code-narrowing.md

my-awesome-typescript-project

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my-awesome-typescript-project / src/code-narrowing

src/code-narrowing

Code narrowing.

Table of contents

• 1. Basic narrowing
• 2. Predicate-based narrowing
• 3. Union-based narrowing

1. Basic narrowing

whichPrimitiveType()

function whichPrimitiveType(v): string;

Defined in: src/code-narrowing.ts:26

typeof and instanceof based narrowing

Parameters

Parameter	Type
v

Returns

string

Remarks

• Test argument value against all supported types.
• Refer to the handbook for the supported narrowing patterns.

2. Predicate-based narrowing

isExtended()

function isExtended(o): o is extendedShape;

Defined in: src/code-narrowing.ts:59

Type predicate

Parameters

Parameter	Type
o

Returns

o is extendedShape

Remarks

• Cannot extract declaration : the type predicate must be present in the function signature.
• If isExtended returns a boolean, o cannot be narrowed to extendedShape (see below).

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runPredicate()

function runPredicate(o): symbol;

Defined in: src/code-narrowing.ts:73

Narrowing with predicate

Parameters

Parameter	Type
o

Returns

symbol

Remarks

• Using predicates is part of building an efficient type system.
• The commented pattern is an equivalent implementation but considered unsafe by the compiler.
• As a result, type assertions should be paired with predicates as often as possible.
• See also assertion based narrowing for node typescript.
• See also this example that combines this with type predicates for narrowing.

3. Union-based narrowing

processEvent()

function processEvent(o): string;

Defined in: src/code-narrowing.ts:88

Narrowing with discriminated unions

Parameters

Parameter	Type
o	event

Returns

string

Remarks

• This allows DRY for similar types and code narrowing in functions that accept the union as a parameter.
• o evaluates to never in the default case.