last.js

const entra = "10+62*7*+";

function flaten (input, carch) {
	var a = 0, output = [];

	if(!carch)
		return input;

	var last = input[ input.length - 1 ];
	var past = input[ input.length - 2 ];

	if( (!last || !past ) && last != 0 && past != 0 )
		return [ -1 ];
	
	eval( 'a = ' + last + carch  + past  );

	delete input[ input.length - 1 ];
	delete input[ input.length - 2 ];

	for (var i = input.length - 1; i >= 0; i--)
		if( input[i] ) output.push(input[i]);

	output.push(a);

	return output;
}

function solution(s) {
	var arr = [];
	for (var i = 0; i < s.length; i++) {
		var isNum =  parseInt(s[i]);
		if(isNum || isNum === 0 ) arr.push(isNum);
		if(/\*|\-|\+|\/|\^/.test(s[i]) )
			arr = flaten(arr, s[i]);
		console.log(arr);
	}

	return arr[0];
}