JavaScript Object Notation (JSON) Path
draft-ietf-jsonpath-base-00
The information below is for an old version of the document.
Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 9535.
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Authors | Glyn Normington , Edward Surov , Marko Mikulicic , Stefan Gössner | ||
Last updated | 2021-03-07 | ||
RFC stream | Internet Engineering Task Force (IETF) | ||
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IESG | IESG state | Became RFC 9535 (Proposed Standard) | |
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draft-ietf-jsonpath-base-00
JSONPath WG G. Normington, Ed. Internet-Draft VMware, Inc. Intended status: Standards Track E. Surov, Ed. Expires: 8 September 2021 TheSoul Publishing Ltd. M. Mikulicic, Ed. InfluxData, Inc. S. Gössner Fachhochschule Dortmund 7 March 2021 JavaScript Object Notation (JSON) Path draft-ietf-jsonpath-base-00 Abstract JSONPath defines a string syntax for identifying values within a JavaScript Object Notation (JSON) document. Contributing This document picks up the popular JSONPath specification dated 2007-02-21 and provides a normative definition for it. In its current state, it is a strawman document showing what needs to be covered. Comments and issues may be directed to this document's github repository (https://github.com/ietf-wg-jsonpath/draft-ietf-jsonpath- jsonpath). Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 8 September 2021. Normington, et al. Expires 8 September 2021 [Page 1] Internet-Draft JSONPath March 2021 Copyright Notice Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 1.2. Inspired by XPath . . . . . . . . . . . . . . . . . . . . 4 1.3. Overview of JSONPath Expressions . . . . . . . . . . . . 5 2. JSONPath Examples . . . . . . . . . . . . . . . . . . . . . . 8 3. JSONPath Syntax and Semantics . . . . . . . . . . . . . . . . 11 3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 11 3.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 11 3.3. Implementation . . . . . . . . . . . . . . . . . . . . . 11 3.4. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 12 3.5. Semantics . . . . . . . . . . . . . . . . . . . . . . . . 12 3.6. Selectors . . . . . . . . . . . . . . . . . . . . . . . . 13 3.6.1. Dot Child Selector . . . . . . . . . . . . . . . . . 13 3.6.2. Union Selector . . . . . . . . . . . . . . . . . . . 14 3.6.2.1. Syntax . . . . . . . . . . . . . . . . . . . . . 14 3.6.2.2. Semantics . . . . . . . . . . . . . . . . . . . . 14 3.6.2.3. Child . . . . . . . . . . . . . . . . . . . . . . 14 3.6.2.4. Array Selector . . . . . . . . . . . . . . . . . 16 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 5. Security Considerations . . . . . . . . . . . . . . . . . . . 20 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 20 6.1. Normative References . . . . . . . . . . . . . . . . . . 20 6.2. Informative References . . . . . . . . . . . . . . . . . 20 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 21 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22 Normington, et al. Expires 8 September 2021 [Page 2] Internet-Draft JSONPath March 2021 1. Introduction This document picks up the popular JSONPath specification dated 2007-02-21 [JSONPath-orig] and provides a normative definition for it. In its current state, it is a strawman document showing what needs to be covered. JSON is defined by [RFC8259]. JSONPath is not intended as a replacement, but as a more powerful companion, to JSON Pointer [RFC6901]. [insert reference to section where the relationship is detailed. The purposes of the two syntaxes are different. Pointer is for isolating a single location within a document. Path is a query syntax that can also be used to pull multiple locations.] 1.1. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. The grammatical rules in this document are to be interpreted as ABNF, as described in [RFC5234]. ABNF terminal values in this document define Unicode code points rather than their UTF-8 encoding. For example, the Unicode PLACE OF INTEREST SIGN (U+2318) would be defined in ABNF as "%x2318". The terminology of [RFC8259] applies. Data Item: A structure complying to the generic data model of JSON, i.e., composed of containers, namely JSON objects and arrays, and of atomic data, namely null, true, false, numbers, and text strings. Also called a JSON value. Object: A JSON object as defined in [RFC8259]. Never used in its generic sense, e.g., for programming language objects. Member: A name/value pair in a JSON object. (Not itself a