Is it possible to bind this in bluebird map? - node.js

Question

So I've tried to write up an example as best I could of what I'm trying to do, this isn't a very practical example but I tried to simplify it, but I feel like I may have complicated this in trying to make an example.
class add {
constructor(baseValue) {
this.base = baseValue;
return new Promise((resolve, reject) => {
resolve(this);
});
}
addBase(num) {
return new Promise((resolve, reject) => {
resolve(this.base + num);
});
}
}
const values = [1,2,3,4,5];
Promise.try(() => {
return new add(5);
}).then((add) => {
// we want to find if a 5 exists in the results
const addPromise = Promise.resolve(values).map(add.addBase, {concurrency: 1});
return Promise.try(() => {
return addPromise;
}).then((results) => {
for(let i = 0; i < results.length; i++) {
if(results[i] === 10) {
return i;
}
}
// doesn't exist
return null;
});
}).then((result) => {
if(result === null) {
console.log('10 does not exist');
} else {
console.log('10 is at position ' + result);
}
})
<script src="https://cdn.jsdelivr.net/bluebird/latest/bluebird.min.js"></script>
If you run this you'll get an error that you can't get base of undefined, this is because of the mapping in bluebird. const addPromise = Promise.resolve(values).map(add.addBase, {concurrency: 1}); Is there a way on this line to bind the add object to this when making these calls?

Answer 1

This is actually a little simpler than you're making it I think. You are passing the raw function into map(), but you should probably be passing an arrow function instead. Consider this simple class and code that tries to use map() by passing add():
class Test{
constructor(n) {
this.n = n
}
add(k) {
return this.n + k
}
}
let t = new Test(10)
let arr = [1, 2, 3]
// error TypeError: undefined is not an object (evaluating 'this.n')
arr.map(t.add)
This throws an error because map isn't calling add() from the object, it just thinks it's a function. An easy fix is to call map like this:
class Test {
constructor(n) {
this.n = n
}
add(k) {
return this.n + k
}
}
let t = new Test(10)
let arr = [1, 2, 3]
let mapped = arr.map((n) => t.add(n))
console.log(mapped)
You could also use:
let mapped = arr.map(t.add.bind(t))
but to me that's harder to read and understand quickly. I'm not sure what's going on with all the immediately resolved promises in your code, but changing the way you call map() makes that error go away. (there's another error later where you reference i that's not in scope.)

Answer 2

You've pretty much answered your own question…
class add {
constructor(baseValue) {
this.base = baseValue;
return new Promise((resolve, reject) => {
resolve(this);
});
}
addBase(num) {
return new Promise((resolve, reject) => {
resolve(this.base + num);
});
}
}
const values = [1,2,3,4,5];
Promise.try(() => {
return new add(5);
}).then((add) => {
// we want to find if a 5 exists in the results
const addPromise = Promise.resolve(values).map(add.addBase.bind(add), {concurrency: 1});
return Promise.try(() => {
return addPromise;
}).then((results) => {
for(let i = 0; i < results.length; i++) {
if(results[i] === 10) {
return i;
}
}
// doesn't exist
return null;
});
}).then((result) => {
if(result === null) {
console.log('10 does not exist');
} else {
console.log('10 is at position ' + result);
}
})
<script src="https://cdn.jsdelivr.net/bluebird/latest/bluebird.min.js"></script>