Instanceof operator
The instanceof operator tests whether an object or reference is compatible with a specified runtime type or constructor according to the programming language’s rules.
Secrets of the JavaScript Ninja, Second Edition
A JavaScript operator that tests whether an object was created by, or inherits from, a particular constructor via its prototype chain.
- How it is used
- Tests whether an object participates in a constructor’s prototype-based inheritance chain.
- How deep it goes
- Works through prototype-chain matching and shows how replacing a constructor’s prototype reverses an existing object’s result.
- Examples
- Contrasts copied methods with inherited prototypes and tests an existing instance before and after prototype replacement.
- What you can do after
- Diagnose/debug with it: explain a surprising type-check result by inspecting the constructor’s current prototype and the object’s chain.
Read how Secrets of the JavaScript Ninja, Second Edition explains instanceof operator →
100 Java Mistakes and How to Avoid Them
- How it is used
- Guards downcasts and dispatches runtime-type-specific behavior while preventing overlapping conditional branches.
- How deep it goes
- Explains subtype matching through branch-order failures and contrasts it with exact runtime-class comparisons.
- Examples
- Uses BigInteger and BigDecimal conversion, date conversion, ArrayList checks, and Calendar instances.
- What you can do after
- Diagnose/debug with it by finding unreachable type branches and choosing subclass-compatible versus exact-class tests.
Read how 100 Java Mistakes and How to Avoid Them explains instanceof operator →
Related concepts
- prototype chainuses · A prototype chain is the linked sequence of JavaScript prototype objects consulted during property lookup when an object lacks the requested property itself.
- pattern matchingused by · Pattern matching is the process of comparing values, structures, or other data with specified or learned patterns to determine correspondence, often enabling classification, extraction, or selection of behavior.
- classuses · A class is a programming-language construct that defines a type’s structure or behavior and provides a basis for creating or representing objects.
- getClasscontrasts with · getClass is the Java Object method that returns the runtime class object of the instance on which it is invoked.
- constructor methoduses · A constructor is a type-associated operation or callable that creates or initializes an instance and establishes its initial state.
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