Creating an array from another array

Question

I'm learning Javascript and I'm wondering what the most elegant way to convert this: [1,8] into this:[1,2,3,4,5,6,7,8]?

Thanks a lot!

Answer 1

const argarray = [1, 8]

const countToN = (array) => {
  // init results array
  let res = []
  // start at the first value array[0] go *up to* the second array[1]
  for (let i = array[0]; i <= array[1]; i++) {
    res.push(i)
  }
  // return the result
  return res
}

console.log(countToN([1, 10]))

This would accommodate what you're trying to do, but it's fairly brittle. You'd have to check that it's an array and that it has only 2 values. If you had other requirements, I could amend this to account for it.

Answer 2

Here's a solution without loops. Note that this only works with positive numbers. It supports arrays of any length, but will always base the result off of the first and last values.

const case1 = [1, 8];
const case2 = [5, 20];

const startToEnd = (array) => {
    const last = array[array.length - 1];
    const newArray = [...Array(last + 1).keys()];

    return newArray.slice(array[0], last + 1);
};

console.log(startToEnd(case1));
console.log(startToEnd(case2));

Here's a solution that works for negative values as well:

const case1 = [-5, 30];
const case2 = [-20, -10];
const case3 = [9, 14];

const startToEndSolid = (array) => {
    const length = array[array.length - 1] - array[0] + 1;

    if (length < 0) throw new Error('Last value must be greater than the first value.');

    return Array.from(Array(length)).map((_, i) => array[0] + i);
};

console.log(startToEndSolid(case1));
console.log(startToEndSolid(case2));
console.log(startToEndSolid(case3));

Answer 3

A simple for loop will do it. Here's an example that has error checking and allows you to range both backwards and forwards (ie [1, 8], and also [1, -8]).

function range(arr) {

  // Check if the argument (if there is one) is
  // an array, and if it's an array it has a length of
  // of two. If not return an error message.
  if (!Array.isArray(arr) || arr.length !== 2) {
    return 'Not possible';
  }

  // Deconstruct the first and last elements
  // from the array
  const [ first, last ] = arr;

  // Create a new array to capture the range
  const out = [];

  // If the last integer is greater than the first
  // integer walk the loop forwards
  if (last > first) {
    for (let i = first; i <= last; i++) {
      out.push(i);
    }

  // Otherwise walk the loop backwards
  } else {
    for (let i = first; i >= last; i--) {
      out.push(i);
    }
  }

  // Finally return the array
  return out;

}

console.log(range([1, 8]));
console.log(range('18'));
console.log(range());
console.log(range([1]));
console.log(range([-3, 6]));
console.log(range([9, 16, 23]));
console.log(range([4, -4]));
console.log(range([1, -8, 12]));
console.log(range(null));
console.log(range(undefined));
console.log(range([4, 4]));

Additional documentation

• Destructuring assignment

Answer 4

Use Array#map as follows:

const input = [1,8],

      output = [...Array(input[1] - input[0] + 1)]
      .map((_,i) => input[0] + i);
      
      
console.log( output );