Javascript中的链接方法错误

Question

I would like to know the reason why this simple piece of code fails:

var arr = [1, 2, 3];
arr.push(arr[0]).shift();
console.log(arr);

it returns in firebug console "TypeError: arr.push(...).shift is not a function"

I think it happens because I invoke the shift() method not on an array but on the pushed element.

Is there a more elegant way to obtain the same result that,

var arr = [1, 2, 3];
arr.push(arr[0]);
arr.shift();
console.log(arr);

produce ?

Thanks in advance!

Answer 1

From the MDN :

The push() method adds one or more elements to the end of an array and returns the new length of the array.

arr.push(arr[0]) doesn't return the array but a number which, obviously, has no shift function.

To my knowledge, there's no simple expression pushing an element to an array and returning that array. But in your case you may simply reverse the operations and do

arr.push(arr.shift());

Answer 2

I think it happens because I invoke the shift() method not on an array but on the pushed element.

Almost. push returns the new length of the array. A number obviously doesn't have a shift() method.

Your method of putting it on two lines is the simplest way.

Answer 3

Essentially this question is saying, can I somehow "elegantly" express the notion of moving the first item of an array to the end. Luckily, JS is a Turing-complete language, which allows us to define functions, so the "elegant" answer is just

rotate(arr)

Now it merely remains to define rotate . To rotate is to drop the first element in the result of adding the head element to the end:

function rotate(arr) { return drop(add(arr, head(arr))); }

Now drop is

function drop(arr) { return arr.shift(), arr; }

and head of course is

function head(arr) { return arr[0]; }

and add is

function add(arr, elt) { return arr.push(elt), arr; }

Another approach

I could also write a function to move n elements from position i to position j , using splice , as follows:

function move(arr, n, i, j) {
    arr.splice.apply(arr, [j-n+1, 0].concat(arr.splice(i, n)));
    return arr;
}

Then to rotate is to move one element at the beginning to the end :

function rotate(arr) { return move(arr, 1, 0, 999); }