JSON.stringify() with replacer
ES5+Uses a replacer function to filter or transform values during JSON stringification.
Syntax
JSON.stringify(value, replacer)Parameters
value any The value to convert to a JSON string
replacer Function A function that receives key and value, and returns the replacement value
Return Value
A JSON string with transformed values
Examples
const obj = {
name: 'John',
password: 'secret123',
age: 30
};
const result = JSON.stringify(obj, (key, value) => {
if (key === 'password') return undefined;
return value;
});
console.log(result);📌 When to Use
Use a replacer function when you need to transform values during stringification, such as filtering sensitive data, converting custom types, or handling circular references.
⚠️ Common Mistakes
Returning undefined from the replacer removes the property entirely, which may not be the intended behavior.
✅ Best Practices
Use a replacer function for complex transformations and a replacer array for simple property filtering.
⚡ Performance Notes
The replacer function is called for every property, so keep it simple for large objects. Consider pre-processing data instead of using complex replacers.
🌍 Real World Example
Logging Sanitizer
Sanitize objects for logging by removing sensitive data and truncating long strings.
function sanitizeForLogging(obj) {
const sensitivePatterns = /password|secret|token|key|auth/i;
const maxStringLength = 100;
const replacer = (key, value) => {
if (sensitivePatterns.test(key)) {
return '[HIDDEN]';
}
if (typeof value === 'string' && value.length > maxStringLength) {
return value.substring(0, maxStringLength) + '... [truncated]';
}
return value;
};
return JSON.stringify(obj, replacer, 2);
}
const request = {
url: '/api/users',
body: { password: 'secret123' },
longField: 'a'.repeat(200)
};
console.log(sanitizeForLogging(request));