How do you fix the lost this bug in ES6 classes in JavaScript?
Three ways: bind in the constructor (this.handleClick = this.handleClick.bind(this)), use arrow class fields (handleClick = () => { ... }), or use arrow wrappers in callbacks (setTimeout(() => this.method(), 1000)).
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More FAQs in this in ES6 Classes in JavaScript
this in the constructor is the new instance. this in methods is the instance when called as instance.method(). But detaching a method (passing to setTimeout or addEventListener) loses this. Fix with bind, arrow class fields, or arrow wrappers.
Because the method is detached from the instance when passed as a reference. The callback is called as a plain function, so this is not the instance. Fix by binding in the constructor or using arrow class fields.
Class fields defined as arrows (handleClick = () => { ... }). They capture this from the constructor's scope (the instance), so they are always bound. They are per-instance (not on the prototype), so they cost a bit more memory.
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