编辑代码

let i = 0;
let timer = null;
timer = setInterval(function(){
	console.log(i++);
	if(i === 10) {
		clearInterval(timer);
		console.log('clearInterval inside!', i);
	}
}, 1000);

if(timer === null) {
	clearInterval(timer);
	console.log('clearInterval outside!', i);
}