Is there any downside to checking if the first element of an array exists as a condition, rather tha-CodePudding

When ever I have to check if an array has elements in it I always default to checking its length for example...

const array = [];

if (array.length > 0) do something...

However today I thought couldn't I just check if the zero index of the array is not undefined rather than checking the length, I find this saves some typing?

// if the zero index element exists

if (array[0]) do something..

Is there any edge cases or scenarios in which doing this would be bad practice or result in different results than just checking the length of the array?

CodePudding user response：

Even if in most cases the two methods will produce the same result, I strongly recommend using the first one. The only way of telling if an array is empty or not, is by checking its length.

if (array.length) {
  // array is not empty
}

Here are an example where the second method would not work:

const array = [null, "I think", "therefore I am"]

console.log(array.length) // 3
console.log(array[0]) // null, and will evaluate to false

Another case that might occur more often:

const array = [1, 2, 3, 4, 5]

delete array[0]

console.log(array.length) // 5
console.log(array[0]) // undefined, and will evaluate to false

Performance

Interestingly, if the array is empty, it's faster to check if the first element exists:

let array = []

console.time('lengthCheck')
console.log(array.length)
console.timeEnd("lengthCheck")
// about 0.06s in my browser

console.time('itemCheck')
console.log(array[0])
console.timeEnd("itemCheck")
// about 0.025s in my browser

CodePudding user response：

For an empty array, array[0] will throw an index_out_of_bounds exception, whereas array.length will return 0.