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NOT NOT, Not Working As Expected

rfornal profile image bob.js ・1 min read

First, NOT NOT ...

The single ! converts a value to its truthy or falsey value, which is technically a boolean. But if you need to a real boolean representation of a value for your expression you must convert it to a real boolean value using a double not, !!.

In my head, I could see the conversion. I hear myself evaluating it as "does this object exist." Knowing that was wrong, I still dug into the code to find out why things were breaking in other areas.

Here's a simple example of the faulty (logic) code.

const data = { params: { type: '' } };

if (!!data.params && !!data.params.type) {
  // do something here

This code refused to go inside the IF-BLOCK.

After digging into the console, I realized ...

// true

// false

What I quickly realized is that I got bit by a simple logic issue. An empty string equates to false, while a string with something in it equates to true.

A better set of logic would have been to use the IN operator.

const data = { params: { type: '' } };

if (('params' in data) && ('type' in data.params)) {
  // do something here

Then, the inner code for the IF-BLOCK would have worked properly.

Another method that can be used is the hasOwnProperty method ...

const data = { params: { type: '' } };

if (data.hasOwnProperty('params') && data.params.hasOwnProperty('type')) {
  // do something here

Generally, I prefer the first of the two solutions. To me, this seems more readable, but that's my preference.

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rfornal profile



REAL, FUN, GEEK who is passionate about Front-End!


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Hey! Just a couple of things:

In your example with if ('params' in data) && ('type' in data.params)) { you're missing a ( from the if statement.

The True and False are more python things, maybe you can use true and false.

Good article! Keep it up!


I'll make the adjustments now. Thanks for catching the missing paren.