es6 padstart方法不兼容处理

	/** * String.padStart() * version 1.0.1 * Feature Chrome Firefox Internet Explorer Opera Safari Edge * Basic support 57 51 (No) 44 10 15 * ------------------------------------------------------------------------------- */
if (!String.prototype.padStart) { 

String.prototype.padStart = function padStart(targetLength, padString) { 

targetLength = targetLength >> 0; //floor if number or convert non-number to 0;
padString = String(typeof padString !== 'undefined' ? padString : ' ');
if (this.length > targetLength) { 

return String(this);
} else { 

targetLength = targetLength - this.length;
if (targetLength > padString.length) { 

padString += padString.repeat(targetLength / padString.length); //append to original to ensure we are longer than needed
}
return padString.slice(0, targetLength) + String(this);
}
};
}