diff --git a/javascript/packages/analysis/src/affected-templates.ts b/javascript/packages/analysis/src/affected-templates.ts
new file mode 100644
index 000000000..0deee26f1
--- /dev/null
+++ b/javascript/packages/analysis/src/affected-templates.ts
@@ -0,0 +1,74 @@
+import type { TemplateDependencies } from "./template-dependencies"
+
+export interface TemplateGraph {
+  dependencies: Map<string, TemplateDependencies>
+  filesForPartial(partialName: string): string[]
+}
+
+export function expressionReferences(expression: string | undefined, name: string): boolean {
+  if (!expression || !name) return false
+  if (name.startsWith("@")) return expression.includes(name)
+
+  return new RegExp(`\\b${name.replace(/[.*+?^${}()|[\]\\]/g, "\\$&")}\\b`).test(expression)
+}
+
+export function affectedTemplates(graph: TemplateGraph, entryPoint: string, state: string): string[] {
+  const entry = graph.dependencies.get(entryPoint)
+  if (!entry) return []
+
+  if (!entry.instanceVariables.includes(state) && !entry.constants.includes(state)) {
+    return []
+  }
+
+  const affected = new Set<string>([entryPoint])
+  const carrying = new Map<string, Set<string>>([[entryPoint, new Set([state])]])
+  const visited = new Set<string>()
+  const queue = [entryPoint]
+
+  while (queue.length > 0) {
+    const file = queue.shift()!
+
+    if (visited.has(file)) continue
+
+    visited.add(file)
+
+    const dependencies = graph.dependencies.get(file)
+    if (!dependencies) continue
+
+    const carried = [...(carrying.get(file) ?? [])]
+
+    for (const call of dependencies.renderCalls) {
+      const flowing = Object.entries(call.locals).filter(([, expression]) => carried.some(name => expressionReferences(expression, name))).map(([local]) => local)
+      const collectionFlows = carried.some(name => expressionReferences(call.collection, name))
+
+      if (flowing.length === 0 && !collectionFlows) {
+        continue
+      }
+
+      for (const partialFile of graph.filesForPartial(call.partial)) {
+        const names = carrying.get(partialFile) ?? new Set<string>()
+
+        for (const local of flowing) {
+          names.add(local)
+        }
+
+        if (collectionFlows) {
+          names.add(itemNameFor(call.partial))
+        }
+
+        carrying.set(partialFile, names)
+
+        if (!affected.has(partialFile)) {
+          affected.add(partialFile)
+          queue.push(partialFile)
+        }
+      }
+    }
+  }
+
+  return [...affected].sort()
+}
+
+function itemNameFor(partialName: string): string {
+  return partialName.split("/").pop() ?? partialName
+}
diff --git a/javascript/packages/analysis/src/index.ts b/javascript/packages/analysis/src/index.ts
index f79042853..118b7941f 100644
--- a/javascript/packages/analysis/src/index.ts
+++ b/javascript/packages/analysis/src/index.ts
@@ -1,6 +1,7 @@
-export * from "./partial-index"
+export * from "./affected-templates"
+export * from "./ancestor-attributes"
 export * from "./partial-callers"
+export * from "./partial-index"
 export * from "./partial-resolution"
 export * from "./render-expression"
-export * from "./ancestor-attributes"
 export * from "./template-dependencies"
diff --git a/javascript/packages/analysis/test/affected-templates.test.ts b/javascript/packages/analysis/test/affected-templates.test.ts
new file mode 100644
index 000000000..670ed4dab
--- /dev/null
+++ b/javascript/packages/analysis/test/affected-templates.test.ts
@@ -0,0 +1,125 @@
+import { describe, test, expect, beforeAll } from "vitest"
+
+import { Herb } from "@herb-tools/node-wasm"
+
+import { affectedTemplates, expressionReferences } from "../src/affected-templates"
+import { collectTemplateDependencies } from "../src/template-dependencies"
+
+import type { TemplateGraph } from "../src/affected-templates"
+
+describe("affectedTemplates", () => {
+  beforeAll(async () => {
+    await Herb.load()
+  })
+
+  function graphOf(files: Record<string, string>): TemplateGraph {
+    const dependencies = new Map(
+      Object.entries(files).map(([file, source]) => [file, collectTemplateDependencies(Herb, file, source)])
+    )
+
+    const byPartial = new Map<string, string[]>()
+
+    for (const file of Object.keys(files)) {
+      const name = file.replace(/^app\/views\//, "").replace(/\/_/, "/").replace(/\.html\.erb$/, "")
