Implementation:Cypress io Cypress RewriteJs

Knowledge Sources	Cypress
Domains	AST_Transformation, Proxy
Last Updated	2026-02-12 00:00 GMT

Overview

Concrete tool for rewriting JavaScript source code via AST transformation to enable cross-origin testing provided by the Cypress rewriter package.

Description

The rewriteJs function takes a URL and JavaScript source string, parses it into an AST using recast and ast-types, applies transformation rules (modifying window references, location access patterns, etc.), and returns the rewritten JavaScript. It handles source map rewriting for debugging support. On parse errors, it gracefully returns the original JavaScript unchanged.

Usage

This function is called by the HTTP proxy for every JavaScript file served to the browser during test execution. It enables Cypress to instrument application code for cross-origin testing support.

Code Reference

Source Location

• Repository: cypress-io/cypress
• File: packages/rewriter/lib/js.ts
• Lines: L98-107

Signature

export function rewriteJs(
  url: string,
  js: string,
  deferSourceMapRewrite?: DeferSourceMapRewriteFn
): string

Import

import { rewriteJs } from '@packages/rewriter'

I/O Contract

Inputs

Name	Type	Required	Description
url	string	Yes	URL of the JavaScript file being rewritten
js	string	Yes	JavaScript source code to rewrite
deferSourceMapRewrite	DeferSourceMapRewriteFn	No	Callback to defer source map rewriting

Outputs

Name	Type	Description
rewritten JS	string	Modified JavaScript with Cypress instrumentation, or original JS on error

Usage Examples

Proxy Integration

import { rewriteJs } from '@packages/rewriter'

// In the HTTP proxy response handler
const originalJs = responseBody.toString()
const rewrittenJs = rewriteJs(requestUrl, originalJs, deferSourceMapRewrite)
response.body = rewrittenJs

Related Pages

Implements Principle

• Principle:Cypress_io_Cypress_Browser_Test_Execution