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Overview

The JavaScript SDK lets you collect card payments inside your own web checkout. Customers stay on your site, and the card details are entered into secure Modulus-hosted fields that your page never touches. An integration has two pieces:

Payment Intents API

A server-side REST call that represents an amount the customer has agreed to pay. Your backend creates the intent with your secret key.

JavaScript SDK

A browser library that renders the card fields, runs 3D Secure, and confirms the payment with your publishable key.
Card data is entered into Modulus-hosted iframes. It never enters your frontend code or DOM.

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End to end

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How it works

1

Create a Payment Intent (server)

When the customer reaches checkout, your server calls Create a Payment Intent with the amount and currency, using your secret key. You receive an id and a client_secret.
2

Hand the intent to the browser

Pass the id and client_secret to your checkout page. Your server never sees card data from here on.
3

Mount the card fields

Load the SDK, initialize it with your publishable key, and mount the secure card fields into your checkout layout.
4

Confirm the payment

Call confirmPayment. The SDK collects the card securely, runs 3D Secure if the bank requires it, and resolves with the final result.

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Prerequisites

You are provisioned two keys at onboarding, with separate sandbox and live sets:

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Base URLs

This documentation targets the sandbox environment. Use keys provisioned for sandbox; no real money is moved. See Testing for the sandbox card numbers. Amounts are integers in the currency’s smallest unit (for PHP, centavos).