How to Validate JSON

Problem

You have a JSON string from an untrusted or hand-edited source and need to verify it is syntactically correct.

Prerequisites

• Understanding of JSON syntax
• Ability to read error messages

Steps

1. 1. Parse the JSON string

   Attempt to parse the string as JSON. A standards-compliant parser will throw an error if the syntax is invalid. Common errors include trailing commas, single-quoted strings, and missing brackets.

   Input

   {"name":"John",}

   Output

   Error: Unexpected token } at position 15

2. 2. Interpret the error location

   If parsing fails, the error message typically includes the position or line number of the problem. Go to that location in the input and fix the syntax issue. Repeat until parsing succeeds.

   Input

   position 15

   Output

   trailing comma before }

3. 3. Optionally validate against a schema

   If you have a JSON Schema, validate the parsed JSON against it to check not just syntax but structure (required fields, types, value ranges). Schema validation catches semantic errors that syntax parsing cannot.

   Input

   JSON + schema

   Output

   valid or schema errors

Expected result

Confirmation that the JSON is valid, or a clear error message identifying the problem and its location.

Common errors

Trailing comma in object or array

JSON does not allow trailing commas. Remove the comma after the last element.

Single-quoted strings

JSON requires double quotes for strings and keys. Replace single quotes with double quotes.

Troubleshooting

• If the error position seems wrong, check for invisible characters like BOM or zero-width spaces.
• Use a JSON validator with line and column numbers for easier debugging.

Security limits

• Parsing untrusted JSON is generally safe, but do not use eval() to parse JSON — it can execute arbitrary code. Use JSON.parse instead.