diff --git a/README.md b/README.md
index d26d35d6..6b06de8b 100644
--- a/README.md
+++ b/README.md
@@ -28,18 +28,21 @@ In the browser without module support:
 
 ## Documentation
 
-WebIDL2 provides two functions: `parse` and `write`.
+WebIDL2 provides these functions:
 
 * `parse`: Converts a WebIDL string into a syntax tree.
 * `write`: Converts a syntax tree into a WebIDL string. Useful for programmatic code
   modification.
+* `merge`: Merges partial definitions and interface mixins.
+* `validate`: Validates that the parsed syntax against a number of rules.
 
 In Node, that happens with:
 
 ```JS
-const { parse, write, validate } = require("webidl2");
+const { parse, write, merge, validate } = require("webidl2");
 const tree = parse("string of WebIDL");
 const text = write(tree);
+const merged = merge(tree);
 const validation = validate(tree);
 ```
 
@@ -48,14 +51,16 @@ In the browser:
 <script>
   const tree = WebIDL2.parse("string of WebIDL");
   const text = WebIDL2.write(tree);
+  const merged = WebIDL2.merge(tree);
   const validation = WebIDL2.validate(tree);
 </script>
 
 <!-- Or when module is supported -->
 <script type="module">
-  import { parse, write, validate } from "./webidl2/index.js";
+  import { parse, write, merge, validate } from "./webidl2/index.js";
   const tree = parse("string of WebIDL");
   const text = write(tree);
+  const merged = merge(tree);
   const validation = validate(tree);
 </script>
 ```
@@ -140,6 +145,13 @@ var result = WebIDL2.write(tree, {
 
 "Wrapped value" here will all be raw strings when the `wrap()` callback is absent.
 
+`merge()` receives an AST or an array of AST, and TODO:
+
+```js
+const merged = merge(tree);
+// TODO example
+```
+
 `validate()` receives an AST or an array of AST, and returns semantic errors as an
 array of objects. Their fields are same as [errors](#errors) have, with one addition:
 
diff --git a/index.js b/index.js
index ed5785fa..476083a0 100644
--- a/index.js
+++ b/index.js
@@ -1,4 +1,5 @@
 export { parse } from "./lib/webidl2.js";
 export { write } from "./lib/writer.js";
+export { merge } from "./lib/merge.js";
 export { validate } from "./lib/validator.js";
 export { WebIDLParseError } from "./lib/tokeniser.js";
diff --git a/lib/merge.js b/lib/merge.js
new file mode 100644
index 00000000..c478b620
--- /dev/null
+++ b/lib/merge.js
@@ -0,0 +1,140 @@
+import { ExtendedAttributes } from "./productions/extended-attributes.js";
+import { Tokeniser } from "./tokeniser.js";
+
+// Remove this once all of our support targets expose `.flat()` by default
+function flatten(array) {
+  if (array.flat) {
+    return array.flat();
+  }
+  return [].concat(...array);
+}
+
+// https://heycam.github.io/webidl/#own-exposure-set
+function getOwnExposureSet(node) {
+  const exposedAttr = node.extAttrs.find((a) => a.name === "Exposed");
+  if (!exposedAttr) {
+    return null;
+  }
+  const exposure = new Set();
+  const { type, value } = exposedAttr.rhs;
+  if (type === "identifier") {
+    exposure.add(value);
+  } else if (type === "identifier-list") {
+    for (const ident of value) {
+      exposure.add(ident.value);
+    }
+  }
+  return exposure;
+}
+
+/**
+ * @param {Set?} a a Set or null
+ * @param {Set?} b a Set or null
+ * @return {Set?} a new intersected set, one of the original sets, or null
+ */
+function intersectNullable(a, b) {
+  if (a && b) {
+    const intersection = new Set();
+    for (const v of a.values()) {
+      if (b.has(v)) {
+        intersection.add(v);
+      }
+    }
+    return intersection;
+  }
+  return a || b;
+}
+
+/**
+ * @param {Set?} a a Set or null
+ * @param {Set?} b a Set or null
+ * @return true if a and b have the same values, or both are null
+ */
+function equalsNullable(a, b) {
+  if (a && b) {
+    if (a.size !== b.size) {
+      return false;
+    }
+    for (const v of a.values()) {
+      if (!b.has(v)) {
+        return false;
+      }
+    }
+  }
+  return a === b;
+}
+
+/**
+ * @param {Container} target definition to copy members to
