Fannie  Zemlak

Fannie Zemlak


Typing Arrays in TypeScript

Roles of Arrays

Arrays are used in the following two roles in JavaScript (and sometimes a mix of the two):

  • Lists: All elements have the same type. The length of the Array varies.
  • Tuple: The length of the Array is fixed. The elements do not necessarily have the same type.

TypeScript accommodates these two roles by offering various ways of typing arrays. We will look at those next.

Ways of typing Arrays

Array role “list”: array type literals vs. interface type Array

An Array type literal consists of the element type followed by []. In the following code, the Array type literal is string[]:

const myStringArray: string[] = ['fee', 'fi', 'fo', 'fum'];

An Array type literal is a shorthand for using the global generic interface type Array:

const myStringArray: Array<string> = ['fee', 'fi', 'fo', 'fum'];

If the element type is more complicated, then you need to put them in parentheses:

(() => boolean)[]

In this case, I prefer Array:

Array<() => boolean>

Both array type literals and Array require all elements to have the same type. That’s how we know that they are for Arrays-as-lists.

Array role “tuple”: tuple type literals

If the Array has a fixed length and each element has a different, fixed type that depends on its position, then we can use tuple type literals such as [string, string, boolean]:

const yes: [string, string, boolean] = ['oui', 'sí', true];

Objects that are also Array-ish: interfaces with index signatures

If an interface has only an index signature, we can use it for Arrays:

interface StringArray {
  [index: number]: string;
const strArr: StringArray = ['Huey', 'Dewey', 'Louie'];

An interface that has both an index signature and property signatures, only works for objects (because indexed elements and properties need to be defined at the same time):

interface FirstNamesAndLastName {
  [index: number]: string;
  lastName: string;

const ducks: FirstNamesAndLastName = {
  0: 'Huey',
  1: 'Dewey',
  2: 'Louie',
  lastName: 'Duck',

Pitfall: type inference doesn’t always get Array types right

Inferring types of Arrays is difficult

Due to the two roles of Arrays, it is impossible for TypeScript to always guess the right type. As an example, consider the following Array literal that is assigned to the variable fields:

const fields = [
  ['first', 'string', true],
  ['last', 'string', true],
  ['age', 'number', false],

What is the best type for fields? The following are all reasonable options:

type Fields1 = Array<[string, string, boolean]>;
type Fields2 = Array<[string, ('string'|'number'), boolean]>;
type Fields3 = Array<Array<string|boolean>>

type Fields4 = [
  [string, string, boolean],
  [string, string, boolean],
  [string, string, boolean],
type Fields5 = [
  [string, 'string', boolean],
  [string, 'string', boolean],
  [string, 'number', boolean],
type Fields6 = [

// Etc.

Type inference for non-empty Array literals

When we use non-empty Array literals, TypeScript’s default is to infer list types (not tuple types):

// %inferred-type: number[]
const arr = [123];

Alas, that’s not always what we want:

type Pair = [string, boolean];

// %inferred-type: (str: string) => (string | boolean)[]
const callback = (str: string) => [str, Boolean(str)];

// @ts-ignore: Type '(string | boolean)[][]' is not assignable to
// type 'Pair[]'.
//   Type '(string | boolean)[]' is missing the following properties
//   from type 'Pair': 0, 1 (2322)
const pairs: Array<Pair> = ['hello', '', '!!!'].map(callback);

The inferred type for callback should be a tuple type. It isn’t and that’s why its results don’t match the type parameter Pair of Array<> in the last line.

We can fix this by specifying the return type of callback explicitly (instead of relying on type inference):

const callback2 = (str: string): [string, boolean] =>
  [str, Boolean(str)];
const pairs2: Array<Pair> = ['hello', '', '!!!'].map(callback2);

We also could have used Pair instead of [string, boolean] for the result of callback2.

#arrays #java #javascript #coding

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Typing Arrays in TypeScript
Verdie  Murray

Verdie Murray


What Is The Main Difference Of TYPES Vs INTERFACES in TypeScript

TypeScript has two ways of declaring structures of your objects in the form of  #types (type aliases) and #interfaces.

In this lesson we will look at the technical differences between these two, when you should use which, along with real world #TypeScript code analysis, and community thoughts


Arvel  Parker

Arvel Parker


Basic Data Types in Python | Python Web Development For Beginners

At the end of 2019, Python is one of the fastest-growing programming languages. More than 10% of developers have opted for Python development.

