Variables declared outside the if statement cannot be used inside the if statement.

letel='Moji'  

if(el){
el = 'Characters';
console.log(el);  
}

class thumbnail {
    constructor(){
        
        This.DOM = {}
        This.DOM.inputURL= document.querySelectorAll('.dlcard__url>input');
        This.DOM.dlcardWrap= document.querySelector(".dlcardWrap";
        This._addEvent();
    }

    _putThumbnail(){
        This.DOM.dlcardIMGs= document.querySelectorAll('.dlcard__thumbnail>img');
        This.DOM.dlcardIMGs.forEach(el)=>{
            el.addEventListener();
            });
    }

    _addEvent(){
        This.params={}
        This.DOM.dlcardWrap.addEventListener("blur", 
            function(elm){
                const input =elm.target;
                if(input.parentElement.classList.contains("dlcard__url")}
                    const url = new URL (input.value)
                    let pairs=url.search.substring(1).split('&');
                    let params = {}
                    for (let pair of pairs) {
                        let kv = pair.split('=');
                        params[kv[0]]=kv[1];
                    }
                    let videoID = params.v;
                    let dlcard = input.closeest('.dlcard');
                    let thumbnailIMG = dlcard.childNodes[3].childNodes[1].src;
                    if(videoID){
                        let thumbnailURL=`https://img.youtube.com/vi/$ {videoID}/maxresdefault.jpg`;
                        thumbnailIMG = thumbnailURL;
                    } else {
                        console.log('through')
                    }
                    
                }
            }, true);
        
    }
}

new thumbnail();