How to Effectively Use Type Predicates in TypeScript

(v): v is number => v !== null && v !== undefined

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🧠 What Is a Type Predicate?

A type predicate is a way to tell TypeScript:

“If this condition is true, then you can safely treat v as a specific type.”

The syntax is:

(parameter): parameter is Type => condition

So in your case:

• v is the parameter

• v is number is the type predicate

• v !== null && v !== undefined is the condition

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🔍 What It Does

This function is used inside .filter() to remove null and undefined, and tell TypeScript that the remaining values are definitely number.

Without Type Predicate:

const arr = [1, null, 2, undefined];

const filtered = arr.filter(v => v !== null && v !== undefined);
// TypeScript still thinks: (number | null | undefined)[]

With Type Predicate:

const filtered = arr.filter(
  (v): v is number => v !== null && v !== undefined
);
// TypeScript now knows: number[]

✅ You can now safely use .toLocaleString() or other number methods without type errors.

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🧪 Real Example

const salaries = [50000, null, undefined, 75000];

const cleanSalaries = salaries
  .filter((v): v is number => v !== null && v !== undefined)
  .map(s => `$${s.toLocaleString()}`);

Without the type predicate, TypeScript would complain about s.toLocaleString() because s might still be null or undefined.

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🧠 TL;DR

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