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//
// Call this function and pass in the name of the font you want to check for availability.
//
function doesFontExist(fontName) {
    // creating our in-memory Canvas element where the magic happens
    var canvas = document.createElement("canvas");
    var context = canvas.getContext("2d");
     
    // the text whose final pixel size I want to measure
    var text = "abcdefghijklmnopqrstuvwxyz0123456789";
     
    // specifying the baseline font
    context.font = "72px monospace";
     
    // checking the size of the baseline text
    var baselineSize = context.measureText(text).width;
     
    // specifying the font whose existence we want to check
    context.font = "72px '" + fontName + "', monospace";
     
    // checking the size of the font we want to check
    var newSize = context.measureText(text).width;
     
    // removing the Canvas element we created
    delete canvas;
     
    //
    // If the size of the two text instances is the same, the font does not exist because it is being rendered
    // using the default sans-serif font
    //
    if (newSize == baselineSize) {
        return false;
    } else {
        return true;
    }
}

doesFontExist("Comic Sans MS");