+ * @param {Container} source definition to copy members from
+ */
+function copyMembers(target, source) {
+  const targetExposure = getOwnExposureSet(target);
+  const parentExposure = intersectNullable(
+    targetExposure,
+    getOwnExposureSet(source)
+  );
+  // TODO: extended attributes
+  for (const orig of source.members) {
+    const origExposure = getOwnExposureSet(orig);
+    const copyExposure = intersectNullable(origExposure, parentExposure);
+
+    // Make a copy of the member with the same prototype and own properties.
+    const copy = Object.create(
+      Object.getPrototypeOf(orig),
+      Object.getOwnPropertyDescriptors(orig)
+    );
+
+    if (!equalsNullable(targetExposure, copyExposure)) {
+      let value = Array.from(copyExposure.values()).join(",");
+      if (copyExposure.size !== 1) {
+        value = `(${value})`;
+      }
+      copy.extAttrs = ExtendedAttributes.parse(
+        new Tokeniser(` [Exposed=${value}] `)
+      );
+    }
+
+    target.members.push(copy);
+  }
+}
+
+/**
+ * @param {*[]} ast AST or array of ASTs
+ * @return {*[]}
+ */
+export function merge(ast) {
+  const dfns = new Map();
+  const partials = [];
+  const includes = [];
+
+  for (const dfn of flatten(ast)) {
+    if (dfn.partial) {
+      partials.push(dfn);
+    } else if (dfn.type === "includes") {
+      includes.push(dfn);
+    } else if (dfn.name) {
+      dfns.set(dfn.name, dfn);
+    } else {
+      throw new Error(`definition with no name`);
+    }
+  }
+
+  // merge partials (including partial mixins)
+  for (const partial of partials) {
+    const target = dfns.get(partial.name);
+    if (!target) {
+      throw new Error(
+        `original definition of partial ${partial.type} ${partial.name} not found`
+      );
+    }
+    if (partial.type !== target.type) {
+      throw new Error(
+        `partial ${partial.type} ${partial.name} inherits from ${target.type} ${target.name} (wrong type)`
+      );
+    }
+    copyMembers(target, partial);
+  }
+
+  return Array.from(dfns.values());
+}
diff --git a/test/merge.js b/test/merge.js
new file mode 100644
index 00000000..e5d204e6
--- /dev/null
+++ b/test/merge.js
@@ -0,0 +1,72 @@
+import expect from "expect";
+import { parse, write, merge } from "webidl2";
+
+// Collapse sequences of whitespace to a single space.
+function collapse(s) {
+  return s.trim().replace(/\s+/g, " ");
+}
+
+expect.extend({
+  toMergeAs(received, expected) {
+    received = collapse(received);
+    expected = collapse(expected);
+    const ast = parse(received);
+    const merged = merge(ast);
+    const actual = collapse(write(merged));
+    if (actual === expected) {
+      return {
+        message: () =>
+          `expected ${JSON.stringify(
+            received
+          )} to not merge as ${JSON.stringify(expected)} but it did`,
+        pass: true,
+      };
+    } else {
+      return {
+        message: () =>
+          `expected ${JSON.stringify(received)} to merge as ${JSON.stringify(
+            expected
+          )} but got ${JSON.stringify(actual)}`,
+        pass: false,
+      };
+    }
+  },
+});
+
+describe("merge()", () => {
+  it("empty array", () => {
+    const result = merge([]);
+    expect(result).toHaveLength(0);
+  });
+
+  it("partial dictionary", () => {
+    expect(`
+      dictionary D { };
+      partial dictionary D { boolean extra = true; };
+    `).toMergeAs(`
+      dictionary D { boolean extra = true; };
+    `);
+  });
+
+  it("partial interface", () => {
+    expect(`
+      interface I { };
+      partial interface I { attribute boolean extra; };
+    `).toMergeAs(`
+      interface I { attribute boolean extra; };
+    `);
+  });
+
+  it("partial interface with [Exposed]", () => {
+    expect(`
+      [Exposed=(Window,Worker)] interface I { };
+      [Exposed=Worker] partial interface I {
+        attribute boolean extra;
+      };
+    `).toMergeAs(`
+      [Exposed=(Window,Worker)] interface I {
+        [Exposed=Worker] attribute boolean extra;
+      };
+    `);
+  });
+});