Async & callbacks

Module 1 - Day 8

Functions

At the nomenclature level, there are:

• Nominal functions: they have declaration and invocation, with high reusability.

  function sayHi(name){
    console.log(`Hello, ${name}!`)
  }

• Anonymous functions: no declaration, only invocation. Used in a single context.

  names.forEach(function(elm) => {
     console.log(`Hello, ${elm}!`)
  })

At the structure level, there are:

• Function statement: they have the reserved word function, parentheses for their parameters and block:

  function getTotals(subtotal, tax){
    return subtotal + tax
  }

• Arrow function: they lack the reserved word function, the parenthesis in their parameters can be omitted in front of a single parameter, and they have an arrow before the opening of their block (block can be omitted in functions of a single instruction).

  const getTotals = (subtotal, tax) => subtotal + tax

Functions can recieve arguments as parameters and return data. It is possible to implement as a last parameter the propagation operator to create an array with the remaining parameters:

const getInvoiceTotal = (tax, ...products) => tax + products.reduce((acc, product) => acc + product.price, 0)

Variable scope

• Variables declared inside a block or function are called local or private, accessible only within that context.
• Variables declared outside any block or function are called global or public, accessible from anywhere in the script.

Async programming

• Javascript is a single-threaded language (single-threaded), so it cannot perform multiple tasks at the same time.
• It is also synchronous, since tasks are executed one after the other following the program sequence, where high-cost tasks in terms of time generate natural bottlenecks in the flow.
• Based on this nature, there are several asynchronous resources available to manage scheduled and/or parallel tasks. Among them, we find non-blocking methods such as .setTimeout() or .setInterval().

Callbacks

A callback is a function that receives as parameter another function, invoking it inside:

function foo = callback => callback()