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ignacio ibarra
Asked: 2020-04-23 13:08:22 +0800 CST 2020-04-23 13:08:22 +0800 CST

How to save value of a Promise

  • 772

Here I have the code:

function request(data) {
...
    var result;
    var req = fetch(url).then((resp) => resp.json()).then(function(dat) {
        result = dat[0][0][0];
    return result;
})

r = request("...")
// Aqui ^ obtengo "undefined"

What I want is to save the value dat[0][0][0]and then return it, to use it again. Can somebody help me?

javascript

2 Answers

  1. Best Answer

    What you should do is use the logic of what you want to do inside the function, since since you are using .then the value will only exist inside it. That is why you get undefined when you call the function:

        function request(data) {
    ...
        var result;
        var req = fetch(url).then((resp) => resp.json()).then(function(dat) {

        //tu codigo aquí en lugar de retornar

        })
    }

    The same thing happened to me and this is how I solved it.

    • 1

  2. A combination must be made between async and await.

    In the function you have request(), you should return the fetch promise and add the async keyword to the fetch definition.

    async function request(data) {
    // ...
    return fetch(url)
    .then((resp) => resp.json())
    .then(function(dat) {
        result = dat[0][0][0];
        return result;
    })
}

    Here's a page showing usages with examples

    EDIT: To make use of await, it must be used inside an async function, so you have two options.

    1. Use the return value in the same request method
    2. Get the return value in another async function like so:
    // para el caso 1 dentro el método request sin hacer el return
async function request(data) {
  // ...
  let r = await fetch(url)
    .then((resp) => resp.json())
    .then(function(dat) {
      result = dat[0][0][0];
      return result;
    })
}

// para el caso 2 dejas el método request con el asnyc y el return
async function request(data) {
    // ...
    return fetch(url)
    .then((resp) => resp.json())
    .then(function(dat) {
        result = dat[0][0][0];
        return result;
    })
}

// en otro método async utilizas el await
async function otraFuncion() {
    let r = await request("...");
}
    • 0