jquery - Checking to see if a value exists in Javascript - Stack Overflow

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How do I prevent a Javascript alert from firing if the alert value is undefined? In other words, something like this:

if (alert(message) != 'undefined') {
        alert(message);
    }

How do I prevent a Javascript alert from firing if the alert value is undefined? In other words, something like this:

if (alert(message) != 'undefined') {
        alert(message);
    }

• javascript
• jquery

Share Improve this question Follow asked Dec 21, 2010 at 23:49 sehummelsehummel 5,5682525 gold badges9292 silver badges141141 bronze badges

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1 Answer 1

Sorted by: Reset to default 10

Use typeof:

if (typeof message !== 'undefined')

Don't put alert(message) into the if expression, otherwise you will execute alert (which we want to avoid before we know the type of message) and the return value (which is also undefined btw ;)) will be pared to undefined.

Update Clarification for !==:

This operator not only pares the value of two operands but also the type. That means no type coercion is done:

42 == "42" // true
42 === "42" // false

In this case it is not really necessary because we know that typeof always returns a string but it is good practice and if you use it thoroughly and consistently, it is more clear where you really want to have type coercion and where not.

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