JSON Formatter

JSON (JavaScript Object Notation) is a lightweight, text-based data interchange format that is easy for humans to read and write and easy for machines to parse and generate. While JSON is designed to be readable, it is often transmitted or stored in compact form with no extra whitespace to save bandwidth and storage space. This compact form is extremely difficult for developers to read, debug, and modify manually. Formatting adds consistent indentation, line breaks, and visual structure that makes the data hierarchy immediately apparent. This is essential for debugging API responses (where errors may be hidden deep in nested structures), reviewing configuration files (where a single typo can cause application failures), analyzing log data (where JSON-formatted logs need to be searchable and comprehensible), and creating documentation examples (where clarity is paramount).

This tool detects comprehensive JSON syntax errors including: missing or extra commas between array elements or object properties, trailing commas (the last comma before a closing bracket or brace), unquoted or improperly quoted property names (JSON requires double quotes), missing opening or closing brackets and braces (structural mismatch), unclosed string literals (missing terminating quote), invalid escape sequences in strings (like \q which is not a valid escape), duplicate keys within the same object (technically valid but often unintended), invalid number formats (like 1.2.3 or trailing decimal points), unrecognized tokens or characters outside of valid JSON syntax, and BOM (Byte Order Mark) characters that interfere with parsing. For each error, the tool reports the exact line number, column position, and a clear description of what is wrong with suggestions for how to fix it.

Standard JSON (RFC 8259) does not allow comments or trailing commas, so pure JSON validation will flag these as errors. However, many practical JSON formats like JSON5 and configuration files (TypeScript tsconfig.json, ESLint configs, VS Code settings) use comments and trailing commas. Our tool detects when these non-standard features are present and offers a 'Parse as JSON5' mode that correctly handles single-line comments (//), multi-line comments (/* */), trailing commas in arrays and objects, unquoted keys, single-quoted strings, and multi-line strings. This allows you to format and validate these files properly. After formatting as JSON5, you can optionally convert to strict standard JSON by removing comments and trailing commas for environments that require strict compliance.

For JSON files up to approximately 5MB, the tool processes entirely in your browser without any server upload, ensuring your data never leaves your machine. For larger files, the browser-based parser may slow down depending on your device memory and processing power. The formatter uses streaming JSON parsing to handle large inputs efficiently, and the tree view only renders visible nodes to keep the DOM lightweight. If you experience performance issues with very large files, try enabling the 'Compact mode' which disables syntax highlighting, or process the file in smaller sections. For extremely large JSON documents (50MB+), consider using command-line tools like jq for initial filtering before formatting subsets in our tool.

JSON (RFC 8259) is the strict standard: only double-quoted strings, property names must be quoted, no comments, no trailing commas. JSON5 is an extension that relaxes these rules to be more human-friendly: allows single and double quotes, unquoted keys (if valid identifiers), comments (single-line and multi-line), trailing commas, and leading or trailing decimal points in numbers. JSON5 is commonly used for configuration files. JSON Lines (JSONL) is a format where each line is a separate valid JSON value (usually an object), used for log files, streaming data, and large datasets where each record should be independently parseable. Our tool detects the format automatically and provides appropriate parsing and formatting rules for each. JSONL files are formatted with one pretty-printed JSON object per line, maintaining the line-delimited structure while adding indentation within each object.