+
+      if (!file.split("/").pop()!.startsWith("_")) continue
+
+      byPartial.set(name, [...(byPartial.get(name) ?? []), file])
+    }
+
+    return { dependencies, filesForPartial: name => byPartial.get(name) ?? [] }
+  }
+
+  test("traces state through the render graph", () => {
+    const graph = graphOf({
+      "app/views/posts/show.html.erb": `<%= @post.title %><%= render "posts/header", post: @post %>`,
+      "app/views/posts/_header.html.erb": `<h1><%= post.name %></h1>`,
+    })
+
+    const affected = affectedTemplates(graph, "app/views/posts/show.html.erb", "@post")
+
+    expect(affected).toContain("app/views/posts/show.html.erb")
+    expect(affected).toContain("app/views/posts/_header.html.erb")
+  })
+
+  test("does not include unrelated templates", () => {
+    const graph = graphOf({
+      "app/views/posts/show.html.erb": `<%= @post.title %><%= render "posts/header", post: @post %>`,
+      "app/views/posts/_header.html.erb": `<h1><%= post.name %></h1>`,
+      "app/views/pages/about.html.erb": `<h1>About</h1>`,
+    })
+
+    expect(affectedTemplates(graph, "app/views/posts/show.html.erb", "@post")).not.toContain("app/views/pages/about.html.erb")
+  })
+
+  test("traces through nested renders", () => {
+    const graph = graphOf({
+      "app/views/posts/show.html.erb": `<%= render "posts/header", post: @post %>`,
+      "app/views/posts/_header.html.erb": `<%= render "posts/title", title: post.title %>`,
+      "app/views/posts/_title.html.erb": `<h1><%= title %></h1>`,
+    })
+
+    const affected = affectedTemplates(graph, "app/views/posts/show.html.erb", "@post")
+
+    expect(affected).toContain("app/views/posts/show.html.erb")
+    expect(affected).toContain("app/views/posts/_header.html.erb")
+    expect(affected).toContain("app/views/posts/_title.html.erb")
+  })
+
+  test("handles constants", () => {
+    const graph = graphOf({ "app/views/posts/index.html.erb": `<%= Post.count %>` })
+
+    expect(affectedTemplates(graph, "app/views/posts/index.html.erb", "Post.count")).toContain("app/views/posts/index.html.erb")
+  })
+
+  test("returns nothing when the entry point does not read the state", () => {
+    const graph = graphOf({
+      "app/views/posts/show.html.erb": `<%= @other.title %><%= render "posts/header", post: @other %>`,
+      "app/views/posts/_header.html.erb": `<h1><%= post.name %></h1>`,
+    })
+
+    expect(affectedTemplates(graph, "app/views/posts/show.html.erb", "@post")).toEqual([])
+  })
+
+  test("returns nothing for a template it does not know", () => {
+    expect(affectedTemplates(graphOf({}), "app/views/posts/show.html.erb", "@post")).toEqual([])
+  })
+
+  test("stops where the state stops flowing", () => {
+    const graph = graphOf({
+      "app/views/posts/show.html.erb": `<%= @post.title %><%= render "posts/header", title: "static" %>`,
+      "app/views/posts/_header.html.erb": `<h1><%= title %></h1>`,
+    })
+
+    const affected = affectedTemplates(graph, "app/views/posts/show.html.erb", "@post")
+
+    expect(affected).toEqual(["app/views/posts/show.html.erb"])
+  })
+
+  test("follows a collection through to the partial", () => {
+    const graph = graphOf({
+      "app/views/posts/index.html.erb": `<%= @posts.count %><%= render partial: "posts/post", collection: @posts %>`,
+      "app/views/posts/_post.html.erb": `<%= post.title %>`,
+    })
+
+    expect(affectedTemplates(graph, "app/views/posts/index.html.erb", "@posts")).toContain("app/views/posts/_post.html.erb")
+  })
+
+  describe("expressionReferences", () => {
+    test("matches an instance variable literally", () => {
+      expect(expressionReferences("@post.title", "@post")).toBe(true)
+      expect(expressionReferences("@poster.title", "@post")).toBe(true)
+    })
+
+    test("matches a local on word boundaries", () => {
+      expect(expressionReferences("post.title", "post")).toBe(true)
+      expect(expressionReferences("posts.count", "post")).toBe(false)
+      expect(expressionReferences("post_id", "post")).toBe(false)
+    })
+
+    test("is false for nothing", () => {
+      expect(expressionReferences(undefined, "post")).toBe(false)
+      expect(expressionReferences("post.title", "")).toBe(false)
+    })
+  })
+})