Client-Side Testing

Last updated 28 October 2026

Quick answer

Client-side testing assigns the A/B variant in the visitor's browser. A script loads, picks the variant, and changes the page.

Key takeaways

Client-side means the browser decides the variant and changes the DOM. Server-side means the server decides before HTML is sent.
Visible copy, layout, and CTA tests fit client-side. Pricing logic, search, and checkout rules usually need the server.
The trade-off guide is the server-side versus client-side post. The other glossary entry is server-side testing.

Definition

Client-side testing assigns the A/B variant in the visitor's browser. A script loads, picks the variant, and changes the page. Server-side testing assigns the variant before the response is sent.

What Client-Side Testing means in A/B testing

Client-side is where the decision runs, not a type of idea. The page can arrive as the control. JavaScript then swaps a headline, hides a block, or inserts a component. Marketers can ship those changes from a visual editor without a backend release. The same script cannot reliably test logic that never reaches the browser: the price the server charges, the search ranking, or which recommendation the API returns. Those need the server to choose the variant while it builds the response.

Why Client-Side Testing matters

Picking one mode for every test is how teams get stuck. A headline test does not need an engineering ticket. A checkout-price test does not belong in a DOM swap. The comparison post's rule of thumb is: if the change is visible on the page, client-side is enough; if money, search, or a rule in the backend is involved, use the server; if the same person must keep the variant across sessions in a way the browser cannot hold, use the server. Flicker and single-page navigation are the client-side costs to plan for.

Example of Client-Side Testing

A homepage headline test is client-side. The snippet changes the text after load, and the goal is a click. A test of which shipping price the cart API returns is server-side. The browser only displays the number the server already chose. Running the shipping test by rewriting the price in the DOM would show one price and charge another.

How to use Client-Side Testing

Use it for copy, layout, images, and other changes the browser can apply to HTML you already sent. Put the snippet in the head. Preview the variant for a flash of the control, and use a URL-change or DOM-change trigger if the site navigates without a full load. Move the test server-side when the variant is a backend rule. Read the comparison post before you standardize on one mode.

Common mistake

A common mistake is client-side testing a single-page app with a script that runs only on the first load. The next route paints the control again. Another is assuming a client-side price change is the price the order will use. Confirm the charged amount, not only the number on the screen.

How Mida works with client-side testing

Mida's website experiments run from the JavaScript snippet. The visual editor applies text, image, and layout changes in the browser, and SPA support covers URL-change and DOM-change triggers so a client-rendered route can be tested again after navigation. The experiment FAQ also says Mida can run server-side or full-stack workflows when the test has to live outside the browser. Feature flags are documented the same way: web flags in the page, and server-side flags when the decision is not in the DOM. Choose the mode per test using the comparison post.

Related A/B testing terms

FAQ

Can a client-side test change a price that is calculated on the server?

It can change the number the browser displays. It cannot, by itself, change the price the server charges. If the displayed price and the charged price differ, the test is measuring a lie. Pricing, tax, and inventory rules belong in a server-side test, which is the split the comparison post recommends when money is involved.

Why do client-side tests flicker?

The first HTML is often the control. The script then swaps in the variant. If that swap happens after paint, the visitor sees the control flash. Anti-flicker hides the page briefly, and it has to live in the head with a timeout. A server-rendered variant does not have that flash, because the browser never receives the control HTML.

Does Mida only run client-side?

No. The website snippet and visual editor are client-side. The feature-flag and experiment FAQs also describe server-side or full-stack workflows when the decision has to happen outside the browser. The comparison post is the place to choose per test, not a single mode for the whole program.