From 81325d07452f61a578853b926eb84f3b56ef8676 Mon Sep 17 00:00:00 2001
From: Jake Leahy <jake@leahy.dev>
Date: Wed, 19 Aug 2026 16:24:25 +1000
Subject: [PATCH] Add checks to `fromJson` when trying to convert to an array
 (#26109)

Issue popped up when using `fromJson` into an array but the JSON passed
is an object

```nim
import std/[jsonutils, json]

let data = parseJson """
{"key": "value"}
"""
var foo: seq[int]
foo.fromJson(data)
echo foo #> @[0]
```
Basically the `setLen` would set the size to be equal to the number of
keys, but `getElems` just returns an empty array if the JSON isn't an
array which lead to it just creating zero'd items in the seq without
letting the user know.

Felt adding the checks was better than just skipping the `setLen` since
it lets the user know that there is a problem with the JSON
---
 changelog.md                | 1 +
 lib/std/jsonutils.nim       | 2 ++
 tests/stdlib/tjsonutils.nim | 6 ++++++
 3 files changed, 9 insertions(+)

diff --git a/changelog.md b/changelog.md
index 024d607dee..d6561b42d2 100644
--- a/changelog.md
+++ b/changelog.md
@@ -99,6 +99,7 @@ parameter and result types, not just their source-level shape. Use
   works without single-quoting.
 - `std/uri`: The `?` operator now appends query parameters to an existing query
   string instead of replacing it. Fixes [#19782](https://github.com/nim-lang/Nim/issues/19782).
+- `std/jsonutils`: `fromJson` now throws an exception when converting to `array`/`seq` if the JSON isn't an array instead of silently failing
 
 ## Language changes
 
diff --git a/lib/std/jsonutils.nim b/lib/std/jsonutils.nim
index d378812b69..688bbdb565 100644
--- a/lib/std/jsonutils.nim
+++ b/lib/std/jsonutils.nim
@@ -238,6 +238,7 @@ proc fromJson*[T](a: var T, b: JsonNode, opt = Joptions()) =
       a = T()
       fromJson(a[], b, opt)
   elif T is array:
+    checkJson b.kind == JArray
     checkJson a.len == b.len, "Json array size doesn't match for " & $T
     var i = 0
     for ai in mitems(a):
@@ -248,6 +249,7 @@ proc fromJson*[T](a: var T, b: JsonNode, opt = Joptions()) =
     for val in b.getElems:
       incl a, jsonTo(val, E)
   elif T is seq:
+    checkJson b.kind == JArray
     a.setLen b.len
     for i, val in b.getElems:
       fromJson(a[i], val, opt)
diff --git a/tests/stdlib/tjsonutils.nim b/tests/stdlib/tjsonutils.nim
index b57cb3e914..3411faa8b6 100644
--- a/tests/stdlib/tjsonutils.nim
+++ b/tests/stdlib/tjsonutils.nim
@@ -451,6 +451,12 @@ template fn() =
       let json = inner.toJson(ToJsonOptions(enumMode: joptEnumSymbol))
       doAssert $json == """{"x":"hello","y":"A"}"""
 
+    block arrayTypeCheck:
+      let json = """{"key": "value"}""".parseJson()
+      var output: seq[int]
+      doAssertRaises(ValueError):
+        output.fromJson(json)
+
     block: # bug #21638
       type Something = object