Test Data Builder

Problem

Test data setup becomes verbose and brittle when each test manually constructs complex objects. When a required field is added, dozens of tests break. Tests become hard to read because most of the data is irrelevant boilerplate, obscuring what the test is actually verifying.

Solution

Create reusable builders that construct test data with sensible defaults and allow customization. This makes tests more readable and easier to maintain.

Example

This demonstrates creating reusable test data builders with sensible defaults and fluent APIs, making tests more readable by focusing on what’s unique to each test case.

// Builder with sensible defaults
class UserBuilder {
  constructor() {
    // Provide default values so tests only specify what matters
    this.user = { name: 'John', email: 'john@example.com', age: 30 };
  }

  // Fluent API for customizing specific fields
  withName(name) {
    this.user.name = name;
    return this;  // Return this for chaining
  }

  withAge(age) {
    this.user.age = age;
    return this;
  }

  build() {
    return this.user;
  }
}

// Test only specifies what's unique, using defaults for everything else
const user = new UserBuilder().withName('Alice').withAge(25).build();

Benefits

• Makes tests more readable by hiding irrelevant boilerplate data.
• Reduces maintenance burden when object structure changes.
• Provides sensible defaults so tests only specify what matters.
• Enables easy creation of complex test scenarios.

Tradeoffs

• Adds indirection that can make test data construction less obvious.
• Requires creating and maintaining builder classes.
• Can hide important details if defaults aren’t well chosen.
• May be overkill for simple objects with few fields.