unique merge array customisation fix (#162)

Closes #161.

README.md

@@ -150,6 +150,8 @@ The first `<field>` is the config property to look through for duplicates.
 
 `<fields>` represents the values that should be unique when you run the field => field function on each duplicate.
 
+When the order of elements of the `<field>` in the first configuration differs from the order in the second configuration, the latter is preserved. 
+
 ```javascript
 const { mergeWithCustomize, unique } = require("webpack-merge");
 

src/unique.ts

@@ -3,26 +3,19 @@ function mergeUnique(
  uniques: string[],
  getter: (a: object) => string
 ) {
+ let uniquesSet = new Set(uniques)
  return (a: [], b: [], k: string) =>
- k === key && [
- ...difference(a, b, (item) => uniques.indexOf(getter(item))),
- ...b,
- ];
-}
-
-function difference(a: object[], b: object[], cb: (v: object) => number) {
- const ret = a.filter((v, i) => {
- const foundA = cb(v);
- const foundB = cb(b[i] || {});
-
- if (foundA >= 0 && foundB >= 0) {
- return foundA !== foundB;
- }
-
- return true;
- });
-
- return ret;
+ (k === key) && Array.from(
+ [...a, ...b]
+ .map((it: object) => ({ key: getter(it), value: it }))
+ .map(({ key, value }) => ({ key: (uniquesSet.has(key) ? key : value), value: value}))
+ .reduce(
+ (m, { key, value}) => {
+ m.delete(key); // This is required to preserve backward compatible order of elements after a merge.
+ return m.set(key, value)
+ },
+ new Map<any, any>())
+ .values())
 }
 
 export default mergeUnique;

test/unique.test.ts

@@ -59,7 +59,7 @@ describe("Unique", function () {
  assert.deepStrictEqual(output, expected);
  });
 
- it("should not lose any plugins", function () {
+ it("should not lose any trailing plugins", function () {
  const output = mergeWithCustomize({
  customizeArray: unique(
  "plugins",
@@ -89,6 +89,36 @@ describe("Unique", function () {
  assert.deepStrictEqual(output, expected);
  });
 
+ it("should not lose any leading plugins", function () {
+ const output = mergeWithCustomize({
+ customizeArray: unique(
+ "plugins",
+ ["HotModuleReplacementPlugin"],
+ (plugin) => plugin.constructor && plugin.constructor.name
+ ),
+ })(
+ {
+ plugins: [
+ new webpack.DefinePlugin({}),
+ new webpack.HotModuleReplacementPlugin(),
+ ],
+ },
+ {
+ plugins: [new webpack.HotModuleReplacementPlugin()],
+ }
+ );
+ // The HMR plugin is picked from the last one due to
+ // default ordering!
+ const expected = {
+ plugins: [
+ new webpack.DefinePlugin({}),
+ new webpack.HotModuleReplacementPlugin(),
+ ],
+ };
+
+ assert.deepStrictEqual(output, expected);
+ });
+
  it("should check only against named plugins (#125)", function () {
  const output = mergeWithCustomize({
  customizeArray: unique(