Anagram Javascript

Question with below code.
If I’m decreasing the object value and when it comes to zero how it is satisfying this condition if(!obj1[letter]) because letter still exist in obj, its value is just going down.

function validAnagram(a, b){
if(a.length !== b.length){
    return false;
}
if(a.length === 0 && b.length === 0 ){
    return true;
}
    let obj1 = {};

  // add whatever parameters you deem necessary - good luck!
    for(let i= 0; i<a.length; i++){
        if(obj1[a[i]]>0){
            obj1[a[i]]++
        }else{
                   obj1[a[i]] = 1;
        }
 

        
    }
    for(let i =0; i<b.length; i++){
        let letter = b[i];
        if(!obj1[letter]){
            return false
        } else {
            obj1[letter]--
        }
    }
    return true;
}



validAnagram('aza', 'zaz')