JavaScript scripting

When a rule editor can’t express what you need, write it. A script is two plain functions:

function onRequest(context, url, request) {
  request.headers["X-Debug"] = "1";
  return request;
}

function onResponse(context, url, request, response) {
  if (response.body && typeof response.body === "object") {
    response.body.isPremium = true;
  }
  return response;
}

HTTPGlass calls them for every message that matches the script’s pattern and sends on whatever you return. JSON bodies arrive already parsed, so response.body.isPremium = true is the whole edit. Return nothing to leave a message exactly as it was.

Familiar hooks

The hook names, argument order, and object shapes follow Proxyman’s scripting API, so scripts written for it are a good starting point here.

In the editor

• Templates: start from “Add an auth header”, “Force an error response”, “Edit a JSON field”, “Strip tracking parameters”, “Log every response size”, or a blank starter
• Test pane: run a script against a sample request and see the result and any console.log or context.log output before it touches live traffic
• Per-hook switches: run only onRequest, only onResponse, or both
• Can’t stall your traffic: a hook that runs longer than two seconds is abandoned and the message goes through unchanged

Scripts run after Rewrite rules and before breakpoints, so a script sees what a rule did and can override it.

Adding a script

Select Scripts in the sidebar and pick a template. Then give the script a pattern, using the shared rule syntax, and optionally limit it to one HTTP method. A script with no pattern never runs, and the list says so. The Run onRequest and Run onResponse switches turn either hook on or off without editing the code.

Bodies and return values

• A JSON body arrives already parsed. Change fields on the object and HTTPGlass re-serialises it and updates Content-Length for you.
• A text body arrives as a string.
• A binary body arrives as body: null with the bytes in bodyBase64. Set bodyBase64 to replace it.
• Return the request or response object to apply your changes. Return null or nothing to leave the message exactly as it was.

Limits and behaviour

• If a hook throws, HTTPGlass logs the error with the script’s name and the message continues unchanged.
• When several scripts match, they run in list order, and each receives the previous one’s output.
• When a script declares onResponse, HTTPGlass asks the server for an uncompressed response on matching requests so the body can be edited.
• Scripts have no fetch and no network access. A script edits the message in front of it and nothing else.

Free and Pro

Scripting is part of Pro. If a subscription ends, your scripts stay saved but stop running until you activate a licence again.

Related

• Scripting in the docs: the full object reference
• Rewrite: no-code edits for the cases a rule can express
• Breakpoints: edit one message by hand
• Map Local: answer with a response you define