What is "Falsy" in JavaScript?

Oct 28, 2019

In JavaScript, a value is falsy if JavaScript's built-in type coercion converts it to false. For example, consider the below if statement:

if (v) {
  console.log('v is not falsy');
}

The console.log() will only run if v is not one of the below values:

These 7 values are the only falsy values in JavaScript. Any value that is not falsy is truthy.

In particular, a non-null object is always truthy, even if its valueOf() function returns a falsy value.

function isFalsy(v) {
  return !v;
}

// `false`. The object form of `0` is truthy, even though 0 is falsy.
isFalsy(new Number(0));

Recommendations

Using truthy/falsy for implicit type coercions in if statements is typically messy. It is rare to find a case that the 7 falsy values are exactly the set of values that you want to look out for.

For example, suppose you're implementing a function that checks that a string is shorter than 25 characters.

function checkLength(v) {
  if (!v) {
    throw new Error('Must provide a string!');
  }
  return v.length < 25;
}

Unfortunately, checkLength('') will throw an error because empty string is falsy. Instead, you should check if v is a string:

function checkLength(v) {
  if (typeof v !== 'string') {
    throw new Error('Must provide a string!');
  }
  return v.length < 25;
}

Nullish Values

Instead of checking for truthy/falsy values, you usually want to check for "nullish" values. One of the common use cases for falsy checks is making sure that you don't get a TypeError: Cannot read property 'prop' of null error when accessing a property of a value v.

It is it is safe to access v.prop unless v is strictly equal to null or undefined. Even NaN.prop is fine.

const x = Number('abc');
x; // NaN
x.prop; // undefined

Checking if v == null is equivalent to v === null || v === undefined. In other words, a value is loosely equal to null only if it is strictly equal to null or undefined. So checking if v == null is often more accurate than checking for truthy or falsy values.


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