Trying to implement a SIMPLE promise in Reactjs

Question

Just trying out Promises for the first time in React. I have a basic promise working (ripped from someone else's code), but don't know how to adapt it to be useful.

What I have so far (within my render() function)

  var promise = new Promise( (resolve, reject) => {

     let name = 'Dave'

     if (name === 'Dave') {
        resolve("Promise resolved successfully");
     }
     else {
        reject(Error("Promise rejected"));
     }
  });

  promise.then(function(result) {
     console.log(result); // "Promise resolved successfully"
  }, err => {
     console.log(err); // Error: "Promise rejected"
  });

Sure enough, as the promise conditional is matched (name === 'Dave'), my console logs Promise resolved successfully.

However, I don't know how to assign a value to a variable when using the promise. For example:

  promise.then(function(result) {
     var newName = 'Bob'
  }, function(err) {
     var newName = 'Anonymous'
  });

And then when I try to return this value in render()'s return statement, like so:

<h2>{newName}</h2>

It says newName is undefined.

I have also tried:

  promise.then(function(result) {
     var newName = result
  }, function(err) {
     var newName = error
  });

...expecting this would assign the resolve or error string into newName variable, but nope.

Am I thinking about this the wrong way? How can I make this more useful than just logging a string when my conditional is met?

Any help would be appreciated, because this is really making my head hurt...

Update

 render() {

      var promise = new Promise( (resolve, reject) => {

         let name = 'Paul'

         if (name === 'Paul') {
            resolve("Promise resolved successfully");
         }
         else {
            reject(Error("Promise rejected"));
         }
      });

      let obj = {newName: ''};

      promise.then( result => {
         obj["newName"] = result
      }, function(error) {
         obj["newName"] = error
      });

      console.log(obj.newName)

    return (
      <div className="App">
         <h1>Hello World</h1>
         <h2>{obj.newName}</h2>
      </div>
    );
  }

For starters, you're declaring a new variable in each of those functions. You would need to declare `newName` outside of your callbacks in order for them to refer to the same variable. — Mike Cluck, Oct 13 '16 at 20:01
try `(result) => { ... ` might get you the right closure context — Meirion Hughes, Oct 13 '16 at 20:03
@MeirionHughes What difference would that make? They aren't even using `this`. — Mike Cluck, Oct 13 '16 at 20:05
@Paulos3000 You're still declaring `newName` inside of your callbacks. `var newName` declares a new variable. That's what `var` does. — Mike Cluck, Oct 13 '16 at 20:08
possible duplicate of [Rendering React components with promises inside the render method](http://stackoverflow.com/q/33242378/1048572)? — Bergi, Oct 13 '16 at 20:21

score 20 · Accepted Answer · answered Oct 13 '16 at 20:59

You are using React in a wrong way. A Promise is designed to return result at a later point of time. By the time your promise has been resolved or rejected, your render would have finished execution and it wont update when the promise completes.

render method should only depend on props and/or state to render the desired output. Any change to prop or state would re-render your component.

First, identify where your Promise should go in the life cycle of the component(here)

In your case i would do the following

Initialize an state inside your constructor(ES6) or via getInitialState

constructor(props) {
  super(props);
  this.state = {
    name: '',
  };
}

Then on componentWillMount or componentDidMount which ever suits you, call the promise there

componentWillMount() {
 var promise = new Promise( (resolve, reject) => {

  let name = 'Paul'

  if (name === 'Paul') {
   resolve("Promise resolved successfully");
  }
  else {
   reject(Error("Promise rejected"));
  }
 });

 let obj = {newName: ''};

 promise.then( result => {
  this.setState({name: result});
 }, function(error) {
  this.setState({name: error});
 });
}

Then in render method write something similar to this.

render() {
 return (
  <div className="App">
   <h1>Hello World</h1>
   <h2>{this.state.name}</h2>
  </div>
 );
}

That worked! I actually tried that before but it didn't return the correct state value, not sure why. Maybe a typo/syntax error... Anyway, thanks! — Paulos3000, Oct 14 '16 at 07:10

score 3 · Answer 2 · answered Oct 13 '16 at 20:07

You need to be thinking about the scope you're in. When you are in the function you're passing to promise.then, you are in a new block scope and therefore any var you create in the function won't exist outside of it's scope. I'm not sure how you're defining your React components, but assuming you have a newName variable on your component, there are two ways you could solve the scope problem - bind and arrow functions:

promise.then(function(result) {
  this.newName = result; //or what you want to assign it to
}.bind(this))

and then you could reference it using {this.newName}

Or with an arrow function:

promise.then((result) => {
  this.newName = result; //or what you want to assign it to
}.bind(this))

I would recommend watching this egghead video to help you understand the this keyword in javascript.

Trying to implement a SIMPLE promise in Reactjs

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