Choosing columns

One columns option selects, orders, and labels columns. Keys are checked against your type, so a typo will not compile.

import { stringify } from 'csv-pipe';

type User = { name: string; email: string; age: number };

const users: User[] = [
  { name: 'Alex Johnson', email: 'alex@example.com', age: 29 },
  { name: 'Carlos Herrera', email: 'carlos@example.com', age: 24 }
];

// An array of keys: each key is also its header.
stringify(users, { columns: ['name', 'email'] });
// name,email
// Alex Johnson,alex@example.com
// Carlos Herrera,carlos@example.com

// A map of key to label: also sets the header text.
stringify(users, { columns: { name: 'Full name', email: 'Email address' } });
// Full name,Email address
// Alex Johnson,alex@example.com
// Carlos Herrera,carlos@example.com

Without columns, the columns are the stable union of all record keys, in first-seen order, so reordered or partial records never shift.

A map orders columns by JavaScript object key order: integer-like keys ("0", "1", ...) come first in ascending numeric order, then the rest in insertion order. If your keys are integer-like and the order matters, use the array form, which always keeps the order you write.

TIP

A missing key and an explicit undefined are treated the same. Declaring columns also makes streaming fully incremental, since the header is known up front.