TypeScript Method Override: override Keyword and super Calls
A subclass replaces a parent method by overriding it. Learn the override keyword, noImplicitOverride, super calls and the rules the compiler checks.
TypeScript: Create an Object From an Interface (7 Ways)
An interface cannot be instantiated with new, but any object with the right shape matches it. Learn seven ways to create such objects and when to use each.
TypeScript instanceof: Classes, Arrays, Errors, Interfaces
instanceof checks whether an object was created by a class and narrows its type. Learn how it works, where it fails, and how to check interfaces with in and type guards.
TypeScript Interface vs Class: Differences and When to Use
An interface describes a shape and disappears after compilation; a class describes a shape, holds code and exists at runtime. Compare them feature by feature.
TypeScript Generics: Function, Class and Interface Examples
TypeScript generics let one function, class or interface work with many types while the compiler checks every value. Examples cover constraints, keyof, defaults and const type parameters.
TypeScript Extend Interface: extends, Merging and & Types
Extend a TypeScript interface with extends, combine several parents, merge declarations, and learn when an intersection type (&) is the better fit.
TypeScript Interface: Declare, Implement and Use (Examples)
A TypeScript interface names the shape of an object. Learn to declare one, use optional and readonly properties, implement it in a class, and choose between an interface and a type alias.
TypeScript Exception Handling: try/catch and Custom Errors
TypeScript exception handling uses try, catch, finally and throw, with an unknown catch variable under strict. Examples cover custom Error classes, cause, instanceof and async errors.
TypeScript Record vs Map: Differences and When to Use Each
A Record types a plain object; a Map is a runtime collection. Compare them on keys, order, JSON, type checks and user input, and convert one into the other.
TypeScript Record: Check Keys, Iterate and Type Objects
Record
TypeScript vs JavaScript: Key Differences with Examples
TypeScript checks types at compile time and outputs plain JavaScript; JavaScript checks them only at runtime. A side-by-side comparison with real compiler errors.
TypeScript Optional and Default Parameters (with Examples)
Optional parameters may be left out and are undefined inside the function; default parameters fill in a value instead. Examples cover undefined vs null, order rules and options objects.
TypeScript Remove Item from Array: splice, filter, pop
Remove items from a TypeScript array by position or by value, in place or on a copy, with the common bugs (-1 index, splice in a loop, delete) shown and fixed.
TypeScript Add Element to Array: push, unshift, splice
Add items to the end, start or any index of a TypeScript array, or to a new array with spread, concat() and toSpliced(), with the compiler errors explained.
TypeScript Date: Current Date, Add Days, Compare, Format
Get the current date in TypeScript, create dates, add days and months, compare and format them, and avoid the time zone and JSON pitfalls of the Date object.
TypeScript Tutorial: All Articles Grouped by Topic
All TypeScript tutorials on howtodoinjava.com, grouped by topic: basics, types, operators, arrays, collections, functions, interfaces and classes.
JavaScript Check if Variable is a Number (and NaN Traps)
Number.isFinite() is the reliable way to check if a variable is a number in JavaScript. Learn why typeof and the global isNaN() let bad values through, how to validate numeric strings from forms, and how to write a TypeScript type guard.
Callback Function in TypeScript: Parameters, Promises, Await
A callback function in TypeScript is a function passed to another function and called later. Examples cover callback types, passing parameters, error-first callbacks, Promises and async/await.
TypeScript Function Overloading: Signatures and Examples
Function overloading in TypeScript gives one function several call signatures and a single implementation. Examples cover classes, constructors, call signatures and the simpler alternatives.
TypeScript Rest Parameters and Function Types with Examples
A rest parameter collects any number of arguments into one typed array, and a spread argument expands an array back into arguments. Examples cover function types, tuples and typed wrappers.
TypeScript Template Strings: Interpolation, Tags and Types
How template strings work in TypeScript: placeholders and expressions, multi-line text, tagged templates and String.raw, and template literal types that check string patterns at compile time.
