[SalesForce] cannot get aura:id

Sorry can't comment yet.

But i have the same problem when using cmp.find() to get a dynamically created component the return value is always undefined.

I even tried it with the code provided in the documentation(https://developer.salesforce.com/docs/atlas.en-us.lightning.meta/lightning/js_cb_dynamic_cmp_async.htm).

<!--c:createComponent-->
<aura:component>
    <aura:handler name="init" value="{!this}" action="{!c.doInit}"/>

    <p aura:id="test">Dynamically created button</p>
    {!v.body}
</aura:component>


/*createComponentController.js*/
({
    doInit : function(cmp) {
        $A.createComponent(
            "ui:button",
            {
                "aura:id": "findableAuraId",
                "label": "Press Me",
                "press": cmp.getReference("c.handlePress")
            },
            function(newButton, status, errorMessage){
                //Add the new button to the body array
                if (status === "SUCCESS") {
                    var body = cmp.get("v.body");
                    body.push(newButton);
                    cmp.set("v.body", body);
                }
                else if (status === "INCOMPLETE") {
                    console.log("No response from server or client is offline.")
                    // Show offline error
                }
                else if (status === "ERROR") {
                    console.log("Error: " + errorMessage);
                    // Show error message
                }
            }
        );
    },

    handlePress : function(cmp) {
        console.log("button pressed");
        console.log(cmp.find("findableAuraId"));
    }
})

The first console log works, the second one returns 'undefined'

A possible workaround : cmp.find({ instancesOf : "auradocs:sampleComponent" }) which returns an array of all "auradocs:sampleComponent"

handlePress : function(cmp) {
    var button = cmp.find({ instancesOf : "ui:button" })[0];
    button.set("v.label", "test");
}

The id will let you get a reference to a specific DOM element. aura:id will let you get a reference to a Component via component.find("yourAuraId"). Once you have a reference to the Aura component, you can call .getElement() or .getElements() to get the DOM that that component will render. It's probably better to go through the framework APIs and use aura:id, though both will do the job to let you manipulate the specific DOM elements.

Note that you can also use the $A.util.addClass() API to add a CSS class on a Component. There's also has, remove, and toggleClass APIs.