json/no-duplicate-keys

When an object has the same key twice, JSON.parse keeps the last value and silently drops the others. Someone reading the file may only notice the first one. It's usually left over from a merge or a copy and paste.

An escape sequence counts as the character it stands for. In JSON5, a, 'a', and "a" are the same key.

Configure#

Off by default. It needs an ESLint config that says which files are in the language:

import json from "@eslint/json";

export default [
  {
    files: ["**/*.json"],
    plugins: { json },
    language: "json/json",
    rules: { "json/no-duplicate-keys": "error" },
  },
];

It also works with json/jsonc and json/json5.

Examples#

{
  "name": "app",
  "private": true,
  "name": "server"
}

{
  port: 3000,
  "port": 8080,
}

{
  "name": "app",
  "author": { "name": "Ada" }
}