If whitespaces don't matter, this is a solution:

const checkString = (bigString, subString) => {
  const split = bigString.split(subString);
  const onlyOccurances = split.filter(v => v === '');

  return split.length === onlyOccurances.length
}

checkString("abc", "abc") // true
checkString("abcabcabc", "abc") // true
checkString("abcdef", "abc") // false

bigString.split(subString) will split the big string into an array of empty strings if there's perfect match, and the length of it will be exactly how many occurances there are. If any of the values in the array are not empty strings, it means there is not a perfect match so there will be a difference between the length of the filtered by empty and the length of the splited values.

Hope it makes sense.

This method will return an object and found will be true if the value is found in the string and multipleFound will be true if found more than once;

const checkString = function(str, v) {
let found = false,
    multi = false,
    index;
index = str.indexOf(v);
if (index !== -1) {
    found = true;
    index = str.indexOf(v, index + v.length);
    if (index !== -1) {
        multi = true;
    }
}
return {
    found : found,
    multipleFound : multi
};

};

This would be one way to check against the pattern abc:

const rx=/^(abc)+$/;
console.log(["abc","abcabcabc","abcdef"].map(t=>
 `${t} ${rx.test(t)}`))