Safe JSON Parse (Fallback on Error)

easy

By FrontendAtlas Editorial · Updated Feb 3, 2026

Implement safeJsonParse(text, fallback = null) to safely parse JSON. It should return the parsed value for valid JSON strings and the fallback for invalid JSON or non-string inputs. This is a common edge-case strategy for parsing unknown inputs.

Arguments

text: string — The JSON string to parse.
fallback: any — Value to return when parsing fails. Defaults to null.

Returns

any —

Parsed JSON value or the fallback.

Examples

safeJsonParse('{"ok":true}') // => { ok: true }

safeJsonParse('not json', []) // => []

safeJsonParse(42, { ok: false }) // => { ok: false }

Solution

Overview

Guard the input type, then try JSON.parse in a try/catch. On any error, return the fallback. Keep the behavior predictable so tests can cover invalid inputs and boundary cases.

Approach: Guard + try/catch

Steps:

1) If text is not a string, return fallback.

2) Attempt JSON.parse.

3) On error, return fallback.

Complexity: O(n) time for parsing, O(1) extra space.

export default function safeJsonParse(text, fallback = null) {
  if (typeof text !== 'string') return fallback;
  try {
    return JSON.parse(text);
  } catch {
    return fallback;
  }
}

export default function safeJsonParse(text: string, fallback: any = null): any {
  if (typeof text !== 'string') return fallback;
  try {
    return JSON.parse(text);
  } catch {
    return fallback;
  }
}

Notes & Pitfalls

Pitfalls

Assuming JSON.parse always returns an object (it can return numbers, strings, booleans, or null).
Catching errors but returning undefined, which makes tests ambiguous.
Parsing non-strings without guarding can throw or behave unexpectedly.

Edge cases

Valid primitives like '0', 'true', and 'null' should parse correctly.
Whitespace around JSON is valid and should parse.
Invalid JSON should always return the fallback without throwing.

Techniques

Type guards before parsing.
Deterministic fallback behavior for tests.
Focused unit tests per edge case.

Resources

MDN – JSON.parse()

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