CODEWITHSUNDEEP
node.jspromisebind
Pano
Pano

Reputation: 2189

node bind and promise

I googled some bind explanation but still very confused. I am using bind with promise in the following way. It's working, but I don't know why. Hope someone could explain and help me with a small issue I got. Here is the sample code:

var temp1 = function(){
    return new Promise(function(resolve, reject){
        resolve("hello");
    });
};

var temp2 = function(para1, para2){
    console.log("1:", para1);
    console.log("2:", para2)
    return new Promise(function(resolve, reject){
        resolve(para1+" "+para2);
    });
};

temp1().then(temp2.bind(null,"world")).then(function(out){
   console.log(out); 
});

Output:

1: world \ 2: hello \ world hello \

I use "bind" because it can make the code cleaner and easier for error handling, otherwise I have to use the following:

temp1().then(function(out1){
    return temp2(out1, "world").then(function(out2){
        console.log(out2);
    });
});

Besides how bind works, I have the following question:

temp1().then(temp2.bind(null,/*the second param is a property of the output from temp1*/)).then(function(out){
   console.log(out); 
});

I know I can always break the chain or add additional chain to achieve it: for example

temp1().then(function(out1){
    return temp2(out1, (out1.length).to_String()).then(function(out2){
        console.log(out2);
    });
});

But it would be awesome not to break the chain.

So my two questions: 1. how bind works, 2. how to keep the chain(also no additional chain) while I can still access the previous output's properties in the params of next function inside then. Thanks!

Upvotes: 0

Views: 1663

Answers (2)

Matt Altepeter
Matt Altepeter

Reputation: 956

bind() is used to set the appropriate context for a function to be called later. I don't think you need to use it in this scenario. If you are using es6 you can do something like so:

temp1()
    .then(out1 => temp2(out1, out1.length.toString()))
    .then(console.log)
    .catch(console.log);

This uses an implicit return. This works because temp2() also returns a promsie so we can chain it to the next promise resolver

Upvotes: 0

Nghia Tran
Nghia Tran

Reputation: 819

Function.prototype.bind()

The bind() method creates a new function that, when called, has its this keyword set to the provided value, with a given sequence of arguments preceding any provided when the new function is called.

So technically, temp2.bind(null,"world") will return a function and pass null,"world" as parameter. Then, those parameters are merged with the result of temp1 which is "hello" at index 0. Therefore, the actual parameters are passed to temp2 are "hello", "world".

To keep the chain you can write:

temp1()
  .then(function (out) {
    return temp2(out,"world")
  })
  .then(function(out){
    console.log(out); 
  });

This is why people created Promise so that you can chain them nicely. The example you gave works but it is similar to Callback Hell.

Upvotes: 1

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