In the programming world, Data types play an important role. Each Variable is stored in different data types and responsible for various functions. Python had two different objects, and They are mutable and immutable objects.

Table of Contents  hide

I Mutable objects

II Immutable objects

III Built-in data types in Python

Mutable objects

The Size and declared value and its sequence of the object can able to be modified called mutable objects.

Mutable Data Types are list, dict, set, byte array

Immutable objects

The Size and declared value and its sequence of the object can able to be modified.

Immutable data types are int, float, complex, String, tuples, bytes, and frozen sets.

id() and type() is used to know the Identity and data type of the object







Built-in data types in Python

a**=str(“Hello python world”)****#str**














Numbers (int,Float,Complex)

Numbers are stored in numeric Types. when a number is assigned to a variable, Python creates Number objects.

#signed interger




Python supports 3 types of numeric data.

int (signed integers like 20, 2, 225, etc.)

float (float is used to store floating-point numbers like 9.8, 3.1444, 89.52, etc.)

complex (complex numbers like 8.94j, 4.0 + 7.3j, etc.)

A complex number contains an ordered pair, i.e., a + ib where a and b denote the real and imaginary parts respectively).


The string can be represented as the sequence of characters in the quotation marks. In python, to define strings we can use single, double, or triple quotes.

# String Handling

‘Hello Python’

#single (') Quoted String

“Hello Python”

# Double (") Quoted String

“”“Hello Python”“”

‘’‘Hello Python’‘’

# triple (‘’') (“”") Quoted String

In python, string handling is a straightforward task, and python provides various built-in functions and operators for representing strings.

The operator “+” is used to concatenate strings and “*” is used to repeat the string.


output**:****‘Hello python’**

"python "*****2

'Output : Python python ’

#python web development #data types in python #list of all python data types #python data types #python datatypes #python types #python variable type

Christa  Stehr

Christa Stehr


Nominal typing in Typescript

Nominal & structural typing

Type systems are typically categorized as either structural or nominal. Languages like Java and Scala have primarily nominal type systems, whereas a language like Typescript has a structural type system. Let’s take a brief look at both systems.

Nominal Typing

In a nominal typing system, type compatibility is checked using the name of the types. If they do not have the same name, then they are not compatible; end of story. **If **Typescript had a nominal typing system the type check for the last line would fail:

Image for post

Structural typing

Typescript uses structural typing to decide whether two types are compatible with one another or not. What do we mean by structural typing? Well, let’s consider the following code snippet:

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To determine whether the type of the constant color(RGBA) is compatible with the type of serializeColor’s parameter x(RGB) the type system must verify that each member of RGB has a corresponding compatible member in RGBA. In this case, RGB has a single member color for which RGBA has a corresponding member with the same type — [number, number, number] — and so it passes the type check. Notice how the type system ignores the additional members that exist on RGBA (alpha).

#typescript #type-safe #type-systems

Joseph  Murray

Joseph Murray


How to Print an Array in Java


Printing an array is a quick way to give us visibility on the values of the contents inside. Sometimes the array values are the desired output of the program.

In this article, we’ll take a look at how to print an array in Java using four different ways.

While the “best way” depends on what your program needs to do, we begin with the simplest method for printing and then show more verbose ways to do it.

#java #array #how to print an array in java #array in java #print an array in java #print

Flo  D'Amore

Flo D'Amore


Types, Type Systems and TypeScript

What is it all about?

type system is a set of rules for performing various consistency and correctness checks in a program. There are various definitions and classifications for type systems. A good working definition is the following:

_In programming languages, a type system is a logical system comprising a set of rules that assigns a property called a “type” to the various constructs of a computer program, such as variables, expressions, functions or modules. — _Wikipedia

There are different categorizations of type systems including:

  • Static vs. Dynamic
  • Structural vs. Nominal
  • Strong vs. Weak (this is more contentious)

Click here for a comparison of various programming languages and their type systems

Structural vs. Nominal

In a _Nominal type system, _all types are either declared or named explicitly. This type of system is used both for determining equivalence between two items, as well as sub type relationships. The majority of statically typed languages (including Java, C, C#, and Rust) use this type of system.

The_ structural type system _is based on properties of the type and any item matching the properties can pass the equivalence test. **TypeScript **(_Go is another language) _uses a structural type system.

To better understand this, let’s define an interface to capture any I/O response and also implement a class HttpResponse for this interface.

Now a function that will take IOResponse as an argument and print the details.

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#type-systems #programming #typescript