How to create an array containing 1…N

Question

I\'m looking for any alternatives to the below for creating a JavaScript array containing 1 through to N where N is only known at runt

Answer 1

Multiple ways using ES6

Using spread operator (...) and keys method

[ ...Array(N).keys() ].map( i => i+1);

Fill/Map

Array(N).fill().map((_, i) => i+1);

Array.from

Array.from(Array(N), (_, i) => i+1)

Array.from and { length: N } hack

Array.from({ length: N }, (_, i) => i+1)

Note about generalised form

All the forms above can produce arrays initialised to pretty much any desired values by changing i+1 to expression required (e.g. i*2, -i, 1+i*2, i%2 and etc). If expression can be expressed by some function f then the first form becomes simply

[ ...Array(N).keys() ].map(f)

Examples:

Array.from({length: 5}, (v, k) => k+1); 
// [1,2,3,4,5]

Since the array is initialized with undefined on each position, the value of v will be undefined

Example showcasing all the forms

let demo= (N) => {
  console.log(
    [ ...Array(N).keys() ].map(( i) => i+1),
    Array(N).fill().map((_, i) => i+1) ,
    Array.from(Array(N), (_, i) => i+1),
    Array.from({ length: N }, (_, i) => i+1)
  )
}

demo(5)

More generic example with custom initialiser function f i.e.

[ ...Array(N).keys() ].map((i) => f(i))

or even simpler

[ ...Array(N).keys() ].map(f)

let demo= (N,f) => {
  console.log(
    [ ...Array(N).keys() ].map(f),
    Array(N).fill().map((_, i) => f(i)) ,
    Array.from(Array(N), (_, i) => f(i)),
    Array.from({ length: N }, (_, i) => f(i))
  )
}

demo(5, i=>2*i+1)