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Javascript !instanceof If Statement

Posted by: admin November 30, 2017 Leave a comment

Questions:

This is a really basic question really just to satisfy my curiosity, but is there a way to do something like this:

if(obj !instanceof Array) {
    //The object is not an instance of Array
} else {
    //The object is an instance of Array
}

The key here being able to use the NOT ! in front of instance. Usually the way I have to set this up is like this:

if(obj instanceof Array) {
    //Do nothing here
} else {
    //The object is not an instance of Array
    //Perform actions!
}

And its a little annoying to have to create an else statement when I simply want to know if the object is a specific type.

Answers:

Enclose in parentheses and negate on the outside.

if(!(obj instanceof Array)) {
    //...
}

Questions:
Answers:
if (!(obj instanceof Array)) {
    // do something
}

Is the correct way to check for this – as others have already answered. The other two tactics which have been suggested will not work and should be understood…

In the case of the ! operator without brackets.

if (!obj instanceof Array) {
    // do something
}

In this case, the order of precedence is important (https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Operators/Operator_Precedence). The ! operator precedes the instanceof operator. So, !obj evaluated to false first (it is equivalent to ! Boolean(obj)); then you are testing whether false instanceof Array, which is obviously negative.

In the case of the ! operator before the instanceof operator.

if (obj !instanceof Array) {
    // do something
}

This is a syntax error. Operators such as != are a single operator, as opposed to a NOT applied to an EQUALS. There is no such operator as !instanceof in the same way as there is no !< operator.

Questions:
Answers:

It is easy to forget the parenthesis (brackets) so you can make a habit of doing:

if(obj instanceof Array === false) {
    //The object is not an instance of Array
}

or

if(false === obj instanceof Array) {
    //The object is not an instance of Array
}

Try it here