What techniques can be used to maintain the value of 'this' when utilizing async.apply?

Question

What techniques can be used to maintain the value of 'this' when utilizing async.apply?

Employing async.parallel to simultaneously run 2 functions, initiated from a static function within a mongoose model. In this code snippet (where the model contains a static function named verifyParent), I utilize this to access the model and its functions:

async.parallel([
      async.apply(content, {slug: slug}),
      async.apply(this.verifyParent, req.body.reply),
    ], (err, result) => {
          //results
});

However, within the this.verifyParent function, attempting to use this references my express app rather than the mongoose model. It seems that async.apply is causing this behavior, and I am struggling to maintain the usual value of this.

In the verifyParent function, I am querying mongodb. When calling this.findOne(), it returns an error stating it's not a function, which suggests that it is referencing the app rather than the model.

javascript express mongoose mongoose-schema async.js

Answer 1

Answer №1

Based on your query, it appears that the model is referenced using the this keyword.

If you want to make any changes to the code, you can modify it as follows:

var model = this;
var verify = function(reply) {
  model.verifyParent(reply);
};
async.parallel([
  async.apply(content, {slug: slug}),
  async.apply(verify, req.body.reply),
], (err, result) => {
      //results
});

It's essential to note that the use of the this keyword is context-specific. Check out Function context for more information.

Answer 2

Based on your query, it appears that the model is referenced using the this keyword.

If you want to make any changes to the code, you can modify it as follows:

var model = this;
var verify = function(reply) {
  model.verifyParent(reply);
};
async.parallel([
  async.apply(content, {slug: slug}),
  async.apply(verify, req.body.reply),
], (err, result) => {
      //results
});

It's essential to note that the use of the this keyword is context-specific. Check out Function context for more information.

Answer 3

Answer №2

To set the function to the current context, you can bind it as follows:

async.parallel([
      async.apply(content, {slug: slug}),
      async.apply(this.verifyParent.bind(this), req.body.reply),
    ], (err, result) => {
          //results
});

Here is the definition of async.apply, which appears to execute the provided function with the given arguments, leading to this referring to the parent scope, in this case, the express app.

This means that the following behavior occurs:

function apply(fn) {
  return fn();
}

var model = {
  prop: "world",
  verifyParent: function () {
    console.log("hello", this.prop)
  }
}

// model context is lost.
apply(model.verifyParent)

// explicitly bind to model.
apply(model.verifyParent.bind(model))

Answer 4

To set the function to the current context, you can bind it as follows:

async.parallel([
      async.apply(content, {slug: slug}),
      async.apply(this.verifyParent.bind(this), req.body.reply),
    ], (err, result) => {
          //results
});

Here is the definition of async.apply, which appears to execute the provided function with the given arguments, leading to this referring to the parent scope, in this case, the express app.

This means that the following behavior occurs:

function apply(fn) {
  return fn();
}

var model = {
  prop: "world",
  verifyParent: function () {
    console.log("hello", this.prop)
  }
}

// model context is lost.
apply(model.verifyParent)

// explicitly bind to model.
apply(model.verifyParent.bind(model))

Answer 5

Answer №3

There are several ways to achieve this:

Utilizing arrow functions:

async.parallel([
      async.apply(content, {slug: slug}),
      async.apply(() => this.verifyParent, req.body.reply),
    ], (err, result) => {
          //results
});

Implementing hard binding:

...
function boundVerifyParent() {
   return this.verifyParent.call(this)
}
...
async.apply(this.boundVerifyParent, req.body.reply),
...

Alternatively, using the bind method:

...
async.apply(this.verifyParent.bind(this), req.body.reply),
...

Answer 6

There are several ways to achieve this:

Utilizing arrow functions:

async.parallel([
      async.apply(content, {slug: slug}),
      async.apply(() => this.verifyParent, req.body.reply),
    ], (err, result) => {
          //results
});

Implementing hard binding:

...
function boundVerifyParent() {
   return this.verifyParent.call(this)
}
...
async.apply(this.boundVerifyParent, req.body.reply),
...

Alternatively, using the bind method:

...
async.apply(this.verifyParent.bind(this), req.body.reply),
...

What techniques can be used to maintain the value of 'this' when utilizing async.apply?

Answer №1

Answer №2

Answer №3

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