Helper for creating an IDBKeyRange for all keys with a given prefix

idbkeyrange_forprefix.js

// Copyright 2019 Google LLC.
// SPDX-License-Identifier: Apache-2.0

// Basic p(r)olyfill for proposed feature

// This defines no successor of empty arrays, so the range for prefix
// [] or [[],[]] has no upper bound.
// An alternate definition would preclude adding additional nesting,
// so the range for prefix [] would have upper bound [[]] and the
// range for prefix [[], []] would have upper bound [[], [[]]].

IDBKeyRange.forPrefix = function(prefix) {
  // Ensure prefix is a valid key itself:
  if (indexedDB.cmp(prefix, prefix) !== 0) throw new TypeError();

  var MAX_DATE_VALUE = 8640000000000000;
  var UPPER_BOUND = {
    NUMBER: new Date(-MAX_DATE_VALUE),
    DATE: "",
    STRING: [],
    ARRAY: undefined
  };

  var upperKey = successor(prefix);
  if (upperKey === undefined)
    return IDBKeyRange.lowerBound(prefix);
  return IDBKeyRange.bound(prefix, upperKey, false, true);

  function successor(key) {

    if (typeof key === 'number') {
      if (key === Infinity)
        return UPPER_BOUND.NUMBER;
      if (key === -Infinity)
        return -Number.MAX_VALUE;
      if (key === 0)
        return Number.MIN_VALUE;
      var epsilon = Math.abs(key);
      while (key + epsilon / 2 !== key)
        epsilon = epsilon / 2;
      return key + epsilon;
    }

    if (key instanceof Date) {
      if (key.valueOf() + 1 > MAX_DATE_VALUE)
        return UPPER_BOUND.DATE;
      return new Date(key.valueOf() + 1);
    }

    if (typeof key === 'string') {
      var len = key.length;
      while (len > 0) {
        var head = key.substring(0, len - 1),
            tail = key.charCodeAt(len - 1);
        if (tail !== 0xFFFF)
          return head + String.fromCharCode(tail + 1);
        key = head;
        --len;
      }
      return UPPER_BOUND.STRING;
    }

    if (Array.isArray(key)) {
      key = key.slice();  // Operate on a copy.
      len = key.length;
      while (len > 0) {
        tail = successor(key.pop());
        if (tail !== undefined) {
          key.push(tail);
          return key;
        }
        --len;
      }
      return UPPER_BOUND.ARRAY;
    }

    throw new TypeError();
  }
};