implementation-prototype-string-indexof

Implementation of the String Prototype indexOf method in Vanilla Javascript.

Definition

The indexOf() method returns the index within the calling String object of the first occurrence of the specified value, starting the search at fromIndex. Returns -1 if the value is not found.

Proposal

The first thing came to my mind were the guard clauses that helped me to isolate the code not related to the algorithm itself. Doing this, instead of nested conditionals we got a "flat" list of conditionals, one after the other. It makes the code easier to read too.

The idea behind my solution is to iterate until we find a match with the first letter of the string we are looking for. If we don't find any, we return -1. If we do, we start to check the other character of the given string.

Once we have found the first character, if the given string has length === 1 it means we can return that index. If there are more characters to check, we go for it.

If the next character to check is different than the character in this we leave from that second loop and we go for another match with the first character of the given word.

String.prototype.indexOfNew = function(searchValue, from = 0) {
  // Truncate `from` because we can't secure that it's an integer
  var _fromTruncated = Math.trunc(from);
  if (searchValue === '' && _fromTruncated < 0) {
    return 0;
  }

  if (searchValue === '' && _fromTruncated >= this.length) {
    return this.length;
  }

  if (searchValue === '') {
    return _fromTruncated;
  }

  // GIVEN:
  //	* A string `this`
  // 	* A string to find in `this`
  // 	* An initial position to start searching
  //
  // THEN:
  //	* Returns -1 if the string to find has not been matched in `this`.
  // 	* Returns the position of the first occurence of the string in `this`
  for (var i = _fromTruncated; i < this.length; i++) {
    var thisCharsLeft = this.length - i;
    if (this[i] === searchValue[0]) {
      var initial = i;
      if (searchValue.length === 1) {
        return initial;

      } else if (thisCharsLeft > searchValue.length - 1) {
        for (var j = 1; j < searchValue.length; j++) {
          if (this[++i] !== searchValue[j]) {
            break;
          } else if (j === (searchValue.length - 1)) {
            return initial;
          }
        }
      }
    }
  }

  return -1;
};