How I Stopped My Jest Snapshots From Churning

// Walk a JS value, replace anything that looks volatile with a stable token.
// Stage 1: pure function, no Jest dependency. You can run this on any object.

const ISO_DATE = /^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}(?:\.\d+)?(?:Z|[+-]\d{2}:?\d{2})$/;
const UUID = /^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/i;
const HEX_HASH = /^[0-9a-f]{32,}$/i;
const REDACT_KEYS = new Set(['createdAt', 'updatedAt', 'requestId', 'traceId', 'sessionId']);

function redact(value, parentKey = '') {
    if (REDACT_KEYS.has(parentKey)) return `<${parentKey}>`;
    if (typeof value === 'string') {
        if (ISO_DATE.test(value)) return '<ISO_DATE>';
        if (UUID.test(value)) return '<UUID>';
        if (HEX_HASH.test(value)) return '<HEX>';
        return value;
    }
    if (Array.isArray(value)) return value.map((v) => redact(v));
    if (value && typeof value === 'object') {
        const out = {};
        for (const [k, v] of Object.entries(value)) out[k] = redact(v, k);
        return out;
    }
    return value;
}

const sample = {
    id: 'a3f1b8d2-4c7e-4f3a-9b81-1234567890ab',
    user: 'alice',
    createdAt: '2024-08-22T14:03:11.521Z',
    requestId: 'req_8f0e',
    payload: {
        traceId: '7c1ad9e0',
        contentHash: '7d865e959b2466918c9863afca942d0fb89d7c9ac0c99bafc3749504ded97730',
    },
};

console.log(JSON.stringify(redact(sample), null, 2));

// Stage 2: the Jest plugin shape.
// In a real project you would put this file path under jest.config.js#snapshotSerializers.
// Here we simulate the test() / serialize() contract Jest expects.

const ISO_DATE = /^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}(?:\.\d+)?(?:Z|[+-]\d{2}:?\d{2})$/;
const UUID = /^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/i;
const REDACT_KEYS = new Set(['createdAt', 'updatedAt', 'requestId']);

function redact(value, parentKey = '') {
    if (REDACT_KEYS.has(parentKey)) return `<${parentKey}>`;
    if (typeof value === 'string') {
        if (ISO_DATE.test(value)) return '<ISO_DATE>';
        if (UUID.test(value)) return '<UUID>';
        return value;
    }
    if (Array.isArray(value)) return value.map((v) => redact(v));
    if (value && typeof value === 'object') {
        const out = {};
        for (const [k, v] of Object.entries(value)) out[k] = redact(v, k);
        return out;
    }
    return value;
}

// Jest expects: `module.exports = { test, serialize }`
const snapshotSerializer = {
    // Apply to plain objects and arrays only; let other serializers handle React, etc.
    test(value) {
        return value !== null && (Array.isArray(value) || (typeof value === 'object' && value.constructor === Object));
    },
    serialize(value, _config, indentation, depth, _refs, printer) {
        const cleaned = redact(value);
        // Defer to Jest's default printer for indentation. Here we approximate.
        return JSON.stringify(cleaned, null, 2)
            .split('\n')
            .map((line, i) => (i === 0 ? line : indentation + line))
            .join('\n');
    },
};

// Simulate Jest calling the serializer.
function simulateSnapshot(value) {
    if (snapshotSerializer.test(value)) {
        return snapshotSerializer.serialize(value, null, '  ', 0, null, null);
    }
    return JSON.stringify(value);
}

const response = {
    user: { id: 'a3f1b8d2-4c7e-4f3a-9b81-1234567890ab', name: 'Alice' },
    requestId: 'req_8f0e',
    createdAt: '2024-08-22T14:03:11.521Z',
};
console.log(simulateSnapshot(response));

// Stage 3: per-test overrides without re-registering the serializer.
// Use a thread-local config object that the serializer reads.

const defaults = {
    keys: new Set(['createdAt', 'updatedAt', 'requestId', 'traceId']),
    patterns: [
        { match: /^\d{4}-\d{2}-\d{2}T/, token: '<ISO_DATE>' },
        { match: /^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/i, token: '<UUID>' },
    ],
};

let currentConfig = defaults;

function withRedaction(overrides, fn) {
    const previous = currentConfig;
    currentConfig = {
        keys: new Set([...previous.keys, ...(overrides.keys || [])]),
        patterns: [...previous.patterns, ...(overrides.patterns || [])],
    };
    try {
        return fn();
    } finally {
        currentConfig = previous;
    }
}

function redact(value, parentKey = '') {
    if (currentConfig.keys.has(parentKey)) return `<${parentKey}>`;
    if (typeof value === 'string') {
        for (const { match, token } of currentConfig.patterns) {
            if (match.test(value)) return token;
        }
        return value;
    }
    if (Array.isArray(value)) return value.map((v) => redact(v));
    if (value && typeof value === 'object') {
        const out = {};
        for (const [k, v] of Object.entries(value)) out[k] = redact(v, k);
        return out;
    }
    return value;
}

// Default: stripe-like ids stay visible.
console.log('default:', redact({ stripeId: 'cus_O7yQ123', userId: 'a3f1b8d2-4c7e-4f3a-9b81-1234567890ab' }));

// Inside a specific test we want stripe ids redacted too.
withRedaction({
    patterns: [{ match: /^(?:cus|sub|pi|in)_[A-Za-z0-9]+$/, token: '<STRIPE_ID>' }],
    keys: ['internalRef'],
}, () => {
    console.log('scoped:', redact({
        stripeId: 'cus_O7yQ123',
        internalRef: 'whatever',
        userId: 'a3f1b8d2-4c7e-4f3a-9b81-1234567890ab',
    }));
});

// After the helper returns, defaults are restored.
console.log('default again:', redact({ stripeId: 'cus_O7yQ999' }));