typescript 38 lines · 7 steps

Building a type-safe CSV writer in TypeScript

A generic class turns rows of any shape into correctly escaped CSV using per-column value extractors.

Explained by highlit
Intermediate generics serialization escaping data-transformation
1type CsvColumn<T> = {
2 header: string;
3 value: (row: T) => string | number | boolean | null | undefined;
4};
5 
6export class CsvWriter<T> {
7 constructor(
8 private readonly columns: CsvColumn<T>[],
9 private readonly delimiter = ",",
10 ) {}
11 
12 toString(rows: Iterable<T>): string {
13 const lines = [this.formatRow(this.columns.map((c) => c.header))];
14 for (const row of rows) {
15 lines.push(this.formatRow(this.columns.map((c) => this.render(c.value(row)))));
16 }
17 return lines.join("\r\n");
18 }
19 
20 private render(value: string | number | boolean | null | undefined): string {
21 if (value === null || value === undefined) return "";
22 return String(value);
23 }
24 
25 private formatRow(fields: string[]): string {
26 return fields.map((field) => this.escape(field)).join(this.delimiter);
27 }
28 
29 private escape(field: string): string {
30 const mustQuote =
31 field.includes(this.delimiter) ||
32 field.includes("\"") ||
33 field.includes("\n") ||
34 field.includes("\r");
35 if (!mustQuote) return field;
36 return `"${field.replace(/"/g, '""')}"`;
37 }
38}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Defining columns as extractor functions decouples the CSV structure from the shape of your data.
  2. 2Correct CSV output hinges on quoting fields that contain delimiters, quotes, or newlines and doubling embedded quotes.
  3. 3Generics let one writer serialize any row type while keeping value extraction fully type-checked.

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