You gonna have to do it the hard way. Parse the string to find all the operator and the associate operand. Then put them in order to perform the calculation.

Do small pieces at a time. Like find the operator.
Then the operand.

Here is an example I just wrote. Seems to work but I didnt tested it for every combination.

//STR TO INT FROM UI
var digits = “”;

//THE OP + NUMS (from digits)
var math = [];

//THE RESULT
var res = 0;

//SCREEN
var view;
view = document.getElementById(‘view’);

//CREATE A NUMBER FROM DIGITS[str]
function getValue(val) {
digits += val;
}

//TAKES CARE OF THE CHRONOLOGY and RETURNS THE RESULT
function opChrono(arr, id) {
//LAST NUMBER EXCEPTION
if (digits) {
math.push(parseFloat(digits));
digits = “”;
}
arr.map(function(val, idx, arr) {
//MULTI FIRST
if (val == “") {
arr.splice((idx - 1), 3, (arr[idx - 1] * arr[idx + 1]));
}
//THEN DIVISION
if (arr.indexOf("”) == -1 && val == “/”) {
arr.splice((idx - 1), 3, (arr[idx - 1] / arr[idx + 1]));
}
})

//no MULTI/DIV
if (arr.indexOf("*") == -1 && arr.indexOf("/") == -1) {
//FIRST NUMBER EXCEPTION
res += arr[0];
//GEN RES
arr.map(function(value, index, arr) {
if (value == "-" || value == "+") {
res += parseFloat(value + arr[index + 1]);
}
})
//SHOW RES
view.innerHTML = res;
//RECU STOP
return arr;
};
//RECU GO
return opChrono(arr, id);

}

function getSign(id) {
//create INT and put into OP ARR
math.push(parseFloat(digits));
//the SIGN
lastSign = document.getElementById(id).id;
if (lastSign != “eq”) {
math.push(lastSign);
}
//show
view.innerHTML = math.join(" ");
//reset to create a new NUMBER
digits = “”;
}