TypeScript type guard

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Type guards are conditional checks that allow types to be narrowed from general types to more specific ones. With type guards, we do run-time type checking and ensure that code is safe.

What is narrowing? Jump to heading

In TypeScript, narrowing is the process of refining broad types into more narrow types. Narrowing is useful because it allows code to be liberal in the types that it accepts. Then, we can use type guards to narrow the type down to something more useful.

These are some common examples of narrowing:

  • unknown or any to string
  • string | object | number to string
  • number | null | undefined to number
  • string to a custom type like NonEmptyString

These are all examples of type guards:

  • typeof value === 'string'
  • 'name' in data
  • value instanceof MouseEvent
  • !value

A type guard is a special kind of expression that changes the type of a variable. We will look at more examples of type guards in practice later.

Custom type guards Jump to heading

String Jump to heading

const isString = (value: unknown): value is string => {
return typeof value === 'string'

Number Jump to heading

This will guard against null, undefined

export const isFiniteNumber = (value: unknown): value is number => {
return Number.isFinite(value) && typeof value === 'number'

isFiniteNumber usage Jump to heading

isFiniteNumber(10) // true

isFiniteNumber tests Jump to heading

describe('isFiniteNumber', () => {
it('should return true', () => {
it('should return false', () => {
expect(isFiniteNumber('Some value')).toBe(false)
expect(isFiniteNumber({ some: 'value' })).toBe(false)
expect(isFiniteNumber(() => 20)).toBe(false)

isDefined Jump to heading

const isDefined = <Value>(value: Value | undefined | null): value is Value => {
return value !== null && value !== undefined

The strengths of each type guard, here is a summary table.

Type guard Usage
Boolean / truthiness Rule out falsy values like null, undefined, '', 0, etc.
Equality Narrow multiple possible types down to a single type
typeof Narrow a type to a primitive type (like string or number)
instanceof Check if a value is an instance of a specific class
in Check if a property can be accessed
Assertion function Assert invariants that should always be true
Custom type guard function Check that a type meets some arbitrary conditions

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