Custom String.prototype.indexOf

string.customIndexOf.js

// String: Custom Index Of
String.prototype.customIndexOf = function(searchValue, fromIndex) {
  var notfound = -1;

  // Validating param: fromIndex
  // fromIndex needs to be type number and needs to be an integer
  if (typeof fromIndex !== 'number' || isNaN(fromIndex)) {
    fromIndex = parseInt(fromIndex);
  }

  if (typeof fromIndex === 'number' && !Number.isInteger(fromIndex)) {
    fromIndex = 0;
  }

  // For fromIndex values lower than 0 or greater than str.length,
  // the search starts at index 0 and str.length respectively.
  if (!fromIndex || fromIndex < 0) {
    fromIndex =  0;
  } else if (fromIndex > this.length) {
    fromIndex = this.length;
  }

  // Validating param: searchValue
  // searchValue needs to be type string
  if (typeof searchValue !== 'string' && !Array.isArray(searchValue)) {
    return notfound;
  }

  // searchValue array quirks
  if (Array.isArray(searchValue)) {
    return 0;
  }

  // searchValue empty string quirks
  if (searchValue === '') {
    if (fromIndex >= this.length) {
      return this.length;
    } else if (fromIndex < 0) {
      return 0;
    } else {
      return fromIndex;
    }
  }

  var stringLength = this.length;
  var searchValueLength = searchValue.length;
  var count = 0;

  if (searchValueLength > stringLength) {
    return notfound;
  }
  
  // Loop through string and searhValue to find index
  for (var i = 0; i <= stringLength - searchValueLength; i++) {
    for (var j = 0; j < searchValueLength; j++) {
      if (this[j+i] === searchValue[j]) {
        count++;
        if (count === searchValueLength && i >= fromIndex) {
          return i;
        }
      } else {
        count = 0;
        break;
      }
    }
  }
  return notfound;
}

window.customIndexOfApp = window.customIndexOfApp || {};

// Compare String.indexOf result and String.customIndexOf
window.customIndexOfApp.assertMethods = function(string, searchValue, fromIndex) {
  var indexOf = string.indexOf(searchValue, fromIndex);
  var customIndexOf = string.customIndexOf(searchValue, fromIndex);
  var styleLog = {
    success: {
      label: 'padding: 10px 0; color: green; font-weight: 700',
      value: 'font-size: 12px; font-weight: 700'
    },
    error: {
      label: 'padding: 10px 4px; color: red; font-weight: 700',
      value: 'color: black; font-size: 12px; font-weight: 700'
    },
    default: {
      value: 'color: black'
    }
  }

  if (indexOf === customIndexOf) {
    console.info('%c Success: '+'%cboth methods the value: '+'%c'+indexOf+'',
      styleLog.success.label,
      styleLog.default.value,
      styleLog.success.value
    );
  } else {
    console.error('%c Error: '+'%cindeOf is: %c'+ indexOf +'%c and customIndexOf is: %c'+customIndexOf+'',
      styleLog.error.label,
      styleLog.default.value,
      styleLog.error.value,
      styleLog.default.value,
      styleLog.error.value
    );
  }
}

// Test cases
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', 'W');
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', 'White');
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', 'GREEN');
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', 'Green');
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', 'green');
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', 'grEEn');
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', 'Bla');
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', 'bLa');
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', 'AXBXC');
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', '1');
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', '123');
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', '');
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', ' ');
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', '  Qwerty');
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', '  Qwerty', 0);
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', '  Qwerty', -1);
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', '  Qwerty', 9999);
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', 'Blue', 0);
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', 'Blue', 1);
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', 'Blue', -1);
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', 'Blue', -100000);
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', 'e', 10);
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', 'xxx', 0);
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', 'XXX', 0);
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', '1B2C3A', 35);
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', 'red', 0.5);
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', 'red', -0.234);
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', 'red', '100');
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', 'red', '100');
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', 'red', '-100000');
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', 'red', '100000');
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', 's', '0');
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', '@');
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', '$', 10);
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', '$', '!');
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', '#', -1);
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', '', []);
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', 'ABC', {});
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', 'S', undefined);
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', undefined);
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', null);
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', []);
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', {});
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', new Function);
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', undefined);
window.customIndexOfApp.assertMethods('White Blue GREEN red Black ABC 123 XXX  Qwerty', null);

How to run

String.prototype.customIndexOf('string', 'searchValue', 'fromIndex');

Tests Cases

To see my tests cases in a cool console.log style please run this on Snippets from Chrome Developer Tools.
See here how to.