fix: handle arrays properly in FormData #1431

omarmciver · 2025-09-23T14:22:30Z

Description

This PR fixes an issue with FormData handling when arrays are involved, particularly for multi-file uploads.

The Problem

When an API endpoint accepts an array of files (or any array in FormData), the current implementation incorrectly converts the entire array to a string representation instead of properly appending each item individually to the FormData object.

Example scenario:

// API expects multiple file attachments
const files = [file1, file2, file3];
api.uploadMultiple({ files });

// Current behavior: FormData contains
// "files" => "[object File],[object File],[object File]" ❌

// Expected behavior: FormData should contain
// "files" => file1
// "files" => file2  
// "files" => file3 ✅

The Solution

This fix modifies the FormData content formatter to:

Detect when a property value is an array
Iterate through each item in the array
Properly append each item individually to FormData
Preserve the correct handling of Blob/File types vs other data types

Changes Made

Updated templates/base/http-clients/fetch-http-client.ejs to handle arrays in FormData
Added test fixture formdata-arrays.json to verify the fix
All existing tests pass with updated snapshots

Use Cases

This fix is essential for:

Multi-file upload endpoints
Batch document processing APIs
Any FormData field that accepts multiple values
Services handling attachments (e.g., agreement management systems)

Testing

✅ All existing tests pass
✅ Snapshots updated to reflect the new array handling
✅ Added test fixture for multi-file upload scenario

Backwards Compatibility

This change is fully backwards compatible:

Single values continue to work as before
Arrays are now handled correctly instead of being stringified
No breaking changes to the API

When FormData contains arrays (e.g., multiple file uploads), the current implementation incorrectly stringifies the entire array instead of appending each item individually. This fix: - Detects when a property is an array - Iterates through each array item - Properly appends each file/blob individually to FormData - Preserves non-file array items by JSON.stringify-ing them individually This enables proper multi-file uploads and array handling in FormData, which is essential for APIs that accept multiple attachments or files.

changeset-bot · 2025-09-23T14:22:34Z

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lc-soft · 2025-10-14T02:23:36Z

templates/base/http-clients/fetch-http-client.ejs

+
+                for (const formItem of propertyContent) {
+                    const isFileType = formItem instanceof Blob || formItem instanceof File;
+                    formData.append(key, isFileType ? formItem : JSON.stringify(formItem));


String type values do not need to be converted to strings again, as this can result in double quotes at both ends of the string

I have tried modifying it this way and it can solve the problem.

formData.append( key, formItem instanceof Blob || typeof formItem === "string" ? formItem : JSON.stringify(formItem) );

lc-soft · 2025-10-14T02:33:22Z

templates/base/http-clients/fetch-http-client.ejs

+                const propertyContent = property instanceof Array ? property : [property];
+
+                for (const formItem of propertyContent) {
+                    const isFileType = formItem instanceof Blob || formItem instanceof File;


Since File extends Blob, you can simplify this check to just formItem instanceof Blob.

@lc-soft

- Remove redundant File instanceof check (File extends Blob) - Add explicit string type check to prevent double-stringification - Strings now passed directly to FormData instead of JSON.stringify Thanks to @lc-soft for the code review and suggestions! 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <[email protected]>

omarmciver · 2025-10-14T16:17:44Z

@lc-soft Thank you for the excellent code review! I've implemented both of your suggestions:

✅ Simplified Blob check: Removed redundant File instanceof check since File extends Blob
✅ Fixed string handling: Added explicit string type check to prevent double-stringification

The updated code now correctly handles:

Blob/File types: Passed directly to FormData ✅
Strings: Passed directly without JSON.stringify ✅
Objects/Arrays: JSON.stringify applied ✅
Numbers/Booleans: JSON.stringify applied ✅

Most importantly: The original bug fix (handling arrays in FormData for multi-file uploads) still works perfectly! 🎉

Changes pushed in commit b9da1cd

Update test snapshots to reflect the simplified FormData handling: - Removed redundant File instanceof check - Added string type check to prevent double-stringification All 133 tests passing. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <[email protected]>

omarmciver · 2025-10-14T16:32:22Z

✅ Tests fixed!

Updated all test snapshots to reflect the refactored FormData handling. All 133 tests now passing.

Changes in commit 6e854b8:

28 snapshot files updated
114 snapshots refreshed
All tests green ✅

Saeid-Za · 2025-10-20T17:32:51Z

Thank you for the PR.
I'm curious, is there a standard way to map a nested object with files to a form data recursively?
so that we could extend it to support nested objects or provide a way for users to define the mapper function.

smorimoto · 2025-10-22T16:34:43Z

@codex review

chatgpt-codex-connector · 2025-10-22T16:37:48Z

Codex Review: Didn't find any major issues. Another round soon, please!

