JavaScript - Create an Array Containing 1...N Numbers

Here are some methods to create an array containing 1...N numbers

1. Using for Loop

The for loop iterates from 1 to N, adding each number to the array.

function create(N) {
    let a = [];
    for (let i = 1; i <= N; i++) {
        a.push(i);
    }
    return a;
}

let n = 12;
let a = create(n);
console.log(a);

[
   1,  2, 3, 4,  5,
   6,  7, 8, 9, 10,
  11, 12
]

In this example :

• Initializes an empty array newArr.
• Uses a for loop to iterate from 1 to N, adding each number to newArr with push().
• Returns the array.

2. Using Spread Operator

We can also use Spread Operator to create an array containing 1...N numbers. We use Array() constructor to create a new array of length N, and then use keys() method to get an iterator over the indices of the array.

function create(n) {
    return [...Array(n).keys()].map(i => i + 1);
}

let n = 12;
let a = create(n);
console.log(a);

[
   1,  2, 3, 4,  5,
   6,  7, 8, 9, 10,
  11, 12
]

In this example :

• Array(n).keys(): Generates an iterator of indices from 0 to N-1.
• [...Array(n).keys()]: Converts the iterator into an array.
• .map(i => i + 1): Maps each index to a number incremented by 1.

3. Using from() Method

The Array.from() method creates an array of length N. It uses a mapping function to fill the array, where each element is set as index + 1, generating a sequence from 1 to N.

function create(n) {
    return Array.from({length: n}, (_, index) => index + 1);
}

let n = 12;
let a = create(n);
console.log(a);

[
   1,  2, 3, 4,  5,
   6,  7, 8, 9, 10,
  11, 12
]

In this example :

• Array.from({ length: N }): Creates an array with N undefined elements.
• (_, index) => index + 1: Maps each index (starting from 0) to a number incremented by 1.

4. Using _.range() method

The _.range() method is used to create an array of numbers progressing from the given start value up to, but not including the end value.

// Requiring the lodash library
const _ = require("lodash");

let range_arr = _.range(1,5);
console.log(range_arr);

Output:

[ 1, 2, 3, 4 ]

In this example :

• _.range(1, 5): Creates an array starting at 1 and ending before 5.
• The resulting array is [1, 2, 3, 4].

5. Using a Generator Function

A generator function generateNumbers(N) yields numbers from 1 to N.

function* genNum(N) {
  for (let i = 1; i <= N; i++) yield i;
}

const create = (N) => [...genNum(N)];
console.log(create(10));

[
  1, 2, 3, 4,  5,
  6, 7, 8, 9, 10
]

In this example :

• function* genNum(N): Declares a generator function that yields numbers from 1 to N.
• [...genNum(N)]: Uses the spread operator to convert the generator's yielded values into an array.
• create(10): Generates an array [1, 2, ..., 10].

6. Using Array.fill() and Array.map()

Array.fill() initializes an array with a specific value, while Array.map() creates a new array by applying a function to each element.

function create(n) {
    return Array(n).fill().map((_, index) => index + 1);
}

let n = 12;
let a = create(n);
console.log(a);

[
   1,  2, 3, 4,  5,
   6,  7, 8, 9, 10,
  11, 12
]

In this example :

• Array(N): Creates an array with N undefined elements.
• .fill(): Fills the array with undefined values (necessary to create an array to map over).
• .map((_, index) => index + 1): Maps each element to its index + 1.