deps/v8/src/js/array.js
function InnerArrayIndexOf(array, element, index, length) {
  if (length == 0) return -1;
  if (IS_UNDEFINED(index)) {
    index = 0;
  } else {
    index = INVERT_NEG_ZERO(TO_INTEGER(index));
    // If index is negative, index from the end of the array.
    if (index < 0) {
      index = length + index;
      // If index is still negative, search the entire array.
      if (index < 0) index = 0;
    }
  }
  var min = index;
  var max = length;
  if (UseSparseVariant(array, length, IS_ARRAY(array), max - min)) {
    %NormalizeElements(array);
    var indices = %GetArrayKeys(array, length);
    if (IS_NUMBER(indices)) {
      // It's an interval.
      max = indices;  // Capped by length already.
      // Fall through to loop below.
    } else {
      if (indices.length == 0) return -1;
      // Get all the keys in sorted order.
      var sortedKeys = GetSortedArrayKeys(array, indices);
      var n = sortedKeys.length;
      var i = 0;
      while (i < n && sortedKeys[i] < index) i++;
      while (i < n) {
        var key = sortedKeys[i];
        if (array[key] === element) return key;
        i++;
      }
      return -1;
    }
  }
  // Lookup through the array.
  if (!IS_UNDEFINED(element)) {
    for (var i = min; i < max; i++) {
      if (array[i] === element) return i;
    }
    return -1;
  }
  // Lookup through the array.
  for (var i = min; i < max; i++) {
    if (IS_UNDEFINED(array[i]) && i in array) {
      return i;
    }
  }
  return -1;
}
function InnerArrayIndexOf(array, element, index, length) {
  if (length == 0) return -1;
  if (IS_UNDEFINED(index)) {
    index = 0;
  } else {
    index = INVERT_NEG_ZERO(TO_INTEGER(index));
    // If index is negative, index from the end of the array.
    if (index < 0) {
      index = length + index;
      // If index is still negative, search the entire array.
      if (index < 0) index = 0;
    }
  }
  var min = index;
  var max = length;
  if (UseSparseVariant(array, length, IS_ARRAY(array), max - min)) {
    %NormalizeElements(array);
    var indices = %GetArrayKeys(array, length);
    if (IS_NUMBER(indices)) {
      // It's an interval.
      max = indices;  // Capped by length already.
      // Fall through to loop below.
    } else {
      if (indices.length == 0) return -1;
      // Get all the keys in sorted order.
      var sortedKeys = GetSortedArrayKeys(array, indices);
      var n = sortedKeys.length;
      var i = 0;
      while (i < n && sortedKeys[i] < index) i++;
      while (i < n) {
        var key = sortedKeys[i];
        if (array[key] === element) return key;
        i++;
      }
      return -1;
    }
  }
  // Lookup through the array.
  if (!IS_UNDEFINED(element)) {
    for (var i = min; i < max; i++) {
      if (array[i] === element) return i;
    }
    return -1;
  }
  // Lookup through the array.
  for (var i = min; i < max; i++) {
    if (IS_UNDEFINED(array[i]) && i in array) {
      return i;
    }
  }
  return -1;
}
For .indexOf, we don't need to pass in the number of arguments at the callsite since ToInteger(undefined) == 0; however, for .lastIndexOf, we need to pass it, since the behavior for passing undefined is 0 but for not including the argument is length-1.