javascript中字符串的数组取消移位

Question

var hello = 'hello';
Array.prototype.unshift.call(hello, '11') // gives error
Array.prototype.join.call(hello, ', ') // works, why??

can someone explain to me why .join works and why .unshift doesn't

Answer 1

Because strings are immutable, and unshift tries to assign to an index (property) of the string, as in

"hello"[4] = '1'

Reference: http://www.ecma-international.org/ecma-262/6.0/#sec-string-exotic-objects :

A String object is an exotic object that encapsulates a String value and exposes virtual integer indexed data properties corresponding to the individual code unit elements of the String value. Exotic String objects always have a data property named "length" whose value is the number of code unit elements in the encapsulated String value. Both the code unit data properties and the "length" property are non-writable and non-configurable.

join doesn't assign anything and only reads properties, therefore it works with any object that has .length .

Answer 2

Try this:

 String.prototype.unshift = function(el) { let arr = [this]; arr.unshift(el); return arr.join(""); } var s = "BCD"; s = s.unshift("A"); console.log(s); // ABCD