参考：https://github.com/miloyip/rapidjson/blob/master/include/rapidjson/internal/itoa.h

fast itoa.js

var fastItoa = (function () {
	// 使用 2n 作为索引
	var table = [
		'0', '0',
		'0', '1',
		'0', '2',
		...
		'9', '8',
		'9', '9'
	];

	return function (num) {

		var ret = '';

		if (num < 10000) {

			var n1 = (num / 100) << 1;	// 高位
			var n2 = (num % 100) << 1;	// 低位

			if (value >= 1000) {
				ret = ret + table[n1];
			}
			if (value >= 100) {
				ret = ret + table[n1 + 1];
			}
			if (value >= 10) {
				ret = ret + table[n2];
			}
			ret = ret + table[n2 + 1];

		} else if (value < 10000 * 10000) {

			var h = num / 10000;
			var l = num % 10000;


			var n1 = (h / 100) << 1;
			var n2 = (h % 100) << 1;
			var n3 = (l / 100) << 1;
			var n4 = (l % 100) << 1;

			if (num >= 1000 * 10000) {
				ret = ret + table[n1];
			}
			if (num >= 100 * 10000) {
				ret = ret + table[n1 + 1];
			}
			if (num >= 10 * 10000) {
				ret = ret + table[n2];
			}
			ret = ret + table[n2 + 1];

			ret = ret + table[d3] + table[d3 + 1] + table[d4] + table[d4 + 1];

		} else {

			var h = num / 100000000;
			num = num % 100000000;

			if (h >= 10) {
				var temp = h << 1;
				ret = ret + table[temp];
				ret = ret + table[temp + 1];
			} else {
				ret = ret + h;
			}

			var m = num / 10000;
			var l = num / 10000;

			var n1 = (m / 100) << 1;
			var n2 = (m % 100) << 1;
			var n3 = (l / 100) << 1;
			var n4 = (l % 100) << 1;

			ret = ret + 
				table[n1] + table[n1 + 1] + 
				table[n2] + table[n2 + 1] + 
				table[n3] + table[n3 + 1] + 
				table[n4] + table[n4 + 1];
		}

		return ret;
	};
}());