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smorimoto · 2025-10-22T16:38:59Z

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cursor · 2025-10-22T16:42:16Z

templates/base/http-clients/fetch-http-client.ejs

+
+                for (const formItem of propertyContent) {
+                    formData.append(key, formItem instanceof Blob || typeof formItem === "string" ? formItem : JSON.stringify(formItem));
+                }


Bug: FormData Serialization Fails with BigInt and Undefined

The FormData serialization logic now uses JSON.stringify for non-Blob/non-string values, which introduces two issues. BigInt values now cause a TypeError. Also, undefined values are passed as the undefined primitive to formData.append(), relying on implicit browser conversion instead of the previous explicit string conversion, which may lead to unexpected behavior.

Copilot

Pull Request Overview

This PR fixes a critical bug in FormData handling where arrays were being incorrectly stringified instead of having each item appended individually to the FormData object. This particularly affects multi-file upload endpoints and any FormData fields accepting multiple values.

Key Changes:

Modified FormData content formatter to detect and properly iterate through arrays
Each array item is now appended individually to FormData with the same key
Simplified type checking logic for Blob/string vs other data types

Reviewed Changes

Copilot reviewed 30 out of 30 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
templates/base/http-clients/fetch-http-client.ejs	Core fix: updated FormData formatter to handle arrays by iterating and appending each item individually
tests/fixtures/formdata-arrays.json	Added test fixture defining multi-file upload endpoint to validate array handling
tests/snapshots/simple.test.ts.snap	Updated snapshots reflecting new array handling logic across multiple test scenarios
tests/snapshots/extended.test.ts.snap	Updated snapshots for extended test cases
tests/spec/*/basic.test.ts.snap	Updated snapshots across all spec test suites to reflect the FormData array handling changes

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Copilot · 2025-10-23T07:11:26Z

templates/base/http-clients/fetch-http-client.ejs

+                const propertyContent = property instanceof Array ? property : [property];
+
+                for (const formItem of propertyContent) {
+                    formData.append(key, formItem instanceof Blob || typeof formItem === "string" ? formItem : JSON.stringify(formItem));


[nitpick] This line is overly long and hard to read. Consider breaking the ternary operation onto multiple lines for better readability, similar to the original formatting style.

Suggested change

formData.append(key, formItem instanceof Blob || typeof formItem === "string" ? formItem : JSON.stringify(formItem));

formData.append(

key,

formItem instanceof Blob || typeof formItem === "string"

? formItem

: JSON.stringify(formItem)

);

Copilot · 2025-10-23T07:11:26Z

templates/base/http-clients/fetch-http-client.ejs

+                const propertyContent = property instanceof Array ? property : [property];
+
+                for (const formItem of propertyContent) {
+                    formData.append(key, formItem instanceof Blob || typeof formItem === "string" ? formItem : JSON.stringify(formItem));


The condition formItem instanceof Blob || typeof formItem === 'string' does not handle File objects correctly. File extends Blob, but numbers, booleans, and other primitive types will be JSON.stringify'd instead of being converted to strings. This changes behavior from the original code which used template literal coercion (${property}) for primitives.

Suggested change

formData.append(key, formItem instanceof Blob || typeof formItem === "string" ? formItem : JSON.stringify(formItem));

formData.append(key, formItem instanceof Blob ? formItem : `${formItem}`);

fix: apply formatter to test fixture

e4c4bae

omarmciver mentioned this pull request Sep 23, 2025

Form Data "files" is not correctly handled with default fetch template #556

Open

lc-soft suggested changes Oct 14, 2025

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lc-soft reviewed Oct 14, 2025

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lc-soft approved these changes Oct 15, 2025

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smorimoto added the bug Something isn't working label Oct 22, 2025

cursor bot reviewed Oct 22, 2025

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smorimoto requested a review from Copilot October 23, 2025 07:10

Copilot AI reviewed Oct 23, 2025

View reviewed changes

	formData.append(key, formItem instanceof Blob \|\| typeof formItem === "string" ? formItem : JSON.stringify(formItem));
	formData.append(key, formItem instanceof Blob ? formItem : `${formItem}`);

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fix: handle arrays properly in FormData #1431

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fix: handle arrays properly in FormData #1431

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omarmciver commented Sep 23, 2025

Description

The Problem

Example scenario:

The Solution

Changes Made

Use Cases

Testing

Backwards Compatibility

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Bug: FormData Serialization Fails with BigInt and Undefined

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Pull Request Overview

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changeset-bot bot commented Sep 23, 2025 •

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