Go home JS, you're drunk

What would be the result of the following code:

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var myArray = undefined; // intended to be an array, but happened to be undefined e.g. due to a missing function parameter
var isCondition = myArray && myArray.length > 0;
console.log(isCondition);

?

Boolean false? Apparently not. The isCondition would be equal to… undefined. What implications does it have?

If we used the isCondition variable in code later on in an if statement like this:

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if(isCondition) { /* ... */ }

the isCondition would eventually evaluate to false and everything’s gonna work as expected. However, if we compared isCondition to false:

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if(isCondition == false) { /* ... */ }
// or even better
if(isCondition === false) { /* ... */ }

the code inside the brackets would not be launched. The isCondition would not be casted to a boolean value and undefined is definately not equal to false so the whole expression becomes falsy. The same principle applies in case of myArray being equal to null by any chance. Suddenly the term Ninja JS Developer got a brand new meaning.

Photo by Eaters Collective on Unsplash.

Tags: #JavaScript

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