TypeScript Set: HashSet Equivalent, Methods and Examples
A TypeScript Set stores unique values in insertion order. Create one, add and check values, remove duplicates from arrays, and compare sets with the ES2025 methods.
TypeScript Map: Create, Iterate, Sort and Convert to JSON
TypeScript Map with tested examples: create, update and iterate entries, handle undefined from get(), convert to JSON, sort, and choose Map vs Record.
TypeScript for Loop: for, for…of, for…in and forEach
The classic for loop, for…of, for…in and forEach() in TypeScript, with break, continue, labels, and iteration over Map, Set and object entries.
tsconfig.json Explained: TypeScript 7 Compiler Options
What each tsconfig.json option does, the real tsc –init output of TypeScript 7.0.2, three stricter options worth enabling, and the old options TypeScript 7 removed.
TypeScript Literal Types: as const, Unions and Validation
Literal types restrict a value to an exact list such as “small” | “medium” | “large”. Learn widening, as const, deriving unions from arrays, checking user input and Record lookups.
TypeScript Enum: Numeric, String, const and as const
Numeric, string and const enums in TypeScript, reverse mapping and string-to-enum conversion, plus why Node.js type stripping and erasableSyntaxOnly reject enums and what to use instead.
Difference Between undefined and null in JavaScript and TS
undefined means a value was never set, null means it was set to “nothing” on purpose. Learn where each comes from, how they differ, and how strictNullChecks, ?? and ?. handle them.
TypeScript Union Types: Narrowing and Discriminated Unions
How TypeScript union types work, how to narrow them with typeof, instanceof, in and type guards, and how discriminated unions with an exhaustive switch catch forgotten cases.
TypeScript == vs ===: Loose vs Strict Equality Explained
The difference between == and === in TypeScript with real results, the type conversion rules of ==, the compiler checks, and the one accepted use of == null.
TypeScript Spread Operator: Arrays, Objects, Rest Syntax
How the three dots copy and merge arrays and objects, pass arguments and collect rest values in TypeScript, with tested examples and the compiler errors that go with them.
TypeScript Logical Operators: &&, ||, !, ?? and ?.
How &&, || and ! short-circuit and return operands in TypeScript, when to use ?? instead of ||, optional chaining, logical assignment, and the compiler checks for always-truthy conditions.
TypeScript Comparison Operators: Numbers, Strings, Dates
How ===, !==, <, >, <= and >= behave in TypeScript for numbers, strings, dates and objects, and which comparisons the compiler rejects.
TypeScript Types: Primitives, any vs unknown, Generics
An overview of the TypeScript type system: primitive types, inference, any vs unknown vs never, arrays and tuples, object types, unions and generics, with examples tested on TypeScript 7.0.2.
Getting Started with TypeScript 7: Install, Compile, Run
Install TypeScript 7 with npm, compile a first program with tsc, set up tsconfig.json, and run .ts files directly with Node.js 22 type stripping.
TypeScript Arithmetic Operators: +, -, *, /, %, ** and ++
Every TypeScript arithmetic operator with real results, plus the cases that surprise beginners: decimal division, negative remainders, ** and the compiler errors for strings and unions.
var vs let vs const in JavaScript and TypeScript Explained
var is function-scoped, let and const are block-scoped, and const cannot be reassigned. Examples cover the temporal dead zone, loop closures, const objects and TypeScript literal types.
JavaScript Variable Hoisting: var, let, const and TDZ
Hoisting makes every declaration exist from the start of its scope. var reads as undefined, let, const and class throw in the temporal dead zone, and function declarations work anywhere.
Truthy and Falsy Values in JavaScript and TypeScript
JavaScript has eight falsy values; everything else, including “0”, [] and {}, is truthy. Learn where the conversion happens, how TypeScript narrows with it, and the bugs it causes.
Transpiler vs Compiler: The Difference, with TypeScript
A transpiler outputs source code at the same level as its input; a classic compiler outputs bytecode or machine code. Real javac, tsc and Node.js output shows the difference.