JSON.parse source text access

2020 February

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https://github.com/gibson042/ecma262-proposal-JSON-parse-with-source

Background

JSON.parse is lossy, even with a reviver function

// Numbers are subject to IEEE 754 precision limits.�JSON.parse(" 9999999999999999")�// → 10000000000000000��// …and reviver functions receive already-parsed values.�JSON.parse(" 9999999999999999", (key, val) => BigInt(val))�// → 10000000000000000n

Reviver functions lack context

// Conditional processing must bubble up to a differentiating level�JSON.parse(`{ "metadata": { "date": "2018-09-25T10:00-04:00" },� "value": { "date": "2018-09-25T10:00-04:00" } }`,� function(key, val) {� if ( key === "value" && "date" in val ) {� val.date = new Date(val.date);� }� return val;� }�)

// → { metadata: { date: "2018-09-25T10:00-04:00" },� value: { date: new Date("2018-09-25T14:00:00.000Z") } }

Proposal

Expose source text to reviver functions

// Numbers are still subject to IEEE 754 precision limits.�JSON.parse(" 9999999999999999")�// → 10000000000000000��// …but reviver functions gain access to the raw source.�JSON.parse(" 9999999999999999", (key, val, {source}) => BigInt(source))�// → 9999999999999999n

Potential enhancement: Supply parent keys

// Conditional processing can take place directly at a deep value.�JSON.parse(`{ "metadata": { "date": "2018-09-25T10:00-04:00" },� "value": { "date": "2018-09-25T10:00-04:00" } }`,� (key, val, {keys}) =>� keys.length === 3 && keys.join(".") === ".value.date" ?� new Date(val) :� val�)

// → { metadata: { date: "2018-09-25T10:00-04:00" },� value: { date: new Date("2018-09-25T14:00:00.000Z") } }

Discussion

Draft spec text

1.1 Runtime Semantics: InternalizeJSONProperty ( holder, name, reviver, parseNode )

The abstract operation InternalizeJSONProperty is a recursive abstract operation that takes twofour parameters: a holder object and, the String name of a property in that object, a reviver function, and a parseNode Parse Node corresponding with the original value of the property. InternalizeJSONProperty uses the value of reviver that was originally passed to the above parse function.

Draft spec text

1. Let context be ! ObjectCreate(%ObjectPrototype%).
2. Let sourceText be the span of source text matched by parseNode.
3. Perform ! CreateDataPropertyOrThrow(context, "source", ! UTF16Encode(sourceText)).

Draft spec text

1. If isArray is true, then
1. Let I be 0.
2. Let len be ? LengthOfArrayLike(val).
3. Repeat, while I < len,
1. Let child be the AssignmentExpression descendant of parseNode corresponding with the element of val at index I.
2. Let newElement be ? InternalizeJSONProperty(val, ! ToString(I), reviver, child).

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Draft spec text

1. Else [If isArray is not true],
1. Let keys be ? EnumerableOwnPropertyNames(val, key).
2. For each String P in keys, do
1. Let child be the AssignmentExpression descendant of parseNode responsible for the value of the property of val with name P.
2. NOTE: In the case of JSON text specifying multiple name/value pairs with the same name for a single object (such as {"a":"lost","a":"kept"}), the value for the corresponding property of the resulting ECMAScript object is specified by the last pair with that name.
3. Let newElement be ? InternalizeJSONProperty(val, P, reviver, child).

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Other questions

Other questions

• Expose parse node indices?
• Support parent keys?
• Extend to include serialization?

Next steps

https://github.com/gibson042/ecma262-proposal-JSON-parse-with-source

Questions?

Feedback?

Advance to Stage 2?

What is Stage 2?