Get value other than null javascript

Question

I have a function in Javascript, and anyway I put it enters the function, I want it not enter if the value has null, because it is giving error. It only needs to enter the function if attachment is different from null. Go on like I’m doing:

   $(function () {
            if (document.getElementById("Anexo").value != "" || document.getElementById("Anexo").value != undefined || document.getElementById("Anexo").value != null) {
                $('#<%=Anexo.ClientID %>').change(function () {
                    var f = this.files[0]

                    if (f.size > 8388608 || f.fileSize > 8388608) {
                        alert("Tamanho excede o limite permitido.")

                        this.value = null;
                    }
                })
            }
        });

Anyway I put, it always returns me the error in this line:

$('#<%=Annex.Clientid %>'). change(Function () {

If the Attachment ta filled does not return error, but it may happen that it is empty. And it cannot return error, so I am trying to use if.

Editing: After many attempts, I arrived at the following error:

Undefined object reference for an object instance. Which occurs on this line: $('#<%=Attachment.Clientid %>'). change(Function () {

I use the language Asp.net C#. Attachment is a Fileupload component, this error occurs only when the component is empty, so you would need to check if it is null to perform the function.

Editing: I tried putting in a variable, like this:

var label = document.getElementById("<%=Anexo.ClientID%>");

When I declare in the variable, then it gives me the error in this line, and the same error of:

Undefined object reference for an object instance.

Answer 1

Considering this condition:

if (obj.value != "" || obj.value != null || obj.value != undefined) {
  console.log("Condição foi satisfeita");
}

The condition will be true when:

1. obj.value is different from ""; or
2. obj.value is different from null; or
3. obj.value is different from undefined;

So if the value is a string empty, condition 2 is met and the code is executed:

const obj = {"value": ""};

if (obj.value != "" || obj.value != null || obj.value != undefined) {
  console.log("Condição foi satisfeita");
}

If the value is null, condition 1 is met and the code is executed:

const obj = {"value": null};

if (obj.value != "" || obj.value != null || obj.value != undefined) {
  console.log("Condição foi satisfeita");
}

Already, if the value is undefined, the condition 1 is also met and the code is executed:

const obj = {"value": undefined};

if (obj.value != "" || obj.value != null || obj.value != undefined) {
  console.log("Condição foi satisfeita");
}

And, finally, if the value does not exist an error will be triggered.

const obj = undefined;

if (obj.value != "" || obj.value != null || obj.value != undefined) {
  console.log("Condição foi satisfeita");
}

I believe that none of the above options should make the code run, so do:

if (obj && obj.value) {
  console.log("Condição foi satisfeita");
}

For thus, if the object does not exist, the first operand of the expression will return false. Already, if the attribute value of the object not existing or being null, the second operand will return false. See below all previous tests on this condition:

const tests = [
  {"value": ""},
  {"value": null},
  {"value": undefined},
  undefined
];

for (obj of tests) {
  if (obj && obj.value) {
    console.log("Condição foi satisfeita");
  }
}

Note that the code does not run in all tests. However, if you set a value for the attribute:

const obj = {"value": "foo"};

if (obj && obj.value) {
  console.log("Condição foi satisfeita");
}

The code is executed.

Answer 2

Lacked some ; in your code. You also have a problem with your verification, if the value were null the verification document.getElementById("Anexo").value != "" would return true, and how you were using the ||(or), if any of the checks returned true the answer would be true. Then replaces the ||(or) by &&(e). That is, the answer will be true only if all checks return true:

$(function () {
    if (document.getElementById("Anexo").value != "" && 
        document.getElementById("Anexo").value != undefined && 
        document.getElementById("Anexo").value != null) {
            $('#<%=Anexo.ClientID %>').change(function () {
                var f = this.files[0];
                if (f.size > 8388608 || f.fileSize > 8388608) {
                    alert("Tamanho excede o limite permitido.");
                    this.value = null;
                }
            });
    }
});