How to code a calculator in javascript without eval

Question

So, I've searched high and low, and I can't find an answer to this. I've attempted it about three times and gotten a basic one cranked out by basically storing the input in an array as a string, parsing the numbers, then switching on the operator, in order to evaluate the integers, but I'm having a really hard time figuring out the chaining logic. Does anyone have any suggestions? Of maybe even just the psuedocode? I really don't want to use eval. Thanks a lot

Answer 1

For a simple calculator with only 5 operators (^, *, /, +, -) and no parentheses, you can do something like this. First, it is convenient to turn the string into an array of numbers and operators. Then, we go through the array looking for each operator in order of precedence, and applying the operator to the numbers preceding and following the it.

function parseCalculationString(s) {
    // --- Parse a calculation string into an array of numbers and operators
    var calculation = [],
        current = '';
    for (var i = 0, ch; ch = s.charAt(i); i++) {
        if ('^*/+-'.indexOf(ch) > -1) {
            if (current == '' && ch == '-') {
                current = '-';
            } else {
                calculation.push(parseFloat(current), ch);
                current = '';
            }
        } else {
            current += s.charAt(i);
        }
    }
    if (current != '') {
        calculation.push(parseFloat(current));
    }
    return calculation;
}

function calculate(calc) {
    // --- Perform a calculation expressed as an array of operators and numbers
    var ops = [{'^': (a, b) => Math.pow(a, b)},
               {'*': (a, b) => a * b, '/': (a, b) => a / b},
               {'+': (a, b) => a + b, '-': (a, b) => a - b}],
        newCalc = [],
        currentOp;
    for (var i = 0; i < ops.length; i++) {
        for (var j = 0; j < calc.length; j++) {
            if (ops[i][calc[j]]) {
                currentOp = ops[i][calc[j]];
            } else if (currentOp) {
                newCalc[newCalc.length - 1] = 
                    currentOp(newCalc[newCalc.length - 1], calc[j]);
                currentOp = null;
            } else {
                newCalc.push(calc[j]);
            }
            console.log(newCalc);
        }
        calc = newCalc;
        newCalc = [];
    }
    if (calc.length > 1) {
        console.log('Error: unable to resolve calculation');
        return calc;
    } else {
        return calc[0];
    }
}
var calculateButton = document.getElementById('calculate'),
    userInput = document.getElementById('userInput'),
    result = document.getElementById('result');
calculateButton.addEventListener('click', function() {
    result.innerHTML = "The answer is " + calculate(parseCalculationString(userInput.value));
});

<input type="text" id="userInput" />
<input type="button" value="Calculate" id="calculate" />
<div id="result"></div>

(jsfiddle). To allow parentheses, you could tell the calculate function to check for parentheses before it starts looking for any of the other operators, then recursively call itself on the expression within each set of parentheses. The parsing function can also be improved e.g. removing any white space and dealing with errors.