Array

You can create an empty array in javascript without using an array literal by using then Array method with an argument of 0.

Array(0) // => []
new Array(0) // => []

You can create an empty array with a length by using a larger length argument to the Array function.

let x = Array(5)
x.length // => 5

You can create a new array and fill it with values from a function using the array spread syntax with a map function. Just using a naked Array(100) value will create a collection of undefined instances that cannot be mapped because the array is empty even if it has a length.

[...Array(100)].map(_ => Math.random());

Methods

You can check if even one element in an array meets a condition using the some method.

let x = [2, 4, 6, 7, 10]
x.some(e => e % 2 === 1) // => true
let x = [2, 4, 6, 8, 10]
x.some(e => e % 2 === 1) // => false

Synchronous vs Async Iteration

You cannot use await and with the forEach method on an array. Each branch of the forEach method will trigger asynchronously and simultaneously, so the loop will not work if it needs to execute synchronously .

You can run a synchronous loop through an array by using the for...of syntax.

let x [1, 2, 3, 4, 5]
for (const n of x) {
  synchronousFunction(n)
}

Index and value iteration

You can iterate an index and value of an array in a for/of loop using the entries function and array destructuring.

for (const [index, value] of ['a', 'b', 'c'].entries()) {
  console.log(index, value) // 1a, 2b, 3c
}