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Norak
Asked: 2020-09-01 03:47:48 +0800 CST 2020-09-01 03:47:48 +0800 CST

Error with promise and for loop

  • 772

I'm starting with Javascript and I still don't handle the asynchronous theme well, promises... etc

The issue is that within a callback I need to access the data of an array (go through it using a for) and this throws me an error that I don't know how to fix.

My code:

var objetoQueGuardo = {'event': 'evento1', 'label': 'valor'};
var dataLayer = [];
dataLayer.push(objetoQueGuardo);

factura.createFactura(id, false, token)
                .then(function successCallback(response) {

            for (var i = 0; dataLayer.length; i++) {
                dataLayer[i].label;
            }

}, function errorCallback(response) {
                    console.warn(response);
});

I have shortened the code a bit, to make the example simpler. The content of dataLayer is an array of objects and I want to keep the label attribute of each one.

The error it shows me:

 TypeError: dataLayer[i] is undefined
 Traza de la pila:
    successCallback@http://api.local/components/api/assets/js/factura.js:150:7
    h/<@http://api.local/components/api/assets/bower_components/angular/angular.min.js:135:278
    $digest@http://api.local/components/api/assets/bower_components/angular/angular.min.js:146:238
    $apply@http://api.local/components/api/assets/bower_components/angular/angular.min.js:149:464

 Possibly unhandled rejection: {}
javascript

1 Answers

  1. Best Answer

    Your problem is the condition of the for loop:

    for (var i = 0; dataLayer.length; i++) {
    dataLayer[i].label; // En este ejemplo esta sentencia realmente no hace nada, pero sirve para detectar el error
}

    is equivalent to writing

    for (var i = 0; !!(dataLayer.length); i++) { // !!1 === true
    dataLayer[i].label;
}

    since dataLayer.lengthis 1, always calculates true. What you need is to compare the length with your index:

    for (var i = 0; i < dataLayer.length; i++) {
    dataLayer[i].label;
}

    By the way, naming callback functions is a good practice because it helps debug errors, as you've seen it appear in the "stacktrace" of the error message.

    • 5