[рони]

Hokage777,
вариант ...

let val1 = "abc5 abc0 abcde abcfg";
const fn = str => {
    let words = str.split(/\s+/).sort((a,b)=> a.length - b.length);
    let word = words[0];
    for(let a of words){
        a = [...a];
        let temp = "";
      for(b of word){
        if(b == a.shift()) temp += b;
        else break;
      }
       if(temp) word = temp;
       else return 0
    }
    return word.length + 1
}
console.log(fn(val1))