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var style = window.getComputedStyle(document.documentElement, null).getPropertyValue('font-size'),
    fontSize = parseInt(style),

    calcUnit = function(fn, unit) {
        return fn(unit);
    },

    px = function(rem) {
        return rem * fontSize;
        // rem -> px
    },

    rem = function(px) {
        return px / fontSize;
        // px -> rem
    };