index.js

var math = require('mathjs');

/*

You One 1s and Four 7s, how do you make these numbers equal 100 with any operation.

*/

// dictionary to create combinations from
var dictionary = ['+', '-', '/', '*'];

// all combinations with 4 letters
var comb = combination(dictionary, 5);


var result = null;

for (var i = 0; i < comb.length; i++) {
  var expression = math.eval(`1 ${comb[i].charAt(0)} 7 ${comb[i].charAt(1)} 7 ${comb[i].charAt(2)} 7 ${comb[i].charAt(3)} 7`);
  console.log(`1 ${comb[i].charAt(0)} 7 ${comb[i].charAt(1)} 7 ${comb[i].charAt(2)} 7 ${comb[i].charAt(3)} 7 = ${expression}`);
  if (expression == 100 || expression == "100") {
    result = `1 ${comb[i].charAt(0)} 7 ${comb[i].charAt(1)} 7 ${comb[i].charAt(2)} 7 ${comb[i].charAt(3)} 7`;
  }
}

if (result) {
  console.log("Result Found: " + result);
} else {
  console.log("No Result was found");
}

function combination(dictionary, length, depth, input, output) {
    if (!depth) {
        output = [];
        input = '';
        depth = 0;
    }

    dictionary.forEach(function(element) {
        if (depth == length - 1) {
            output.push(input + element);
        } else {
            combination(dictionary, length, depth + 1, input + element, output);
        }
    });

    if (!depth)
        return output;

    return;
}