CODEWITHSUNDEEP
javascript
hangry horse
hangry horse

Reputation: 87

Why does `join()` on an array of undefined elements return 1 less element in javascript?

I was initially looking up what happens on Array.prototype.toString(). From the spec i realised this calls .join(). But I don't understand the output on an array with undefined elements.

[,,,].toString() returns ",,". But I expected ",,,". There appears to be 1 element missing in the serialization. What's happening here?

Upvotes: 1

Views: 672

Answers (2)

yqlim
yqlim

Reputation: 7080

Because in JavaScript, the last , (the trailing comma) is ignored if it is followed by nothing.

console.log([1,].length);
// -> 1

console.log([1,,].length);
// -> 2

Therefore, when you are doing [,,,], the last , is ignored. Thus the length is only 3.

console.log([,,,].length);
// -> 3

When you join an array with n items, you get n-1 commas. Hence the result that you are getting.

Upvotes: 5

saeedkazemi
saeedkazemi

Reputation: 331

The last comma in an array is optional (for convenience) and therefore ignored in join()

Upvotes: 3

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