javascript - 为什么 Javascript 在设置变量时速度很慢？

Question

我在 Javascript 中遇到间隔问题。这个例子说明了一切

var foo = {

	counter: function() {
		// static variable
		if( typeof this.totalNumbers === 'undefined' )
			this.totalNumbers = 5;

		if( typeof this.counterInterval === 'undefined' ) {
			document.body.innerHTML +=
				'<p>this is being executed twice and then ' +
				'js decides its not undefined anymore ' +
				'after setting 2 intervals</p>'
			;
			this.counterInterval = setInterval(this.counter, 1000);
			return;
		}
		// now works perfectly but with two intervals...
		this.totalNumbers -= 1;
		document.body.innerHTML += '<p>' + this.totalNumbers + '</p>';

		if( this.totalNumbers === 0 ) {
			delete this.totalNumbers;
			clearInterval( this.counterInterval );
			document.body.innerHTML += 
					'now the last interval was deleted but the function' +
					' keeps running';
		}
	},
};
foo.counter();

Answer 1

您需要先绑定(bind)计数器函数，然后再将其传递给setInterval

this.counterInterval = setInterval(this.counter.bind(this), 1000);

否则第一次调用和第二次调用之间的this是不同的

var foo = {

	counter: function() {
		// static variable
		if( typeof this.totalNumbers === 'undefined' )
			this.totalNumbers = 5;

		if( typeof this.counterInterval === 'undefined' ) {
			document.body.innerHTML +=
				'<p>this is being executed twice and then ' +
				'js decides its not undefined anymore ' +
				'after setting 2 intervals</p>'
			;
			this.counterInterval = setInterval(this.counter.bind(this), 1000);
			return;
		}
		// now works perfectly but with two intervals...
		this.totalNumbers -= 1;
		document.body.innerHTML += '<p>' + this.totalNumbers + '</p>';

		if( this.totalNumbers === 0 ) {
			delete this.totalNumbers;
			clearInterval( this.counterInterval );
			document.body.innerHTML += 
					'now the last interval was deleted but the function' +
					' keeps running';
		}
	},
};
foo.counter();