Understand Evolving Types

Overview

Some variables start with broad types and narrow as TypeScript sees values added.

This is an exception to the rule that types don’t change. Variables initialized without a value, or as empty arrays, can have “evolving” types that narrow based on what you assign to them.

When to Use This Skill

• Variables initialized to null or undefined
• Arrays that start empty and get values pushed
• Variables that start as any and narrow
• Understanding when explicit annotations are better

The Iron Rule

Evolving types work but are fragile.
Prefer explicit annotations for clarity.

Remember:

• Only applies to variables without initial typed values
• Type evolves based on assignments
• Final type is only valid after all assignments
• Explicit annotation is often clearer

Detection: The Evolving any

const result = [];  // any[]
result.push('a');   // string[]
result.push(1);     // (string | number)[]

result
// ^? (string | number)[]

The type evolves with each push.

How Evolving Types Work

Uninitialized Variables

let val;  // any (evolving)
val
// ^? let val: any

if (Math.random() < 0.5) {
  val = /hello/;
  val
  // ^? let val: RegExp
} else {
  val = 12;
  val
  // ^? let val: number
}
val
// ^? let val: number | RegExp

TypeScript tracks assignments and computes the union.

Empty Arrays

const arr = [];  // any[] (evolving)
arr.push(1);
arr
// ^? number[]

arr.push('hello');
arr
// ^? (string | number)[]

null or undefined Initial Value

let x = null;  // any (evolving)
x
// ^? null

x = 12;
x
// ^? number

When Types Stop Evolving

Once a variable leaves its scope or is used in a function, its type is fixed:

function buildArray() {
  const arr = [];
  arr.push(1);
  arr.push(2);
  return arr;  // Type fixed as number[]
}

const myArray = buildArray();
myArray.push('hello');  // Error if return type is number[]

Problems with Evolving Types

Order Matters

const arr = [];
arr.push(1);
// arr is number[] here

// If this line is later:
arr.push('hello');
// arr is (string | number)[] but earlier uses assumed number[]

Implicit any

With noImplicitAny, empty arrays without annotation get any[]:

const values = [];  // Implicit any[] - may cause lint warnings

Fragile Inference

let x = null;
x = 'hello';
x = 42;  // Now it's string | number

// Later, someone adds:
x = true;  // Now it's string | number | boolean

// All code using x must handle all possibilities

Better: Explicit Annotations

// Clear intent, stable type
const result: number[] = [];
result.push(1);
result.push(2);
// result is always number[]

// Prevents accidents
result.push('hello');
//          ~~~~~~~
// Argument of type 'string' is not assignable to 'number'

When Evolving Types Are OK

Short, Simple Loops

const squares = [];
for (let i = 0; i < 5; i++) {
  squares.push(i * i);
}
// squares: number[] is clear from context

Accumulating Known Types

let result = null;
for (const item of items) {
  if (condition(item)) {
    result = item;
    break;
  }
}
// result evolves to ItemType | null

Functional Alternatives

Instead of evolving arrays, prefer functional constructs:

// Don't:
const doubled = [];
for (const n of numbers) {
  doubled.push(n * 2);
}

// Do:
const doubled = numbers.map(n => n * 2);
// ^? number[]

Type is inferred directly, no evolution needed.

Real-World Example

// Evolving (works but fragile)
async function fetchData() {
  let data;
  try {
    const response = await fetch('/api');
    data = await response.json();
  } catch (e) {
    data = null;
  }
  return data;  // any
}

// Better: explicit
async function fetchData(): Promise<Data | null> {
  try {
    const response = await fetch('/api');
    return await response.json();
  } catch (e) {
    return null;
  }
}

Pressure Resistance Protocol

1. “Evolving Types Work”

Pressure: “TypeScript figures it out automatically”

Response: It’s fragile and can change unexpectedly with new code.

Action: Add explicit annotation for stability.

2. “I Don’t Know the Type Yet”

Pressure: “The type depends on runtime conditions”

Response: You know the possible types; declare them.

Action: Use union type: let x: string | number | null = null;

Red Flags – STOP and Reconsider

• const arr = [] without annotation
• let x = null or let x without annotation
• Types that change based on assignment order
• any[] warnings in lint

Common Rationalizations (All Invalid)

Quick Reference

// EVOLVING (works but fragile)
const arr = [];       // any[]
arr.push(1);          // number[]
arr.push('a');        // (string | number)[]

// BETTER (stable and clear)
const arr: number[] = [];
arr.push(1);
arr.push('a');  // Error!

// BEST (functional)
const arr = items.map(item => item.value);
// Type inferred correctly

The Bottom Line

Evolving types are a convenience, not a best practice.

While TypeScript can track types through assignments, explicit annotations are clearer and more robust. Use evolving types only for simple, localized code. For anything else, declare your intent with annotations.

Reference

Based on “Effective TypeScript” by Dan Vanderkam, Item 25: Understand Evolving Types.