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Comments in JSON: Unexpected token /How to Fix: Comments in JSON: Unexpected token /
Standard JSON has no comment syntax. A // line comment or /* block comment */ anywhere in the document makes parsers fail with "Unexpected token /" (JavaScript) or "Expecting value" (Python) at the comment's position.
Comments were deliberately excluded from the JSON specification, partly to prevent them being abused for parsing directives that would fragment the format.
Common causes
1. Documenting a config file with // comments
Natural instinct from every programming language - but it breaks the file for standard parsers.
{
// server port
"port": 8080
}{
"port": 8080
}2. Copying from JSONC files (VS Code settings, tsconfig)
VS Code's settings.json and tsconfig.json are JSONC - a variant that allows comments. Copying their content into real JSON contexts carries the comments along.
How to fix it
{
"_comment": "Server port - keep in sync with the load balancer",
"port": 8080
}# YAML supports comments natively
port: 8080 # keep in sync with the load balancer
// JSON5 (needs a json5 parser library)
{
// keep in sync with the load balancer
port: 8080,
}Frequently asked questions
Why were comments removed from JSON?
Douglas Crockford removed them early in JSON's history after seeing them used to hold parsing directives, which threatened interoperability - and to keep the grammar minimal. Data-only, no annotations, every parser agrees.
Is the "_comment" key convention safe?
Yes - it is just a regular key that consuming code ignores. Downsides: it travels with the data (larger payloads) and only attaches per-object, not per-line. For heavily documented configs, YAML, TOML, or JSONC are better fits.
How do I strip comments from a JSONC file programmatically?
Use a JSONC-aware parser (like the jsonc-parser npm package or strip-json-comments) rather than a regex - naive regexes corrupt strings that contain // or /* sequences, like URLs.
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