fix(csv-stringify): preserve caller column definitions

packages/csv-stringify/lib/api/normalize_columns.js

@@ -32,10 +32,11 @@ const normalize_columns = function (columns) {
             Error('Invalid column definition: property "key" is required'),
           ];
         }
-        if (column.header === undefined) {
-          column.header = column.key;
-        }
-        newcolumns.push(column);
+        newcolumns.push({
+          ...column,
+          key: column.key,
+          header: column.header === undefined ? column.key : column.header,
+        });
       } else {
         return [
           Error("Invalid column definition: expect a string or an object"),

packages/csv-stringify/test/option.columns.js

@@ -1,8 +1,51 @@
 import "should";
 import { stringify } from "../lib/index.js";
+import { stringify as stringifySync } from "../lib/sync.js";
 
 describe("Option `columns`", function () {
   describe("definition", function () {
+    it("accepts frozen column definitions without adding a header", function () {
+      const column = Object.freeze({ key: "a" });
+      const columns = Object.freeze([column]);
+      stringifySync([{ a: "value" }], { columns, header: true }).should.eql(
+        "a\nvalue\n",
+      );
+      Object.keys(column).should.eql(["key"]);
+    });
+
+    const structuralColumns = [
+      ["inherited", Object.create({ key: "a" })],
+      ["non-enumerable", Object.defineProperty({}, "key", { value: "a" })],
+      [
+        "getter",
+        Object.create({
+          get key() {
+            return "a";
+          },
+        }),
+      ],
+    ];
+    for (const [name, column] of structuralColumns) {
+      it(`preserves a ${name} column key`, function () {
+        stringifySync([{ a: "value" }], {
+          columns: [column],
+          header: true,
+        }).should.eql("a\nvalue\n");
+        Object.keys(column).should.eql([]);
+      });
+    }
+
+    it("does not mutate reusable column definitions", function () {
+      const column = { key: "a" };
+      stringifySync([{ a: "first" }], { columns: [column], header: true });
+      column.key = "b";
+      stringifySync([{ b: "second" }], {
+        columns: [column],
+        header: true,
+      }).should.eql("b\nsecond\n");
+      Object.keys(column).should.eql(["key"]);
+    });
+
     it("validates option types", function () {
       (() => {
         stringify([], { columns: true }, () => {});