Jest Testing Best Practices
Quick Start
import { run } from "../src/api";
describe("interpreter", () => {
it("evaluates arithmetic expressions", () => {
expect(run("2 + 3")).toBe(5);
});
it("handles errors gracefully", () => {
expect(() => run("undefined_var")).toThrow(/undefined/i);
});
});
Core Principles
- Arrange-Act-Assert: Structure tests clearly
- One assertion focus: Test one behavior per test
- Descriptive names: Use "should" or behavior-based naming
- Isolation: Tests should not depend on each other
Testing Patterns
Interpreter Tests
describe("builtins", () => {
describe("map", () => {
it("transforms each element", () => {
const code = `[1, 2, 3] /> map((x) -> x * 2)`;
expect(run(code)).toEqual([2, 4, 6]);
});
it("passes index as second argument", () => {
const code = `["a", "b"] /> map((x, i) -> i)`;
expect(run(code)).toEqual([0, 1]);
});
});
});
Parser Tests
describe("parser", () => {
it("parses pipe expressions", () => {
const ast = parse("x /> fn");
expect(ast.body[0]).toMatchObject({
type: "ExprStmt",
expression: {
type: "PipeExpr",
left: { type: "Identifier", name: "x" },
right: { type: "Identifier", name: "fn" }
}
});
});
});
Async Tests
it("resolves async operations", async () => {
const result = await run(`
let fetch = () -> delay(10) /> then(() -> "done") #async
await fetch()
`);
expect(result).toBe("done");
});
Snapshot Tests
it("formats code consistently", () => {
const formatted = format("let x=1+2");
expect(formatted).toMatchSnapshot();
});
Configuration
// jest.config.js
module.exports = {
preset: "ts-jest",
testEnvironment: "node",
testMatch: ["**/__tests__/**/*.test.ts"],
collectCoverageFrom: ["src/**/*.ts", "!src/**/*.d.ts"],
};
Reference Files
- [references/mocking.md](references/mocking.md) - Mocking strategies
- [references/async-testing.md](references/async-testing.md) - Async test patterns
