JavaScript Language Basics

TypeScript const assertions vs satisfies operator

Posted on by admin

I compare as const assertions and satisfies operator in this post.

as const comes with TypeScript 3.4 and generates literal values for more precise and type-safe code.

satisfies comes with TypeScript 4.9 and generates more specific type and validates given object type so that it can catch possible error.

Notice that as const generates a type with readonly fields. So it is an immutable type.

Use case

Notice that satisfies just generates a type from palette object. As you can see below code, red field type is [number, number, number]

As you can see below code, red field type became string instead of [number, number, number]

Use case

satisfies operator catches unlisted field name usage, or wrong field type.

TypeScript const modifier on type parameters

Posted on by admin

It is a really good feature coming with TypeScript 5.0. We could infer type of object as general as shown in line 8 in below code, so, to infer more-specific type, we had to add as const as shown in line 11.

TypeScript 5.0 makes it easier with adding const in front of type parameter declaration in line 13 in below code.

Use case

Assume that Person type has hobbies , and i just want to infer passed values from hobbies field.

TypeScript Union Type and Distributive Conditional Types Part 2

Posted on by admin

I show the reason Distributive Conditional Type needs to be used for Union Type in this post.

If generic type T is passed as Union Type like string | number, then array item will be Union Type as below.

If you need to iterate on each type in Union Type, then Distributive Conditional Type (with extends keywod) needs to be used.

To avoid distributive behaviour in Distibutive Conditional Type, surround each side of extends keyword with square brackets.

Continue reading →

JavaScript Hoisting

Posted on by admin

This topic is very very important in javaScript. It is an unknown behavior of javaScript. Shortly, javaScript compiler moves variables and function declarations

Hoisting is JavaScript's default behavior of moving declarations to the top of the current scope.

Variables and constants declared with let or const are not hoisted!

JavaScript Declarations are Hoisted

In JavaScript, a variable can be declared after it has been used.

In other words, a variable can be used before it has been declared.

Hoisting applies to variable declarations and to function declarations.

function declarations are hoisted before variable declarations.

Hoisting is only possible with declaration but not the initialization. JavaScript will not move variables that are declared and initialized in a single line.

Continue reading →

JavaScript Functions

Posted on by admin

I sometimes try to remember basis of JavaScript Functions. So, I often exercise about it. I followed this link https://www.w3schools.com/js/js_function_definition.asp. I copied some important descriptions from that link. My aim is to follow this post when I need to remember again 🙂

JavaScript Functions

A JavaScript function is a block of code designed to perform a particular task.

A Function is much the same as a Procedure or a Subroutine, in other programming languages.

Why Functions?

You can reuse code: Define the code once, and use it many times.

You can use the same code many times with different arguments, to produce different results.

JavaScript Function Types

  1. Function Declaration (function statement)
  2. Function Expression
  3. Function Constructor
  4. Self-Invoking Function (Immediately Invokable Function Expression IIFE)
  5. Arrow Function
  6. Object Method Shorthand Definition
  7. Generator Function
Continue reading →