JSON value.) Name: The name in a name/value pair constituting a member. (Also known as "key", "tag", or "label".) Element: An item in an array. (Also used with a distinct meaning in XML context for XML elements.) Normington, et al. Expires 8 September 2021 [Page 3] Internet-Draft JSONPath March 2021 Query: Short name for JSONPath expression. Argument: Short name for the JSON data item a JSONPath expression is applied to. Output Path: A simple form of JSONPath expression that identifies a Position by providing a query that results in exactly that position. Similar to, but syntactically different from, a JSON Pointer [RFC6901]. Position: A JSON data item identical to or nested within the JSON data item to which the query is applied to, expressed either by the value of that data item or by providing a Normalized Path Expression as a JSONPath Output Path. Normalized Path Expression: A query in a normalized form that identifies exactly one Position in an Argument; see Section 1.3. 1.2. Inspired by XPath A frequently emphasized advantage of XML is the availability of powerful tools to analyse, transform and selectively extract data from XML documents. [XPath] is one of these tools. In 2007, the need for something solving the same class of problems for the emerging JSON community became apparent, specifically for: * Finding data interactively and extracting them out of [RFC8259] data items without special scripting. * Specifying the relevant parts of the JSON data in a request by a client, so the server can reduce the amount of data in its response, minimizing bandwidth usage. So what does such a tool look like for JSON? When defining a JSONPath, how should expressions look? The XPath expression /store/book[1]/title looks like x.store.book[0].title or x['store']['book'][0]['title'] Normington, et al. Expires 8 September 2021 [Page 4] Internet-Draft JSONPath March 2021 in popular programming languages such as JavaScript, Python and PHP, with a variable x holding the JSON data item. Here we observe that such languages already have a fundamentally XPath-like feature built in. The JSONPath tool in question should: * be naturally based on those language characteristics. * cover only essential parts of XPath 1.0. * be lightweight in code size and memory consumption. * be runtime efficient. 1.3. Overview of JSONPath Expressions JSONPath expressions always apply to a JSON data item in the same way as XPath expressions are used in combination with an XML document. Since a JSON data item is anonymous, JSONPath uses the abstract name "$" to refer to the top level data item of the argument. JSONPath expressions can use the _dot-notation_ $.store.book[0].title or the _bracket-notation_ $['store']['book'][0]['title'] for paths input to a JSONPath processor. [1] Where a JSONPath processor uses JSONPath expressions as output paths, these will always be converted to normalized JSONPath expressions which employ the more general _bracket-notation_. [2] Bracket notation is more general than dot notation and can serve as a canonical form when a JSONPath processor uses JSONPath expressions as output paths. JSONPath allows the wildcard symbol "*" for member names and array indices. It borrows the descendant operator ".." from [E4X] and the array slice syntax proposal "[start:end:step]" [SLICE] from ECMASCRIPT 4. JSONPath was originally designed to employ an _underlying scripting language_ for computing expressions. The present specification defines a simple expression language that is independent from any scripting language in use on the platform. Normington, et al. Expires 8 September 2021 [Page 5] Internet-Draft JSONPath March 2021 JSONPath can use expressions, written in parentheses: "(<expr>)", as an alternative to explicit names or indices as in: $.store.book[(@.length-1)].title The symbol "@" is used for the current item. Filter expressions are supported via the syntax "?(<boolean expr>)" as in $.store.book[?