JSON Parsing Is Only the First Step of Validation

Separate syntax from meaning

JSON.parse answers whether a string can be interpreted as JSON and returns the resulting JavaScript value. It does not answer whether that value matches your feature’s expectations. A string, null or array may be perfectly valid JSON while being unusable as a settings record. Write down the required data shape before treating parsing as successful import.

Build a small import contract

For a theme import, suppose the accepted object needs a nonempty name and a colors array of six-digit HEX strings. Parse the text inside try and catch, then check that the result is a non-null object rather than an array. Validate each required property and array entry separately. Only copy the allowed fields into the internal settings value.

Keep diagnostics specific

A syntax error should explain that the JSON could not be parsed. A shape error should identify the relevant field, such as colors must contain only supported HEX strings. Avoid replacing every failure with Invalid data when the user could easily fix the problem. Keep the original pasted text available so correcting one issue does not require reconstructing the entire import.

Watch number and duplicate-key behavior

JSON numbers become JavaScript numbers in an ordinary parse, which can lose precision for large integers. If exact external identifiers matter, transport them as strings. Also decide how your import policy treats duplicate object keys; a parsed object alone may not preserve evidence of those duplicates. A dedicated parser may be needed when detecting them is part of the contract.

Do not turn parsed data into executable content

Never use eval as a JSON parser. Do not insert imported strings as unrestricted HTML or merge arbitrary external properties into privileged configuration objects. Parsing is not sanitization, authorization or proof that an API response is trustworthy. Keep those boundaries explicit, and validate again on the server for operations whose rules must be enforced centrally.

Verify a useful rejection matrix

Test malformed JSON, null, an array, a missing name, an empty colors list and one invalid color. Include an unusually large input to check responsiveness and an integer outside the safe range. Confirm that a valid import produces a new reviewed value without silently changing unrelated settings. Good validation preserves user control while refusing data the feature cannot interpret reliably.

Further reading

• MDN: JSON.parse