typescript 45 lines · 8 steps

Parsing bracketed form field names into nested objects

Turn HTML form fields like user[roles][] into a nested object by parsing their names and building the structure on the fly.

Explained by highlit
Intermediate parsing recursive-types regex form-handling nested-data
1type NestedValue = string | NestedValue[] | { [key: string]: NestedValue };
2 
3function parseFieldPath(name: string): string[] {
4 const match = name.match(/^([^\[\]]+)((?:\[[^\[\]]*\])*)$/);
5 if (!match) return [name];
6 const [, head, rest] = match;
7 const keys = [head];
8 for (const segment of rest.matchAll(/\[([^\[\]]*)\]/g)) {
9 keys.push(segment[1]);
10 }
11 return keys;
12}
13 
14function assign(target: Record<string, NestedValue>, keys: string[], value: string): void {
15 let cursor: any = target;
16 for (let i = 0; i < keys.length - 1; i++) {
17 const key = keys[i];
18 const nextKey = keys[i + 1];
19 const isArray = nextKey === '' || /^\d+$/.test(nextKey);
20 if (cursor[key] == null) {
21 cursor[key] = isArray ? [] : {};
22 }
23 cursor = cursor[key];
24 }
25 
26 const last = keys[keys.length - 1];
27 if (last === '' && Array.isArray(cursor)) {
28 cursor.push(value);
29 } else {
30 cursor[last] = value;
31 }
32}
33 
34export function serializeForm(form: HTMLFormElement): Record<string, NestedValue> {
35 const result: Record<string, NestedValue> = {};
36 const data = new FormData(form);
37 
38 for (const [name, raw] of data.entries()) {
39 if (typeof raw !== 'string') continue;
40 const keys = parseFieldPath(name);
41 assign(result, keys, raw);
42 }
43 
44 return result;
45}
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STEP 01

Walkthrough

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Three takeaways
  1. 1A recursive type like NestedValue lets you describe arbitrarily deep mixes of objects and arrays in one definition.
  2. 2Peeking at the next key decides whether to create an array or object, so structure emerges from the path shape itself.
  3. 3Splitting name-parsing from value-assignment keeps each function small and independently testable.

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