Are Javascript strings immutable? Modifying string by str[index] won't affect string but won't generate exception either, why? [duplicate]

Question

The following code doesn't affect string str but won't generate any error or exception either, why?

start();

function start(){
  var str = 'abcdef';
  for(var i=0;i<5;i++){
    str[i] = str[i+1];
    console.log(str[i]);
  }
  console.log(str);
}

The output looks like following:

a
b
c
d
e
abcdef

CertainPerformance · Accepted Answer · 2020-10-12 02:33:34Z

2

Yes, they are immutable.

If you enable strict mode, the silent failures will turn into explicit errors:

'use strict';

start();

function start(){
  var str = 'abcdef';
  for(var i=0;i<5;i++){
    str[i] = str[i+1];
    console.log(str[i]);
  }
  console.log(str);
}

(Enabling strict mode is generally a good idea, it can make debugging easier)

This isn't a string-specific issue - trying to assign to any read-only property will throw in strict mode, and will fail silently in sloppy mode:

'use strict';
var str = 'a';
console.log(Object.getOwnPropertyDescriptor(str, '0'));

const obj = {};
Object.defineProperty(obj, 'prop', { value: 'value' });
console.log(Object.getOwnPropertyDescriptor(obj, 'prop'));
obj.prop = 5;

edited Oct 12, 2020 at 2:33

answered Oct 12, 2020 at 2:27

CertainPerformance

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