Update/Add attribute to json array in T-SQL - arrays

Question

All of the examples examples I see dealing with json arrays have the array nested under a top level object. I have a json array in a column:
[{"key": "value1"}, {"key": "value2"}]
I would like to run a sql script to add/update a key for each element in the array, resulting in:
[{"key": "value1", "otherKey": "otherValue"}, {"key": "value", "otherKey": "otherValue"}]
Yes, in my case I want the same value set for each array member. I've tried:
declare #info nvarchar(max)
SET #info = '[{"key": "value1"}, {"key": "value2"}]'
print JSON_MODIFY(#info, '[0].otherKey', '""')
and fails with "JSON path is not properly formatted. Unexpected character '[' is found at position 0."
This is in MSSQL 2017.

Answer 1

The approach, that can be used, depends on JSON structure (I assume, that the count of the items in the JSON array is not fixed):
If the input JSON array has items (JSON objects) with fixed key/keys, you may use a combination of OPENJSON() with explicit schema (to parse this JSON as table) and FOR JSON (to modify and return the rows as JSON)
If the input JSON has items with different structure, you may use a combination of OPENJSON() with default schema, JSON_MODIFY() and STRING_AGG().
JSON:
declare #info nvarchar(max)
SET #info = '[{"key": "value1"}, {"key": "value2"}]'
Statement for fixed structure:
SELECT #info = (
SELECT [key], 'OtherValue' AS OtherKey
FROM OPENJSON(#info) WITH ([key] varchar(100) '$.key')
FOR JSON PATH
)
Statement for variable structure:
SELECT #info = CONCAT('[', STRING_AGG(JSON_MODIFY([value], '$.OtherKey', 'OtherValue'), ','), ']')
FROM OPENJSON(#info)
SELECT #info
Result:
[{"key":"value1","OtherKey":"OtherValue"},{"key":"value2","OtherKey":"OtherValue"}]
Note, that the reason for the error is that the value of path parameter ([0].otherKey) is wrong. The correct path expression is $[0].otherKey for the first item in the JSON array.

Answer 2

Have you tried adding $ before the indexer sign? Like:
declare #info nvarchar(max)
SET #info = '[{"key": "value1"}, {"key": "value2"}]'
SELECT JSON_MODIFY(#info, '$[0].otherKey', '""')
This gives the following output:
[{"key": "value1","otherKey":"\"\""}, {"key": "value2"}]