var becomes undefined inside Promise

Question

This is probably something a very small problem but I can't find its solution so please bear with me!

I don't understand why at const body = ctx.request.body ctx becomes undefined. Before it carried over this from the create() function which calls createEntity().

What am I doing wrong?

I'm calling a function createEntitylike this:

module.exports = {
  createEntity: () => {
    var ctx = this  // both ctx and this are fine
    return new Promise((resolve, reject) => {
      const body = ctx.request.body  // <-- ctx is undefined
      resolve();
    })
  }
}

create() calls createEntity(). It is a generator function wrapped with co()via Koa.js

create: function * () {
    this.body = yield createEntity.call(this)
}

EDIT: Here's a screenshot why I thought this is fine after calling createEntity:

You're using an arrow function, why not just use `const body = this.request.body`? — Nebula, Jul 28 '16 at 16:22
`this` is evaluated incorrectly on the screenshot, it looks like a Jetpack product bug. — Estus Flask, Jul 28 '16 at 16:31
@estus What is Jetpack? I thought VSCode is using the default node debugger. — Hedge, Jul 28 '16 at 16:39
@Hedge Confused it with Webstorm debugger appearance. Sure, Node debugger is used somehow, but this doesn't mean that IDE uses it the right way. — Estus Flask, Jul 28 '16 at 16:50

score 4 · Accepted Answer · edited May 23 '17 at 10:28

4

both ctx and this are fine

I doubt that. You are using an arrow function. You cannot set an arrow function's this via call because an arrow doesn't have an own this ¹.

Use a normal function instead.

See also Arrow function vs function declaration / expressions: Are they equivalent / exchangeable?

1: this inside createEntity refers to the top-level this of the module, which is module.exports:

this;        // <------------|
module.exports = {           |
  createEntity: () => {      |
    var ctx = this     // ---|
  }
}
console.log(this === module.exports); // true

edited May 23 '17 at 10:28

Community

1
1

answered Jul 28 '16 at 16:23

Felix Kling

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Yeah that was a dumb mistake. Please check out my screenshot in the original question why I thought `this` was fine. – Hedge Jul 28 '16 at 16:29
Object doesn't mean they are the same. If you have var object = {} it is also an Object but has nothing inside – LuisPinto Jul 28 '16 at 16:31

score 1 · Answer 2 · edited Sep 14 '17 at 20:34

By assigning an arrow function to createEntity, this inside the arrow function is assigned to the value of this in the scope where the arrow function was declared, which is probably the global object (therefore if you're using use strict, this will be undefined), and it'll stay that way even if you invoke the function using call.

Use a common function expression instead

createEntity: function () {
    ...
}

var becomes undefined inside Promise

2 Answers2