(@.price < 10)].title Here is a complete overview and a side by side comparison of the JSONPath syntax elements with their XPath counterparts. Normington, et al. Expires 8 September 2021 [Page 6] Internet-Draft JSONPath March 2021 +=======+==================+=====================================+ | XPath | JSONPath | Description | +=======+==================+=====================================+ | / | $ | the root element/item | +-------+------------------+-------------------------------------+ | . | @ | the current element/item | +-------+------------------+-------------------------------------+ | / | "." or "[]" | child operator | +-------+------------------+-------------------------------------+ | .. | n/a | parent operator | +-------+------------------+-------------------------------------+ | // | .. | nested descendants (JSONPath | | | | borrows this syntax from E4X) | +-------+------------------+-------------------------------------+ | * | * | wildcard: All elements/items | | | | regardless of their names | +-------+------------------+-------------------------------------+ | @ | n/a | attribute access: JSON data items | | | | do not have attributes | +-------+------------------+-------------------------------------+ | [] | [] | subscript operator: XPath uses it | | | | to iterate over element collections | | | | and for predicates; native array | | | | indexing as in JavaScript here | +-------+------------------+-------------------------------------+ | | | [,] | Union operator in XPath (results in | | | | a combination of node sets); | | | | JSONPath allows alternate names or | | | | array indices as a set | +-------+------------------+-------------------------------------+ | n/a | [start:end:step] | array slice operator borrowed from | | | | ES4 | +-------+------------------+-------------------------------------+ | [] | ?() | applies a filter (script) | | | | expression | +-------+------------------+-------------------------------------+ | n/a | () | expression engine | +-------+------------------+-------------------------------------+ | () | n/a | grouping in Xpath | +-------+------------------+-------------------------------------+ Table 1: Overview over JSONPath, comparing to XPath XPath has a lot more to offer (location paths in unabbreviated syntax, operators and functions) than listed here. Moreover there is a significant difference how the subscript operator works in Xpath and JSONPath: Normington, et al. Expires 8 September 2021 [Page 7] Internet-Draft JSONPath March 2021 * Square brackets in XPath expressions always operate on the _node set_ resulting from the previous path fragment. Indices always start at 1. * With JSONPath, square brackets operate on the _object_ or _array_ addressed by the previous path fragment. Array indices always start at 0. 2. JSONPath Examples This section provides some more examples for JSONPath expressions. The examples are based on a simple JSON data item patterned after a typical XML example representing a bookstore (that also has bicycles): { "store": { "book": [ { "category": "reference", "author": "Nigel Rees", "title": "Sayings of the Century", "price": 8.95 }, { "category": "fiction", "author": "Evelyn Waugh", "title": "Sword of Honour", "price": 12.99 }, { "category": "fiction", "author": "Herman Melville", "title": "Moby Dick", "isbn": "0-553-21311-3", "price": 8.99 }, { "category": "fiction", "author": "J. R. R. Tolkien", "title": "The Lord of the Rings", "isbn": "0-395-19395-8", "price": 22.99 } ], "bicycle": { "color": "red", "price": 19.95 } } } Figure 1: Example JSON data item Normington, et al. Expires 8 September 2021 [Page 8] Internet-Draft JSONPath March 2021 The examples in Table 2 use the expression mechanism to obtain the number of items in an array, to test for the presence of a member in a object, and to perform numeric comparisons of member values with a constant. Normington, et al. Expires 8 September 2021 [Page 9] Internet-Draft JSONPath March 2021 +======================+=========================+==================+ | XPath | JSONPath | Result | +======================+=========================+==================+ | /store/book/author | $.store.book[*].author | the authors of | | | | all books in | | | | the store | +----------------------+-------------------------+------------------+ | //author | $..author | all authors | +----------------------+-------------------------+------------------+ | /store/* | $.store.* | all things in | | | | store, which | | | | are some books | | | | and a red | | | | bicycle | +----------------------+-------------------------+------------------+ | /store//price | $.store..price | the prices of | | | | everything in | | | | the store | +----------------------+-------------------------+------------------+ | //book[3] | $..book[2] | the third book | +----------------------+-------------------------+------------------+ | //book[last()] | "$..book[(@.length-1)]" | the last book | | | "$..book[-1]" | in order | +----------------------+-------------------------+------------------+ | //book[position()<3] | "$..book[0,1]" | the first two | | | "$..book[:2]" | books | +----------------------+-------------------------+------------------+ | //book[isbn] | $..book[?