The answer
Lost this in callbacks, arrow methods on objects (lexical this), forgetting new, this in setTimeout/forEach inside methods, destructured methods (detached), and expecting arrows to have object this.
this Keyword Pitfalls and Fixes in JavaScript
What are common this keyword pitfalls in JavaScript?
More FAQs in this Keyword Pitfalls and Fixes in JavaScript
Use .bind(obj) to permanently bind this, or wrap the method call in an arrow function (() => obj.method()). Arrows inherit this lexically from the enclosing scope, so they preserve the correct binding.
Because an arrow function's this is the enclosing scope's this, not the object. So obj.greet = () => this.name gives undefined. Use regular functions for methods so this is the object.
Use an arrow function for the setTimeout callback. The arrow inherits this from the method (which is the object). Do not use a regular function, which loses this.
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