JavaScript: Wat!

array-custom-prop-not-iterable.js

// A custom property on an array is **not iterable**

const arr = [1,2];
arr[10] = 10; // => [1, 2, undefined*8, 10]
arr['ok'] = 'ok';

// for-in: enumerable properties:
for (const index in arr) console.log(arr[index]); // => 1, 2, 10, 'ok'
// for-of: iterable
for (const value of arr) console.log(value); // => 1, 2, undefined*8, 10
// forEach: strip undefined???
arr.forEach(value => console.log(value)); // => 1, 2, 10

map-parseInt.js

['1','2','3'].map(parseInt) // => [1, NaN, NaN]
// map callback: function(value:any, index:any, arr:[]):any
// parseInt: function(value:string, radix:number):number

['1','2','3'].map(num => parseInt(num, 10)) // => [1, 2, 3]