(@.isbn)] | filter all | | | | books with | | | | isbn number | +----------------------+-------------------------+------------------+ | //book[price<10] | $..book[?(@.price<10)] | filter all | | | | books cheaper | | | | than 10 | +----------------------+-------------------------+------------------+ | //* | $..* | all elements | | | | in XML | | | | document; all | | | | members of | | | | JSON data item | +----------------------+-------------------------+------------------+ Table 2: Example JSONPath expressions applied to the example JSON data item Normington, et al. Expires 8 September 2021 [Page 10] Internet-Draft JSONPath March 2021 3. JSONPath Syntax and Semantics 3.1. Overview A JSONPath is a string which selects zero or more nodes of a piece of JSON. A valid JSONPath conforms to the ABNF syntax defined by this document. A JSONPath MUST be encoded using UTF-8. To parse a JSONPath according to the grammar in this document, its UTF-8 form SHOULD first be decoded into Unicode code points as described in [RFC3629]. 3.2. Terminology A JSON value is logically a tree of nodes. Each node holds a JSON value (as defined by [RFC8259]) of one of the types object, array, number, string, or one of the literals "true", "false", or "null". The type of the JSON value held by a node is sometimes referred to as the type of the node. 3.3. Implementation An implementation of this specification, from now on referred to simply as "an implementation", SHOULD takes two inputs, a JSONPath and a JSON value, and produce a possibly empty list of nodes of the JSON value which are selected by the JSONPath or an error (but not both). If no node is selected and no error has occurred, an implementation MUST return an empty list of nodes. Syntax errors in the JSONPath SHOULD be detected before selection is attempted since these errors do not depend on the JSON value. Therefore, an implementation SHOULD take a JSONPath and produce an optional syntax error and then, if and only if an error was not produced, SHOULD take a JSON value and produce a list of nodes or an error (but not both). Alternatively, an implementation MAY take a JSONPath and a JSON value and produce a list of nodes or an optional error (but not both). For any implementation, if a syntactically invalid JSONPath is provided, the implementation MUST return an error. If a syntactially invalid JSON value is provided, any implementation SHOULD return an error. Normington, et al. Expires 8 September 2021 [Page 11] Internet-Draft JSONPath March 2021 3.4. Syntax Syntactically, a JSONPath consists of a root selector ("$"), which selects the root node of a JSON value, followed by a possibly empty sequence of _selectors_. json-path = root-selector *selector root-selector = "$" ; $ selects document root node The syntax and semantics of each selector is defined below. 3.5. Semantics The root selector "$" not only selects the root node of the input document, but it also produces as output a list consisting of one node: the input document. A selector may select zero or more nodes for further processing. A syntactically valid selector MUST NOT produce errors. This means that some operations which might be considered erroneous, such as indexing beyond the end of an array, simply result in fewer nodes being selected. But a selector doesn't just act on a single node: each selector acts on a list of nodes and produces a list of nodes, as follows. After the root selector, the remainder of the JSONPath is processed by passing lists of nodes from one selector to the next ending up with a list of nodes which is the result of applying the JSONPath to the input JSON value. Each selector acts on its input list of nodes as follows. For each node in the list, the selector selects zero or more nodes, each of which is a descendant of the node or the node itself. The output list of nodes of a selector is the concatenation of the lists of selected nodes for each input node. A specific, non-normative example will make this clearer. Suppose the input document is: "{"a":[{"b":0},{"b":1},{"c":2}]}". As we will see later, the JSONPath "$.a[*].b" selects the following list of nodes: "0", "1". Let's walk through this in detail. The JSONPath consists of "$" followed by three selectors: ".a", "[*]", and ".b". Firstly, "$" selects the root node which is the input document. So the result is a list consisting of just the root node. Normington, et