将一个空元素推入javascript数组

Question

How can I push empty element to an existing Js array, lets assume :

 var arr = [54,77,21]; var target = [54,77,21,,,,36]; arr.push(); //do not append an empty element into the array. arr.push(); console.log(JSON.stringify(arr)); //output: [54,77,21]

How to append empty elements so "arr" will be equivalent to "target" array?

Answer 1

You could address the index directly. This builds a sparse array.

 var arr = [54,77,21]; arr[6] = 36; console.log(JSON.stringify(arr));

Or push undefined until you like to push the value. This returns a filled array.

 var arr = [54,77,21]; arr.push(undefined); arr.push(undefined); arr.push(undefined); arr.push(36); console.log(JSON.stringify(arr));

By using JSON.stringify , you get for undefined or sparse items null , because JSON knows only null instead of undefined .

Answer 2

You can use Array#length :

arr.length++;

You can set the length property to truncate an array at any time. When you extend an array by changing its length property, the number of actual elements increases ; for example, if you set length to 3 when it is currently 2, the array now contains 3 elements, which causes the third element to be a non-iterable empty slot.

But note that JSON does not support sparse arrays. Ie you cannot see empty slots with JSON.stringify .

 var arr = [54,77,21]; arr.length++; arr.length++; arr.length++; arr.push(36); console.log(arr);

(FYI: Stack Snippets do not seem to support sparse arrays correctly. You need to run that code in the browser console instead.)

Answer 3

You could use the array.prototype.concat() method.

 var arr1 = [1, 2, 3, 4]; var arrtarget = [1, 2, 3, 4, , , , 5, 6]; console.log(arr1); console.log(arrtarget); newArr = arr1.concat([, , , 5,6]); console.log(newArr);

Alternatively, you could use the Array.Prototype.push() method as

arr1.push(undefined);