Grouping continuous number as "from~to" strings in JavaScript array

You can do this with Array.reduce() followed by Array.map():

const nums = [1, 2, 3, 5, 8, 9, 10]

const res = nums.reduce((acc, cur, idx, arr) => {
    if (idx === 0 || cur - 1 > arr[idx - 1]) {
        acc.push([cur])
    } else {
        acc[acc.length - 1].push(cur)
    }
    return acc
}, []).map(cur => cur.length > 1 ? cur.shift() + "~" + cur.pop() : cur[0])

console.log(res)

Go thru only one time all elements. Whenever there is pattern of numbers not in sequence, just put them in results array.

var myArray = [1, 2, 3, 5, 8, 9, 10];

let start = myArray[0];
let end = myArray[0];
const results = [];

for (let i = 1; i < myArray.length; i++) {
  if (myArray[i] === (end + 1)) {
    end = myArray[i];
  } else {
    results.push((start === end) ? `${start}` : `${start}~${end}`);
    start = myArray[i];
    end = myArray[i];
  }
}
results.push((start === end) ? `${start}` : `${start}~${end}`);

console.log(results);

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Basically just create a lastValue variable, and iterating the array. If the currentValue - 1 equals the lastValue then it's a continuous number. Create a new group once the condition failed.

In the final you just merge the groups with a string.

var newArray = [];
var tempArray = []; 
var oldArray = [1, 2, 3, 5, 8, 9, 10];
var lastValue = null;

oldArray.forEach(value => {
  if (lastValue != value - 1 || lastValue === null) {
    tempArray.push([value, value]);
  } else {
    tempArray[tempArray.length - 1][1] = value;
  }
  lastValue = value;
});

tempArray.forEach(value => {
  if (value[0] === value[1]) {
    newArray.push(value[0].toString());
  } else {
    newArray.push(value.join("~"));
  }
});

// OUTPUS: (3) ["1~3", "5", "8~10"]
document.write(JSON.stringify(newArray));

For typescript, this function takes a sorted array of numbers and groups them into a single string.

function convertNumberArrayToRangeString(numbers: number[]): string {
    const delimiter = '~';
    return numbers
      .reduce((accumulator: string, currentValue: number, index: number, array: number[]) => {
        if (index > 0 && array[index - 1] + 1 !== currentValue) {
            if (index > 2 && array[index - 1] - array[index - 2] === 1) {
              accumulator +=  delimiter + array[index - 1].toString();
            }
            accumulator += ',' + currentValue.toString();
        } else if (index === 0 || index === array.length - 1) { // first or last
          if (array[index - 1] === currentValue - 1) {
            accumulator +=  delimiter;
          } else if (index > 0) {
            accumulator +=  ',';
          }
          accumulator += currentValue.toString();
        } 
        return accumulator;
      }, '');
  }

const list = [1, 2, 3, 6, 7, 8]; // sorted unique list
const groupedList = convertNumberArrayToRangeString(list);
console.log(groupedList);

Output> "1~3,6~8"