al. Expires 8 September 2021 [Page 12] Internet-Draft JSONPath March 2021 Next, ".a" selects from any input node of type object and selects any value of the input node corresponding to the member name ""a"". The result is again a list of one node: "[{"b":0},{"b":1},{"c":2}]". Next, "[*]" selects from any input node which is an array and selects all the elements of the input node. The result is a list of three nodes: "{"b":0}", "{"b":1}", and "{"c":2}". Finally, ".b" selects from any input node of type object with a member name "b" and selects the value of the input node corresponding to that name. The result is a list containing "0", "1". This is the concatenation of three lists, two of length one containing "0", "1", respectively, and one of length zero. As a consequence of this approach, if any of the selectors selects no nodes, then the whole JSONPath selects no nodes. In what follows, the semantics of each selector are defined for each type of node. 3.6. Selectors 3.6.1. Dot Child Selector Syntax A dot child selector has a member name known as a dot child name or a single asterisk ("*"). A dot child name corresponds to a name in a object. selector = dot-child ; see below for alternatives dot-child = "." dot-child-name / ; .<dot-child-name> "." "*" ; .* dot-child-name = 1*( "-" / ; - DIGIT / ALPHA / "_" / ; _ %x80-10FFFF ; any non-ASCII Unicode character ) DIGIT = %x30-39 ; 0-9 ALPHA = %x41-5A / %x61-7A ; A-Z / a-z More general child names, such as the empty string, are supported by "Union Child" (Section 3.6.2.3). Normington, et al. Expires 8 September 2021 [Page 13] Internet-Draft JSONPath March 2021 Note that the "dot-child-name" rule follows the philosophy of JSON strings and is allowed to contain bit sequences that cannot encode Unicode characters (a single unpaired UTF-16 surrogate, for example). The behaviour of an implementation is undefined for child names which do not encode Unicode characters. Semantics A dot child name which is not a single asterisk ("*") is considered to have a member name. It selects the value corresponding to the name from any object node. It selects no nodes from a node which is not a object. The member name of a dot child name is the sequence of Unicode characters contained in that name. A dot child name consisting of a single asterisk is a wild card. It selects all the values of any object node. It also selects all the elements of any array node. It selects no nodes from number, string, or literal nodes. 3.6.2. Union Selector 3.6.2.1. Syntax A union selector consists of one or more union elements. selector =/ union union = "[" ws union-elements ws "]" ; [...] ws = *" " ; zero or more spaces union-elements = union-element / union-element ws "," ws union-elements ; ,-separated list 3.6.2.2. Semantics A union selects any node which is selected by at least one of the union selectors and selects the concatenation of the lists (in the order of the selectors) of nodes selected by the union elements. 3.6.2.3. Child Syntax A child is a quoted string. Normington, et al. Expires 8 September 2021 [Page 14] Internet-Draft JSONPath March 2021 union-element = child ; see below for more alternatives child = %x22 *double-quoted %x22 / ; "string" "'" *single-quoted "'" ; 'string' double-quoted = dq-unescaped / escape ( %x22 / ; " quotation mark U+0022 "/" / ; / solidus U+002F "\" / ; \ reverse solidus U+005C "b" / ; b backspace U+0008 "f" / ; f form feed U+000C "n" / ; n line feed U+000A "r" / ; r carriage return U+000D "t" / ; t tab U+0009 "u" 4HEXDIG ) ; uXXXX U+XXXX dq-unescaped = %x20-21 / %x23-5B / %x5D-10FFFF single-quoted = sq-unescaped / escape ( "'" / ; ' apostrophe U+0027 "/" / ; / solidus U+002F "\" / ; \ reverse solidus U+005C "b" / ; b backspace U+0008 "f" / ; f form feed U+000C "n" / ; n line feed U+000A "r" / ; r carriage return U+000D "t" / ; t tab U+0009 "u" 4HEXDIG ) ; uXXXX U+XXXX sq-unescaped = %x20-26 / %x28-5B / %x5D-10FFFF escape = "\" ; \ HEXDIG = DIGIT / "A" / "B" / "C" / "D" / "E" / "F" ; case insensitive hex digit Notes: 1. double-quoted strings follow JSON in [RFC8259]. Single- quoted strings follow an analogous pattern. 