Minified JSON from an API response is a single line that is essentially unreadable. Formatting applies consistent indentation and line breaks so you can see the structure, find the field you need, and spot where an unexpected null or empty array is hiding.
The usual culprits are a trailing comma after the last item, single quotes instead of double quotes, unquoted keys, or a stray comment. All four are valid JavaScript and invalid JSON. The error message includes a character position to help you locate it.
No. Only whitespace between tokens changes. Keys, values, types and ordering are all preserved exactly.
Two spaces is the most common convention and keeps deeply nested structures from running off the screen. Four is easier to scan for shallow documents. Minify for anything being transmitted, since whitespace is pure overhead on the wire.
No. API responses routinely contain tokens, personal data and internal identifiers, so this runs entirely in your browser.