Check if JavaScript array is empty

You want to do the check for undefined first. If you do it the other way round, it will generate an error if the array is undefined.

if (array === undefined || array.length == 0) {
  // array empty or does not exist
}

    

The foolproof approach Jump to heading

This takes into account that the variable might not refer to an array, but to some other type of object with a length property.

if (!Array.isArray(array) || !array.length) {
  // array does not exist, is not an array, or is empty
  // ⇒ do not attempt to process array
}

    

The pragmatic approach Jump to heading

In a lot of cases, the above might seem like overkill. Maybe you’re using a higher order language like TypeScript that does most of the type-checking for you at compile-time, or you really don’t care whether the object is actually an array, or just array-like.

In those cases, I tend to go for the following, more idiomatic JavaScript:

if (!array || !array.length) {
  // array or array.length are falsy
  // ⇒ do not attempt to process array
}

    

Or, more frequently, its inverse:

if (array && array.length) {
  // array and array.length are truthy
  // ⇒ probably OK to process array
}

    

With the introduction of the optional chaining operator (Elvis operator) in ECMAScript 2020, this can be shortened even further:

if (!array?.length) {
  // array or array.length are falsy
  // ⇒ do not attempt to process array
}

    

Or the opposite:

if (array?.length) {
  // array and array.length are truthy
  // ⇒ probably OK to process array
}

    

From this brilliant Stack Overflow answer.