performance.md

Item 78: Pay Attention to Compiler Performance

Things to Remember

• There are two forms of TypeScript performance issues: build performance (tsc) and editor latency (tsserver). Recognize the symptoms of each and direct your optimizations accordingly.
• Keep type checking separate from your build process.
• Remove dead code and dependencies, and be on guard for code bloat in type dependencies. Use a treemap to visualize what TypeScript is compiling.
• Use incremental builds and project references to reduce the work tsc does between builds.
• Simplify your types: avoid large unions, use interface extension rather than intersection types, and consider annotating function return types.## Code Samples

// hello.ts
console.log('Hello World!');

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---

function foo() {}
//       ~~~ 'foo' is declared but its value is never read.

export function bar() {}

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// src/fib.ts
export function fib(n: number): number {
  if (n < 2) {
    return n;
  }
  return fib(n - 1) + fib(n - 2);
}

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// test/fib.test.ts
import {fib} from '../src/fib';

describe('fib', () => {
  it('should handle base cases', () => {
    expect(fib(0)).toEqual(0);
    expect(fib(1)).toEqual(1);
  })

  it('should handle larger numbers', () => {
    expect(fib(2)).toEqual(1);
    expect(fib(3)).toEqual(2);
    expect(fib(4)).toEqual(3);
    expect(fib(5)).toEqual(5);
    expect(fib(16)).toEqual(987);
  });
});

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type Digit = '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9';
type Year = `2${Digit}${Digit}${Digit}`;
const validYear: Year = '2024';
const invalidYear: Year = '1999';
//    ~~~~~~~~~~~ Type '"1999"' is not assignable to type
//                '"2000" | "2001" | "2002" | ... 996 more ... | "2999"'.

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