問題：計算兩個數字的和？

分析：眾所周知，Number型別的值有個上限，如果是兩個極大的數字相加求和，便不能使用最簡單的+方法，可以將兩個數字當成字串去處理，從最末尾開始相加，注意遞進的位數，最後得到相加的字串

實現：

//引數a、b均為字串正整數，求和
function add(a, b){
	var aLen = a.length,
		bLen = b.length,
		maxLen = aLen - bLen >= 0 ? aLen : bLen,
		num = 0,		//向上位數遞級
		str = '';
	for(var i = 1; i <= maxLen; i++){
		var aVal = aLen - i >= 0 ? parseInt(a.charAt(aLen - i)) : 0,
			bVal = bLen - i >= 0 ? parseInt(b.charAt(bLen - i)) : 0,
			bitVal = 0,
			addVal = aVal + bVal + num;
		if(addVal >= 10){
			num = parseInt(addVal / 10);
			bitVal = addVal % 10;
		}else{
			num = 0;
			bitVal = addVal;
		}
		str += bitVal;
	}
	return Array.prototype.reduceRight.call(str, function(a, b){return a + b;});
}
 

function add(a, b){
	var aArr = a.split('').reverse(),
		bArr = b.split('').reverse();
	return (function addStep(aArr, bArr, str, num){
		var aVal = aArr.length === 0 ? 0 : parseInt(aArr.shift()),
			bVal = bArr.length === 0 ? 0 : parseInt(bArr.shift()),
			num = num || 0,
			bitVal = 0,
			addVal = aVal + bVal + num,
			str = str || '';
		if(addVal >= 10){
			num = parseInt(addVal / 10);
			bitVal = addVal % 10;
		}else{
			num = 0;
			bitVal = addVal;
		}
		str += bitVal;
		if(aArr.length === 0 && bArr.length === 0){
			console.log(Array.prototype.reduceRight.call(str, function(a, b){return a + b;}));
		}else{
			addStep(aArr, bArr, str, num);
		}
	})(aArr, bArr, '', 0)
}