How to create an array containing 1…N in JavaScript?

To create an array containing the numbers from 1 to N in JavaScript, you can use one of the following methods:

1. Using Array.from() (Recommended)

Most concise and efficient for modern browsers (ES6+):

const N = 5;
const arr = Array.from({ length: N }, (_, i) => i + 1);
// Output: [1, 2, 3, 4, 5]

How it works:

• Array.from({ length: N }) creates an array of N empty slots.
• The second argument is a mapping function that replaces each empty slot with i + 1, where i is the index (starting at 0).

2. Using Spread Operator and keys()

Leverages the spread operator and array indexing:

const N = 5;
const arr = [...Array(N).keys()].map(i => i + 1);
// Output: [1, 2, 3, 4, 5]

Breakdown:

• Array(N) creates an array of N empty slots.
• .keys() generates an iterator of indices [0, 1, ..., N-1].
• map(i => i + 1) increments each index by 1.

3. Using a Loop

Explicitly push values into an array (works in all browsers):

const N = 5;
const arr = [];
for (let i = 1; i <= N; i++) {
  arr.push(i);
}
// Output: [1, 2, 3, 4, 5]

4. Using fill() and map()

Initialize an array with fill() and map indices:

const N = 5;
const arr = Array(N).fill().map((_, i) => i + 1);
// Output: [1, 2, 3, 4, 5]

Note:

• Array(N).fill() fills the array with undefined values.
• .map((_, i) => i + 1) replaces each undefined with i + 1.

Edge Cases

• N = 0: Returns an empty array.
• Negative N: Returns an empty array.

Summary

• Best Practice: Use Array.from({ length: N }, (_, i) => i + 1).
• Legacy Browsers: Use the loop method.
• Avoid new Array(N).map(...) directly (due to sparse arrays).

Example Output for N = 3:

[1, 2, 3]