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Title: »Truthy« and »falsy« values in JavaScript


Date: 1428522382


Authors: rasshofer


Text:

The terms »truthy« and »falsy« are an essential feature (not a bug!) of the JavaScript language, stating that values aren’t just true when talking about »truthy« neither just false when talking about »falsy«. What’s meant is that values coerce to true or false when evaluated in a boolean context. Thus, something which evaluates to true is truthy and something which evaluates to false is falsy.

Let’s have a look at a simple function to test a value for its truthiness/falsiness.

function checkTruthiness(value) {
    console.log(value ? 'Truthy' : 'Falsy');
}

This function takes the value parameter and evaluates it in a boolean context (= the condition of the ternary if-statement). As a result, all of the following values are truthy values.

checkTruthiness(true);

checkTruthiness({});

checkTruthiness([]);

checkTruthiness('Lorem ipsum');

checkTruthiness(1.23);

checkTruthiness(new Date());

While there are a lot of truthy values in JavaScript, there are actually only six falsy values. Thus, memorizing that list is the easiest way to tell whether a value is truthy or falsy.

checkTruthiness(false);

checkTruthiness(null);

checkTruthiness(undefined);

checkTruthiness(NaN);

checkTruthiness(0);

checkTruthiness('');

Looking for an example where truthiness/falsiness is actually helpful? Assuming you want to make sure a user name is set before you’re saving the user to your database, you could do a explicit check on all single possibilities manually…

if (user.name !== undefined && user.name !== null && user.name !== '') {
    users.push(user);
}

…or you make use of the falsiness of those values.

if (user.name) {
    users.push(user);
}

However, there some caveats to keep in mind.

  • If 0 as an username was meaningful, your user wouldn’t be able to register as 0 evaluates to false. Or if a user has to specify how many children he/she has, if (user.children) { … } would exclude all people who validly specified the information but don’t having any children.
  • Truthiness isn’t the same as == true. While if (expression) { … } checks for truthiness, x == y uses conversions to determine equality. It’s not exactly the same concept and the algorithm for loose equality is much more complicated than for truthiness.
  • It’s possible to explicitly wrap a primitive (string, number, null, undefined, boolean) in an object, making it truthy. For example, while 0 is falsy, new Number(0) is truthy. An even scarier example is new Boolean(false) which is also truthy.
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