How do I search text in Firestore when there's no LIKE query?

Every Firestore app eventually gets a search box, and then this question:

// What people try first
db.collection("products").where("name", "LIKE", "%shoe%")   // doesn't exist

Cloud Firestore has no LIKE, no CONTAINS for strings, no full-text search and no case-insensitive matching. Its queries are built on indexes that support equality, ranges, in and array-contains, and that's all. Here are the options that do work, from simplest to most capable.

1. Prefix search with a range query

A string range can express "starts with":

const term = "sho";
db.collection("products")
  .where("nameLower", ">=", term)
  .where("nameLower", "<=", term + "")
  .limit(20);

 is a very high Unicode code point, so the range covers every string that starts with term.

• Comparisons are case-sensitive, so store a normalized copy (nameLower, with accents stripped if needed) and query that.
• It matches the start of the field only. "running shoe" won't match shoe.

It's good enough for autocomplete on names, usernames or SKUs.

2. Keyword arrays with array-contains

Store searchable tokens on each document when you write it: