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JSON Keys must be Valid Strings According to JSON.org, a string is a sequence of zero or more Unicode characters, wrapped in double quotes, using backslash escapes. Any valid string can be used as a JSON key. These keys must be enclosed in the double-quotes ( " ).
JSON objects are written in key/value pairs. JSON objects are surrounded by curly braces { } . Keys must be strings, and values must be a valid JSON data type (string, number, object, array, boolean or null). Keys and values are separated by a colon.
It means that the editor failed to get a response to the server or the response wasn't in a valid JSON format. Basically, if the editor can't communicate with the server, it will show this error message instead.
No. Any valid string is a valid key. It can even have " as long as you escape it:
{"The \"meaning\" of life":42}
There is perhaps a chance you'll encounter difficulties loading such values into some languages, which try to associate keys with object field names. I don't know of any such cases, however.
The following characters must be escaped in JSON data to avoid any problems:
" (double quote)\ (backslash)\n, \t
JSON Parser can help you to deal with JSON.
It is worth mentioning that while starting the keys with numbers is valid, it could cause some unintended issues.
Example:
var testObject = {
"1tile": "test value"
};
console.log(testObject.1tile); // fails, invalid syntax
console.log(testObject["1tile"]; // workaround
Unicode codepoints U+D800 to U+DFFF must be avoided: they are invalid in Unicode because they are reserved for UTF-16 surrogate pairs. Some JSON encoders/decoders will replace them with U+FFFD. See for example how the Go language and its JSON library deals with them.
So avoid "\uD800" to "\uDFFF" alone (not in surrogate pairs).
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