Solution for task "Sum 100" by https://www.diginex.com

100.js

/*Question 1: Sum to 100
Write a function that, given a string of digits, build an expression such that the digits will sum to 100 by inserting +, - anywhere between the digits.
For example, given the input 123456789, one such expression is 1 + 23 - 4 + 5 + 6 + 78 - 9 = 100
Your function should return all possible combinations.
*/
var results = {};

function calc(n, s, part) {

    if (n === 0 && s === '' && part[0] !== '-') return results[part.substring(1)] = null

    if (Math.abs(n) > (1 + Number(s[0])) * Math.pow(10, s.length - 1)) return;// extra line for optimization

    for (var i = 1; i <= s.length; i++) {
        var a = Number(s.substring(0, i))
        calc(n - a, s.substring(i), part + '+' + a)
        calc(n + a, s.substring(i), part + '-' + a)
    }
}


calc(100, '123456789', '') || Object.keys(results).map(r => console.log(r))

/*
1+2+3-4+5+6+78+9
1+2+34-5+67-8+9
1+23-4+5+6+78-9
1+23-4+56+7+8+9
12+3+4+5-6-7+89
12+3-4+5+67+8+9
12-3-4+5-6+7+89
123+4-5+67-89
123-4-5-6-7+8-9
123+45-67+8-9
123-45-67+89
*/