US Federal Income Tax calculator in javascript

fedtax.js

// Weekly Employee Income Withholding Calculator by GitHub#Liath
// Based on IRS Circular E - http://www.irs.gov/pub/irs-pdf/p15.pdf
// Per the "Percentage Method"
var  w = {
    s : { // Single
        0    : { p : 0, s : 0 },
        44   : { p: 0.1, s: 0 },
        222  : { p: 0.15, s: 17.8 },
        764  : { p: 0.25, s: 99.1 },
        1789 : { p: 0.28, s: 355.35 },
        3685 : { p: 0.33, s: 886.23 },
        7958 : { p: 0.35, s: 2296.32 },
        7990 : { p: 0.396, s: 2307.52 }
    },
    m : { // Married
        0    : { p : 0, s : 0 },
        165  : { p : 0.1, s : 0 },
        520  : { p : 0.15, s : 35.5 },
        1606 : { p : 0.25, s : 198.4 },
        3073 : { p : 0.28, s : 565.15 },
        4597 : { p : 0.33, s : 991.87 },
        8079 : { p : 0.35, s : 2140.93 },
        9105 : { p : 0.396, s : 2500.03 }
    }
},
ba = 76.9, // 1 Allowance
c = function(m, a, g) { // Returns Federal Income Tax amount (Married, Allowances, Gross Income)
    g -= (ba*a); // Pay after allowances
    var b = Object.keys(w[(m==1) ? 'm' : 's']); //Married?
    for (var i = 0; i < b.length; i++) { // Find bracket
        if (b[i] > g)  {
            g -= b[i-1]; // Get taxable income
            b = w[(m==1) ? 'm' : 's'][b[i-1]]; // Set bracket
            return ((b.p*(g)) + b.s); // Taxable income * Tax Rate + Base Tax, per IRS Circular E table 5
        }
    }
}

I'm so confused on this one I can't get it to work for the life of me can someone please Help

So, if you're single with one allowance and you make $1000 a week you would run c(0, 1, 1000). 0 for single, 1 is the allowances, and $1000/week. Should result in 138.875.

Thank you for this, Liath! I have websites that rely on this code, so I forked it to keep it updated each year: https://gist.github.com/axcviii/502ef703ec727507e5cb1a5232df7fc7 (currently 2018 rates).