```typescript
/**
 * Parses a semantic version string into its constituent parts.
 */

export function parseSemver(version: string): {
  major: number;
  minor: number;
  patch: number;
  prerelease: string | null;
  build: string | null;
} {
  const match = version.match(/^(\d+)\.(\d+)\.(\d+)(?:-(.*)|+(.+))?$/);
  
  if (match === null) {
    throw new Error('Invalid semantic version string');
  }
  
  return {
    major: parseInt(match[1], 10),
    minor: parseInt(match[2], 10),
    patch: parseInt(match[3], 10),
    prerelease: match[4] !== undefined ? match[4] : null,
    build: match[5] !== undefined ? match[5] : null,
  };
}
```