Array.splice（）是否可以通过在数组的最后一个元素之外的索引处添加元素来创建稀疏数组？

Question

可以通过在数组的最后一个元素之外的索引处添加一个元素来使用Array.splice()创建稀疏数组吗？”我需Array.splice()因为在某些情况下我只想推送到该数组上，但是在其他情况下，需要拼接到数组中，但是尝试使用拼接使数组稀疏是行不通的，尽管在我的特定情况下，我能够实现一些代码来测试是否使用拼接，或者只是在超出索引的位置分配数组元素数组的长度。

Answer 1

No. The ECMAScript specification does not allow a relative start position greater than the array length. From ES2015 on Array.prototype.splice , step 7:

7. ...let actualStart be min( relativeStart , len ).

The variable actualStart is what's actually used for the splice algorithm. It's produced by the minimum of relativeStart (the first argument to the function) and len (the length of the array). If len is less than relativeStart , then the splice operation will use len instead the actual argument provided.

In practical terms, this means that you can only append values onto the end of arrays. You cannot use splice to position a new element past the length index.

Answer 2

It should be noted the length of the array is not necessarily the index of the last item in the array plus 1. It can be greater.

Then, you can't use splice to insert elements beyond the length of the array, but if you make sure the length is large enough, you can insert beyond the last index plus 1.

var arrSplice = ['what', 'ever'];
arrSplice.length = 10; // Increase the capacity
arrSplice.splice(10, 0, 'foobar'); // Now you can insert items sparsely
console.log(arrSplice.length); // 10
console.log(arrSplice[arrSplice.length - 1]); // foobar

Answer 3

Array.splice() cannot be used to create sparse arrays. Instead, if the index argument passed to Array.splice() is beyond the length of the array, it seems that the element just gets appended to the array as if Array.push() had been used.

/* How you might normally create a sparse array */
var arrNoSplice = ['foo', 'bar'];
arrNoSplice[10] = 'baz';
console.log(arrNoSplice.length); // 11

/* Demonstrates that you cannot use splice to create a sparse array */
var arrSplice = ['what', 'ever'];
arrSplice.splice(10, 0, 'foobar');
console.log(arrSplice.length); // 3
console.log(arrSplice[arrSplice.length - 1]); // foobar