What is the value of this in a class constructor? What about in a class method called as a callback?

Q: What is the value of this in a class constructor? What about in a class method called as a callback?

In a JavaScript class constructor, `this` refers to the new instance being created. Properties and methods attached to `this` become instance members. However, when a class method is passed as a callback (e.g., to `setTimeout`), it loses its binding and `this` becomes `undefined` (in strict mode) or the global object. To fix this, you must bind the method in the constructor or use an arrow function class field. Example: `class App { constructor() { this.name = 'App'; } log() { console.log(this.name); } } const app = new App(); setTimeout(app.log, 100); // undefined (context lost)`.

Difficulty: 5/10

this binding, class constructors, callback context

In a class constructor, `this` refers to the instance being created. In a class method called as a callback, `this` becomes `undefined` or the global object unless the method is bound or called with explicit binding.

In a JavaScript class constructor, this refers to the new instance being created. Properties and methods attached to this become instance members. However, when a class method is passed as a callback (e.g., to setTimeout), it loses its binding and this becomes undefined (in strict mode) or the global object. To fix this, you must bind the method in the constructor or use an arrow function class field. Example: class App { constructor() { this.name = 'App'; } log() { console.log(this.name); } } const app = new App(); setTimeout(app.log, 100); // undefined (context lost).

Scenario Questions

0-2 years experience

2-5 years experience

1A React class component's `this.handleClick` loses its context when used as an event handler. Walk me through why that happens and how you'd fix it.

Follow-up Questions

Difficulty: 5/10

this binding, class constructors, callback context

In a class constructor, `this` refers to the instance being created. In a class method called as a callback, `this` becomes `undefined` or the global object unless the method is bound or called with explicit binding.

Scenario Questions

0-2 years experience

2-5 years experience

1A React class component's `this.handleClick` loses its context when used as an event handler. Walk me through why that happens and how you'd fix it.

What is the value of this in a class constructor? What about in a class method called as a callback?

In a class constructor, this refers to the instance being created. In a class method called as a callback, this becomes undefined or the global object unless the method is bound or called with explicit binding.

Scenario Questions

Follow-up Questions

What is the value of this in a class constructor? What about in a class method called as a callback?

In a class constructor, this refers to the instance being created. In a class method called as a callback, this becomes undefined or the global object unless the method is bound or called with explicit binding.

Scenario Questions

Follow-up Questions

In a class constructor, `this` refers to the instance being created. In a class method called as a callback, `this` becomes `undefined` or the global object unless the method is bound or called with explicit binding.

In a class constructor, `this` refers to the instance being created. In a class method called as a callback, `this` becomes `undefined` or the global object unless the method is bound or called with explicit binding.