Javascript : Unexpected empty element in array with different key types?

Question

Can someone explain what's going on in this JavaScript code:

    let arr = [];

    arr["foo"] = "11";
    arr["sd"] = "12";
    arr[1] = "13";

    console.log(arr.length); //2
    console.log(arr) // [empty, "13", foo: "11", sd: "12"]

Why is there an empty element in the array?

Also, why the Array length equals 2 and not 4?

Pointy · Accepted Answer · 2020-01-31 18:09:07Z

6

Property names that are not numeric do not count towards the length of an array in JavaScript. By setting arr[1] you force the length to be 2, because the length is always one greater than the maximum numeric property name. Thus your array implicitly has an empty element 0, an element 1 with a value, and then two other properties with string keys.

answered Jan 31, 2020 at 18:09

Pointy

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1 Comment

Pointy Over a year ago

Note that "numeric" property names must be positive integers that fit in 31 bits.

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