vuejs/vue

warn if $set is used on a property that already exist

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#8,129 opened on May 4, 2018

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Description

Version

2.5.16

Reproduction link

https://codepen.io/JJPandari/pen/gzLVBq?editors=1010

Steps to reproduce

See the codepen snippet. Follow the comment there to change the vm's data and see what happens.

What is expected?

Even if the prop already exists, using set still makes it reactive, thus trigger view update.

What is actually happening?

Using set later doesn't update the view.

Related source code: https://github.com/vuejs/vue/blob/3eb37acf98e2d9737de897ebe7bdb7e9576bcc21/src/core/observer/index.js#L192 I think most users would expect set to make the prop reactive whenever it's used. I initially opened an issue for the api doc because it wasn't clear (for me) about this. But the comment in the source code is.

Contributor guide

Research direction
Investigate the `$set` implementation in vue/src/core/observer/index.js, specifically the logic that checks if a property already exists and how reactivity is set up. The issue suggests that `$set` should make an existing property reactive, but it doesn't. Look at the condition and the call to `defineReactive` to understand why.
Tech stack
javascript
Domain
frontend
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Mostly clear
Prerequisites
Vue
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60