Small snippet for converting strings in Two-Line Element into JSON format (with additional computed values such ass period, semi-major axis, JD and MJD).

tle.js

var TLE = 
(function () {
	function isCorrupted(line) {
		var checksum = parseInt(line[line.length-1]);
		var computedChecksum = line.slice(0,line.length-1)
		.replace(/[\-\-]/g,"1")
		.replace(/[a-zA-Z\.+\s]/g,"")
		.split("")
		.map(function(digit) { return parseInt(digit) })
		.reduce(function(prev, current) {return prev + current;}, 0) % 10;
		
		return checksum !== computedChecksum;
	}
	function computeJD(str) {
		var y = parseInt(str.slice(0,2));
		var day = parseFloat(str.slice(2));
		y += y < 57 ?  2000 : 1900;
		var days = Date.parse( y + "-01-01") / (24 * 60 * 60 * 1000); // days since 01-01-1970
		return (day - 1) + days + 2440587.5; // + JD  of 00:00:00 01-01-1970
	}	
	function calculatePeriod(str) {
		return  86400 / parseFloat(str); // secs in a day devided  by revolutions per day
	}
	function identity(x) {
		return x;
	}
	function floatWithDecimalPointAssumed(x) { 
		x  = x.trim();
		if (x.indexOf("-") > 0) {
			x = x.replace("-","e-");
		}
		return parseFloat("." + x); 
	}
	var converters = {
		firstLineCorrupted : {
			line : 1, col: 1, chars : 69 , f : isCorrupted
		},
		secondLineCorrupted : {
			line : 2, col: 1, chars : 69, f : isCorrupted
		},
		sateliteNumber : {
			line : 1, col: 3, chars : 5,	f : parseInt
		},
		classification : {
			line : 1, col: 8, chars : 1, f : identity
		},
		internationalDesignator : {
			line : 1, col: 10, chars : 8, f : identity
		},
		epoch : {
			line : 1, col: 21, chars : 14,	f : parseFloat
		},
		epochYear : {
			line : 1, col: 19, chars : 2,	f : parseInt
		},
		jd : {
			line : 1, col: 19, chars : 16,	f : computeJD
		},
		mjd : {
			line : 1, col: 19,	chars : 16, f : function(x) { return computeJD(x) - 2400000.5; } // counting from 0h Nov 17, 1858
		},
		firstDerivateOfMeanMotion :  {
			line : 1, col: 34, chars : 10, f : parseFloat
		},
		secondDerivateOfMeanMotion :  {
			line : 1, col: 45, chars : 8, f : floatWithDecimalPointAssumed
		},
		BSTARdragTerm :  {
			line : 1, col: 54, chars : 8, f : floatWithDecimalPointAssumed
		},
		ephemerisType :  {
			line : 1, col: 63, chars : 1, f : parseInt
		},
		elementNumber :  {
			line : 1, col: 65, chars : 4, f : parseInt
		},
		eccentricity : {
			line : 2, col: 27, chars : 7, f : floatWithDecimalPointAssumed
		},
		inclination : {
			line : 2, col: 9,  chars : 8, f : parseFloat
		},
		ascendingNodeRightAscension : {
			line : 2, col: 18, chars : 8, f : parseFloat
		},
		argumetOfPeriapsis : {
			line : 2, col: 35, chars : 8,	f : parseFloat
		},
		meanAnomaly : {
			line : 2, col: 44, chars : 8 , f : parseFloat
		},
		revolutionsPerDay : {
			line : 2, col: 53, chars : 11 ,	f : parseFloat
		},
		period : {
			line : 2, col: 53, chars : 11,	f : calculatePeriod // secs in a day devided  by revolutions per day
		},
		semiMajorAxis : {
			line : 2, col: 53, chars : 11 , // (GM * (T / (2 * PI)) ^ 2) ^ 1/3
			f : function(x) { return Math.pow(398600.4418 * Math.pow( calculatePeriod(x) / (2 * Math.PI),2), 1/3); } 
		}
	};
	function parse(tleString) {
	var tle = { titleLine : "" };
	var lines = tleString.split("\n");
	
	if (lines.length == 3) {
		tle.titleLine = lines[0].trim();
		lines.shift();
	}
	
	for (var property in converters) {
		if (converters.hasOwnProperty(property)) {
			var c = converters[property];
			var begin = c.col - 1,
				end = c.col + c.chars - 1;
			tle[property] = c.f(lines[c.line- 1].slice(begin, end));
		}
	}
	
	return tle;
}
	
	return {parse : parse};
})();