
                    toDisplay.push(x);
                }
            });
        }
    });
    
    for (let i=0, len_i=toFilt.length; i<len_i;++i){
            let iflt=toFilt[i];
        if(toDisplay.includes(i)){
            //iflt.style.display='block';
        }else{
            //iflt.style.display='none';
        }
    }
};
 cOpts.forEach(c=>{
        c.oninput=choiceInp;
    });

p.forEach((i,x)=>{
        i.checked=true;
});
choiceInp();