How I can check 'undefined' variable in AJAX queries?

Question

What is the most appropriate way to test if a variable is undefined in JavaScript?

$.ajax({
    type: method,
    url: url,
    success: function (data) {
        if (form != null || form != undefined){
            data = $(form).serialize();
        }
        var json =  JSON.parse(data),
            list = $(table).html(),
            compileTemplate = Handlebars.compile(list),
            result = compileTemplate(json),
            content = doc.getElementById(element);
        content.innerHTML = result;
    },
    dataType: "text"
});

I'm looking for a more beautiful way for this check.

`if( form )` should check if the variable exists and is defined (but I'm sure there are a bunch of edge cases where this may break) — Doug, Mar 20 '18 at 12:59
To test if a variable is undefined: `if(typeof form === "undefined")` — Damien, Mar 20 '18 at 13:02
Possible duplicate of [Detecting an undefined object property](https://stackoverflow.com/questions/27509/detecting-an-undefined-object-property) — madhead, Mar 20 '18 at 14:21

score 2 · Accepted Answer · answered Mar 20 '18 at 13:06

First check if it is undefined before checking the value so it does not throw an error. Also it needs an AND (&&) not an OR (||).

The official way to do that is to check type.

Change the 'if' statement to say this:

if (typeof(form) !== "undefined" && form != null) {

phuzi · Answer 2 · 2018-03-20T13:56:40.010

0

You could simply test form != null which will be true if form is neither null or undefined.

$.ajax({
    type: method,
    url: url,
    success: function (data) {
        if (form != null){
            data = $(form).serialize();
        }
        var json =  JSON.parse(data),
            list = $(table).html(),
            compileTemplate = Handlebars.compile(list),
            result = compileTemplate(json),
            content = doc.getElementById(element);
        content.innerHTML = result;
    },
    dataType: "text"
});

edited Mar 20 '18 at 13:56

answered Mar 20 '18 at 13:11

phuzi

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In this situation, I can not use this method without variable 'form'. – cxFaust Mar 20 '18 at 13:55
@cxFaust not quite sure what you mean. – phuzi Mar 20 '18 at 13:57
If I could use this code, I would check the variable before the method. But I use this code with a variable "form" and without. – cxFaust Mar 20 '18 at 14:28
The test `if (form != null){` will parse the serialised form data if it's not null or undefined. Is that not what you want? – phuzi Mar 20 '18 at 14:30

How I can check 'undefined' variable in AJAX queries?

2 Answers2