2. "HEXDIG" includes A-F and a-f. Semantics If the child is a quoted string, the string MUST be converted to a member name by removing the surrounding quotes and replacing each escape sequence with its equivalent Unicode character, as in the table below: Normington, et al. Expires 8 September 2021 [Page 15] Internet-Draft JSONPath March 2021 +=================+===================+ | Escape Sequence | Unicode Character | +=================+===================+ | "" %x22 | U+0022 | +-----------------+-------------------+ | "" "'" | U+0027 | +-----------------+-------------------+ | "" "/" | U+002F | +-----------------+-------------------+ | "" "" | U+005C | +-----------------+-------------------+ | "" "b" | U+0008 | +-----------------+-------------------+ | "" "f" | U+000C | +-----------------+-------------------+ | "" "n" | U+000A | +-----------------+-------------------+ | "" "r" | U+000D | +-----------------+-------------------+ | "" "t" | U+0009 | +-----------------+-------------------+ | "" uXXXX | U+XXXX | +-----------------+-------------------+ Table 3: Escape Sequence Replacements The child selects the value corresponding to the member name from any object node that has a member with that name. It selects no nodes from a node which is not a object. 3.6.2.4. Array Selector Syntax An array selector selects zero or more elements of an array node. An array selector takes the form of an index, which selects at most one element, or a slice, which selects zero or more elements. union-element =/ array-index / array-slice An array index is an integer (in base 10). array-index = integer integer = ["-"] ("0" / (DIGIT1 *DIGIT)) ; optional - followed by 0 or ; sequence of digits with no leading zero DIGIT1 = %x31-39 ; non-zero digit Normington, et al. Expires 8 September 2021 [Page 16] Internet-Draft JSONPath March 2021 Note: the syntax does not allow integers with leading zeros such as "01" and "-01". An array slice consists of three optional integers (in base 10) separated by colons. array-slice = [ start ] ws ":" ws [ end ] [ ws ":" ws [ step ] ] start = integer end = integer step = integer Note: the array slices ":" and "::" are both syntactically valid, as are ":2:2", "2::2", and "2:4:". Semantics Informal Introduction This section is non-normative. Array indexing is a way of selecting a particular element of an array using a 0-based index. For example, the expression "[0]" selects the first element of a non-empty array. Negative indices index from the end of an array. For example, the expression "[-2]" selects the last but one element of an array with at least two elements. Array slicing is inspired by the behaviour of the "Array.prototype.slice" method of the JavaScript language as defined by the ECMA-262 standard [ECMA-262], with the addition of the "step" parameter, which is inspired by the Python slice expression. The array slice expression "[start:end:step]" selects elements at indices starting at "start", incrementing by "step", and ending with "end" (which is itself excluded). So, for example, the expression "[1:3]" (where "step" defaults to "1") selects elements with indices "1" and "2" (in that order) whereas "[1:5:2]" selects elements with indices "1" and "3". When "step" is negative, elements are selected in reverse order. Thus, for example, "[5:1:-2]" selects elements with indices "5" and "3", in that order and "[::-1]" selects all the elements of an array in reverse order. Normington, et al. Expires 8 September 2021 [Page 17] Internet-Draft JSONPath March 2021 When "step" is "0", no elements are selected. This is the one case which differs from the behaviour of Python, which raises an error in this case. The following section specifies the behaviour fully, without depending on JavaScript or Python behaviour. Detailed Semantics An array selector is either an array slice or an array index, which is defined in terms of an array slice. A slice expression selects a subset of the elements of the input array, in the same order as the array or the reverse order, depending on the sign of the "step" parameter. It selects no nodes from a node which is not an array. A slice is defined by the two slice parameters, "start" and "end", and an iteration delta, "step". Each of these parameters is optional. "len" is the length of the input array. The default value for "step" is "1". The default values for "start" and "end" depend on the sign of "step", as follows: +===========+=========+==========+ | Condition | start | end | +===========+=========+==========+ | step >= 0 | 0 | len | +-----------+---------+----------+ | step < 0 | len - 1 | -len - 1 | +-----------+---------+----------+ Table 4: Default array slice start and end values Slice expression parameters "start" and "end" are not directly usable as slice bounds and must first be normalized. Normalization is defined as: FUNCTION Normalize(i): IF i >= 0 THEN RETURN i ELSE RETURN len + i END IF The result of