Type Narrowing (Control Flow Analysis)

Type Narrowing is the process by which TypeScript refines broad types (like string | number | null) into narrower, more specific types based on runtime conditionals.

TypeScript uses Control Flow Analysis (CFA) to analyze your code's execution paths (branches, returns, loops, throws) and track type refinements at every line of code.

1. Mechanisms of Control Flow Analysis

TypeScript automatically tracks variable assignments and conditional checks across execution paths:

2. Types of Narrowing

1. Equality Narrowing (===, !==, ==, !=)

2. Truthiness Narrowing (if (val))

Filtering out null, undefined, 0, "", false, and NaN:

3. Early Return / Unreachable Code Narrowing

Control flow analysis recognizes return, throw, and break statements to eliminate types in subsequent lines:

4. Assignment Narrowing

Assigning a specific value to a union variable narrows its type immediately:

3. Interactive Code Playground

Test control flow analysis narrowing below:

4. Summary Table

| typeof Guard | if (typeof x === "number") | All non-number union arms |