array-from.js

// it's array-like, but it's not an array
const notAnArray = {
  length: 3,
  0: 'a',
  1: 'b',
  2: 'c'
}

console.log( Array.isArray( notAnArray ) ) // false

// this common pattern works, but it's still not an array
for( let i = 0; i < notAnArray.length; i++ ){
  console.log( notAnArray[ i ] )
}

// it's "array-like" - so we can make it a real array with Array.from:
const butNowItIs = Array.from( notAnArray )

console.log( Array.isArray( butNowItIs ) ) // true

// here's a neat trick - create an array of a given length and populate it:
const arrayFromLength = Array.from(
  { length: 26 },
  ( _value, key ) => String.fromCharCode( 97 + key )
)

console.log( Array.isArray( arrayFromLength ) ) // true
console.log( arrayFromLength ) // [ 'a', 'b', 'c', ...etc ]

// now lets make it a function:
const createSequence = ( length, mapFn ) =>
  Array.from( { length }, ( _value, key ) => mapFn( key ) )

const theAlphabet = createSequence( 26, i => String.fromCharCode( i + 97 ) )

console.log( Array.isArray( theAlphabet ) ) // true
console.log( theAlphabet ) // [ 'a', 'b', 'c', ...etc ]

/*
  more:

  https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/from
*/