the array indexing expression "[i]" is defined to be the result of the array slicing expression "[i:Normalize(i)+1:1]". Normington, et al. Expires 8 September 2021 [Page 18] Internet-Draft JSONPath March 2021 Slice expression parameters "start" and "end" are used to derive slice bounds "lower" and "upper". The direction of the iteration, defined by the sign of "step", determines which of the parameters is the lower bound and which is the upper bound: FUNCTION Bounds(start, end, step, len): n_start = Normalize(start) n_end = Normalize(end) IF step >= 0 THEN lower = MIN(MAX(n_start, 0), len) upper = MIN(MAX(n_end, 0), len) ELSE upper = MIN(MAX(n_start, -1), len-1) lower = MIN(MAX(n_end, -1), len-1) END IF RETURN (lower, upper) The slice expression selects elements with indices between the lower and upper bounds. In the following pseudocode, the "a(i)" construct expresses the 0-based indexing operation on the underlying array. IF step > 0 THEN i = lower WHILE i < upper: SELECT a(i) i = i + step END WHILE ELSE if step < 0 THEN i = upper WHILE lower < i: SELECT a(i) i = i + step END WHILE END IF When "step = 0", no elements are selected and the result array is empty. Normington, et al. Expires 8 September 2021 [Page 19] Internet-Draft JSONPath March 2021 An implementation MUST raise an error if any of the slice expression parameters does not fit in the implementation's representation of an integer. If a successfully parsed slice expression is evaluated against an array whose size doesn't fit in the implementation's representation of an integer, the implementation MUST raise an error. 4. IANA Considerations TBD: Define a media type for JSON Path expressions. 5. Security Considerations This section gives security considerations, as required by [RFC3552]. 6. References 6.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>. [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November 2003, <https://www.rfc-editor.org/info/rfc3629>. [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, <https://www.rfc-editor.org/info/rfc5234>. [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>. [RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data Interchange Format", STD 90, RFC 8259, DOI 10.17487/RFC8259, December 2017, <https://www.rfc-editor.org/info/rfc8259>. 6.2. Informative References [E4X] ISO, "Information technology — ECMAScript for XML (E4X) specification", ISO/IEC 22537:2006 , 2006. Normington, et al. Expires 8 September 2021 [Page 20] Internet-Draft JSONPath March 2021 [ECMA-262] Ecma International, "ECMAScript Language Specification, Standard ECMA-262, Third Edition", December 1999, <http://www.ecma-international.org/publications/files/ ECMA-ST-ARCH/ECMA- 262,%203rd%20edition,%20December%201999.pdf>. [JSONPath-orig] Gössner, S., "JSONPath – XPath for JSON", 21 February 2007, <https://goessner.net/articles/JsonPath/>. [RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC Text on Security Considerations", BCP 72, RFC 3552, DOI 10.17487/RFC3552, July 2003, <https://www.rfc-editor.org/info/rfc3552>. [RFC6901] Bryan, P., Ed., Zyp, K., and M. Nottingham, Ed., "JavaScript Object Notation (JSON) Pointer", RFC 6901, DOI 10.17487/RFC6901, April 2013, <https://www.rfc-editor.org/info/rfc6901>. [SLICE] "Slice notation", n.d., <https://github.com/tc39/proposal-slice-notation>. [XPath] Berglund, A., Boag, S., Chamberlin, D., Fernandez, M., Kay, M., Robie, J., and J. Simeon, "XML Path Language (XPath) 2.0 (Second Edition)", World Wide Web Consortium Recommendation REC-xpath20-20101214, 14 December 2010, <https://www.w3.org/TR/2010/REC-xpath20-20101214>. Acknowledgements This specification is based on Stefan Gössner's original online article defining JSONPath [JSONPath-orig]. The books example was taken from http://coli.lili.uni- bielefeld.de/~andreas/Seminare/sommer02/books.xml -- a dead link now. Contributors Carsten Bormann Universität Bremen TZI Postfach 330440 D-28359 Bremen Germany Phone: +49-421-218-63921 Email: cabo@tzi.org Normington, et al. Expires 8 September 2021 [Page 21] Internet-Draft JSONPath March 2021 Authors' Addresses Glyn Normington (editor) VMware, Inc. Winchester United Kingdom Email: glyn.normington@gmail.com Edward Surov (editor) TheSoul Publishing Ltd. Limassol Cyprus Email: esurov.tsp@gmail.com Marko Mikulicic (editor) InfluxData, Inc. Pisa Italy Email: mmikulicic@gmail.com Stefan Gössner Fachhochschule Dortmund Sonnenstraße 96 D-44139 Dortmund Germany Email: stefan.goessner@fh-dortmund.de Normington, et al. Expires 8 September 2021 [Page 22]