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
– Doug, Commented Mar 20, 2018 at 12:59
To test if a variable is undefined: if(typeof form === "undefined") — Damien
– Damien, Commented Mar 20, 2018 at 13:02
Possible duplicate of Detecting an undefined object property — madhead
– madhead, Commented Mar 20, 2018 at 14:21

Jeff Clayton · Accepted Answer · 2018-03-20 13:06:35Z

2

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) {

answered Mar 20, 2018 at 13:06

Jeff Clayton

7,3711 gold badge33 silver badges38 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

phuzi · Accepted Answer · 2018-03-20 13:56:40Z

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, 2018 at 13:56

answered Mar 20, 2018 at 13:11

phuzi

13.3k4 gold badges31 silver badges66 bronze badges

4 Comments

cxFaust Over a year ago

In this situation, I can not use this method without variable 'form'.

phuzi Over a year ago

@cxFaust not quite sure what you mean.

cxFaust Over a year ago

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.

phuzi Over a year ago

The test if (form != null){ will parse the serialised form data if it's not null or undefined. Is that not what you want?

Collectives™ on Stack Overflow

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

2 Answers 2

Comments

4 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

4 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related