isJSON
Checks if a string is a valid JSON.
1/**
2 * Checks if a string is a valid JSON.
3 *
4 * @param str - The string to check.
5 * @returns True if the string is valid JSON, false otherwise.
6 */
7export function isJSON(str: string): boolean {
8 if (typeof str !== 'string') return false;
9 try {
10 const result = JSON.parse(str);
11 return typeof result === 'object' && result !== null;
12 } catch {
13 return false;
14 }
15}Safe Parsing with Error Handling
Wraps
JSON.parsein atry/catchblock to gracefully handle malformed input without crashing the application.Object-Only Verification
Ensures that the parsed result is a non-null object, avoiding cases where
JSON.parse("123")would misleadingly returntrue.Input Type Guard
Quickly returns
falsefor non-string inputs, avoiding unnecessary processing or runtime errors.Lightweight Utility
Offers a compact and dependency-free solution for JSON validation, suitable for both frontend and backend contexts.
Tests | Examples
1test('returns true for valid JSON objects', () => {
2 expect(isJSON('{"key":"value"}')).toBe(true);
3 expect(isJSON('{"a":1,"b":[1,2,3],"c":null}')).toBe(true);
4});
5
6test('returns true for valid JSON arrays', () => {
7 expect(isJSON('[1, 2, 3]')).toBe(true);
8 expect(isJSON('[]')).toBe(true);
9});
10
11test('returns false for primitives', () => {
12 expect(isJSON('"string"')).toBe(false);
13 expect(isJSON('123')).toBe(false);
14 expect(isJSON('true')).toBe(false);
15 expect(isJSON('null')).toBe(false);
16});
17
18test('returns false for malformed JSON', () => {
19 expect(isJSON('{key:value}')).toBe(false);
20 expect(isJSON('{')).toBe(false);
21 expect(isJSON('')).toBe(false);
22});
23
24test('returns false for non-string inputs', () => {
25 expect(isJSON(undefined as any)).toBe(false);
26 expect(isJSON(null as any)).toBe(false);
27 expect(isJSON(123 as any)).toBe(false);
28});Common Use Cases
Validating API Payloads
Check if incoming strings (e.g., from HTTP request bodies or query params) are valid JSON before parsing.
Form or User Input Validation
Safely confirm that a string input is valid JSON before applying business logic.
Preprocessing Config Files or Templates
Detect whether configuration files or dynamic text blocks are properly formatted JSON.
Data Import or ETL Pipelines
Filter or reject invalid records before further processing during import/export tasks.
Storage Safety Checks
Ensure that a string being stored to or retrieved from localStorage/sessionStorage is valid JSON.