Cannot call method 'unshift' of undefined

Question

var _array=[];
var someint=44;
var somevalue='a string';
var u=[someint];
_array.unshift(u);
_array[u].unshift('somevalue');//*error

why does this not work?

Uncaught TypeError: Cannot call method 'unshift' of undefined

Answer 1

You've added an element to "_array", and its index will be 0. However, you're trying to use an array as an index into an array, and that won't work. That is,

_array[u]

doesn't make sense. Arrays are indexed numerically. Thus the value of that expression is undefined , and that explains the error message — you're trying to reference the "unshift" property of the value undefined . (Well, undefined isn't really a value, but whatever.)

Answer 2

use

_array[0].unshift();

instead of

_array[u].unshift('somevalue');