A method works when called directly but loses its receiver when passed as a callback. What happened?

Explains call-site-dependent this binding and compares wrapper, bind, and arrow-function fixes.

1Say itAnswer out loud first

2CompareCompare with Core answer

3RefineRefine with Stronger and Advanced

TL;DR

For an ordinary function, this is determined by how the function is called. service.save() supplies service as the receiver, but passing service.save as a callback loses that call-site relationship. In strict mode the callback's this is usually undefined. I can preserve intent with a wrapper such as value => service.save(value) or with service.save.bind(service).

Full answer below

Practice focus

Whether the candidate can diagnose receiver loss without claiming that `this` is lexical in ordinary functions.

Answer first

Answer path

Core answer first, then checkpoints toward a stronger answer.

Start
Core answer
Start here. This is the direct spoken answer to practice first.
Overview
Passing object.method passes a function value. It does not permanently carry the object from the original property access.
For an ordinary function, this is determined by how the function is called. service.save() supplies service as the receiver, but passing service.save as a callback loses that call-site relationship. In strict mode the callback's this is usually undefined. I can preserve intent with a wrapper such as value => service.save(value) or with service.save.bind(service).

Review signals

Use these to catch weak phrasing, shallow reasoning, and risky claims.

Common mistakesCandidateCommon mistakesMistakes that weaken the answer or signal shallow understanding.Open details

Details

Saying this always points to where a function was defined.
Adding bind without checking whether the callback API supplies a receiver.
Confusing an arrow function's lexical this with an ordinary method.

A method works when called directly but loses its receiver when passed as a callback. What happened?

Practice focus

Answer path

Overview

Review signals

Follow-up prompts

Continue practice

Next interviewer turn

Skill areas

Human editorial direction

Was this drill useful?

Found this answer useful?