New: added glossary entry Type Guard

Author: stuartherbert

CHANGELOG.md

@@ -105,6 +105,7 @@ The following changes have been completed, and will be included in the next tagg
   - added Type Alias
   - added Type Casting
   - added Type Guarantee
+  - added Type Guard
   - added Value Object
 
 ### Fixes

TEMPLATE.md

@@ -65,4 +65,5 @@ Table of Contents:
 [Type Alias]: ../../glossary/type-alias.md
 [Type Casting]: ../../glossary/type-casting.md
 [Type Guarantee]: ../../glossary/type-guarantee.md
+[Type Guard]: ../../glossary/type-guard.md
 [Value Object]: ../../glossary/value-object.md

glossary/README.md

@@ -51,6 +51,7 @@ Table of Contents:
 - [Type Alias][Type Alias]
 - [Type Casting][Type Casting]
 - [Type Guarantee][Type Guarantee]
+- [Type Guard][Type Guard]
 
 [ADOPTION]: ../impacted-areas/ADOPTION.md
 [CONTRIBUTIONS]: ../impacted-areas/CONTRIBUTIONS.md

glossary/type-guard.md

@@ -0,0 +1,76 @@
+# Type Guard
+
+A _type guard_ is a function that returns a [type predicate][Type Predicate]. _Type guards_ are a type of [data guard][Data Guard].
+
+We use them in Typescript code to tell the compiler which operations are safe to perform on any piece of data of indeterminate type.
+
+```typescript
+function isValue<T = any>(
+    input: unknown
+): input is Value<T> {
+    return ((input as Value).implementsValue
+        && (input as Value).implementsValue());
+}
+```
+
+End-users rely on correctly-functioning _type guards_ to avoid using Typescript's `as XXX` [type casting][Type Casting] in their code.
+
+It's almost impossible to write a type-safe library or application without creating _type guards_.
+
+[ADOPTION]: ../impacted-areas/ADOPTION.md
+[CONTRIBUTIONS]: ../impacted-areas/CONTRIBUTIONS.md
+[CORRECTNESS]: ../impacted-areas/CORRECTNESS.md
+[GOVERNANCE]: ../impacted-areas/GOVERNANCE.md
+[PROJECT-MAINTENANCE]: ../impacted-areas/PROJECT-MAINTENANCE.md
+[ROBUSTNESS]: ../impacted-areas/ROBUSTNESS.md
+[SECURITY]: ../impacted-areas/SECURITY.md
+[TESTABILITY]: ../impacted-areas/TESTABILITY.md
+[Base Class]: ./base-class.md
+[Branded Type]: ./branded-type.md
+[Caller]: ./caller.md
+[CQRS]: ./CQRS.md
+[Data Bag]: ./data-bag.md
+[Data Guard]: ./data-guard.md
+[Data Guarantee]: ./data-guarantee.md
+[Default Value]: ./default-value.md
+[Defensive Programming]: ./defensive-programming.md
+[Dependency]: ./dependency.md
+[Dependency Injection]: ./dependency-injection.md
+[Docblock]: ./docblock.md
+[End-User]: ./end-user.md
+[Entity]: ./entity.md
+[Exported Item]: ./exported-item.md
+[Flavoured Type]: ./flavoured-type.md
+[Function Prefix]: ./function-prefix.md
+[Function Signature]: ./function-signature.md
+[Hard-Coded]: ./hard-coded.md
+[Identity]: ./identity.md
+[Immutability]: ./immutability.md
+[Inherited Method]: ./inherited-method.md
+[Instantiable Type]: ./instantiable-type.md
+[Mandatory Dependency]: ./mandatory-dependency.md
+[Nominal Typing]: ./nominal-typing.md
+[Optional Input]: ./optional-input.md
+[Overridden Method]: ./overridden-method.md
+[Plain Object]: ./plain-object.md
+[Primitive Type]: ./primitive-type.md
+[Protocol]: ./protocol.md
+[Refined Type]: ./refined-type.md
+[Rest Parameter]: ./rest-parameter.md
+[Reusability]: ./reusability.md
+[Side Effects]: ./side-effects.md
+[Smart Constructor]: ./smart-constructor.md
+[Structural Typing]: ./structural-typing.md
+[Type Alias]: ./type-alias.md
+[Type Casting]: ./type-casting.md
+[Type Guarantee]: ./type-guarantee.md
+[Type Guard]: ./type-guard.md
+[Type Inference]: ./type-inference.md
+[Type Predicate]: ./type-predicate.md
+[Type Signature]: ./type-signature.md
+[User-Supplied Functional Options]: ./user-supplied-functional-options.md
+[User-Supplied Input]: ./user-supplied-input.md
+[User-Supplied Options]: ./user-supplied-options.md
+[User-Supplied Optional Dependencies]: ./user-supplied-optional-dependencies.md
+[Value]: ./value.md
+[Value Object]: ./value-object.md