@mswjs/interceptors/src/utils/createProxy.ts

Low-level HTTP/HTTPS/XHR/fetch request interception library.

import { findPropertySource } from './findPropertySource'

export interface ProxyOptions<Target extends Record<string, any>> {
  constructorCall?(args: Array<unknown>, next: NextFunction<Target>): Target

  methodCall?<F extends keyof Target>(
    this: Target,
    data: [methodName: F, args: Array<unknown>],
    next: NextFunction<void>
  ): void

  setProperty?(
    data: [propertyName: string | symbol, nextValue: unknown],
    next: NextFunction<boolean>
  ): boolean

  getProperty?(
    data: [propertyName: string | symbol, receiver: Target],
    next: NextFunction<void>
  ): void
}

export type NextFunction<ReturnType> = () => ReturnType

export function createProxy<Target extends object>(
  target: Target,
  options: ProxyOptions<Target>
): Target {
  const proxy = new Proxy(target, optionsToProxyHandler(options))

  return proxy
}

function optionsToProxyHandler<T extends Record<string, any>>(
  options: ProxyOptions<T>
): ProxyHandler<T> {
  const { constructorCall, methodCall, getProperty, setProperty } = options
  const handler: ProxyHandler<T> = {}

  if (typeof constructorCall !== 'undefined') {
    handler.construct = function (target, args, newTarget) {
      const next = Reflect.construct.bind(null, target as any, args, newTarget)
      return constructorCall.call(newTarget, args, next)
    }
  }

  handler.set = function (target, propertyName, nextValue) {
    const next = () => {
      const propertySource = findPropertySource(target, propertyName) || target
      const ownDescriptors = Reflect.getOwnPropertyDescriptor(
        propertySource,
        propertyName
      )

      // Respect any custom setters present for this property.
      if (typeof ownDescriptors?.set !== 'undefined') {
        ownDescriptors.set.apply(target, [nextValue])
        return true
      }

      // Otherwise, set the property on the source.
      return Reflect.defineProperty(propertySource, propertyName, {
        writable: true,
        enumerable: true,
        configurable: true,
        value: nextValue,
      })
    }

    if (typeof setProperty !== 'undefined') {
      return setProperty.call(target, [propertyName, nextValue], next)
    }

    return next()
  }

  handler.get = function (target, propertyName, receiver) {
    /**
     * @note Using `Reflect.get()` here causes "TypeError: Illegal invocation".
     */
    const next = () => target[propertyName as any]

    const value =
      typeof getProperty !== 'undefined'
        ? getProperty.call(target, [propertyName, receiver], next)
        : next()

    if (typeof value === 'function') {
      return (...args: Array<any>) => {
        const next = value.bind(target, ...args)

        if (typeof methodCall !== 'undefined') {
          return methodCall.call(target, [propertyName as any, args], next)
        }

        return next()
      }
    }

    return value
  }

  return handler
}