Adding properties to variables in javascript [duplicate]

Question

Consider the following codes:

var str = "abc";
 str.foo = 123;  // write - ignored
123
 str.foo  // read
undefined

Why do I get undefined? Shouldn't that property output 123? What is the reason for it outputting undefined?

Since var str is a string, why don't properties show up a second time?

Cant we add properties and methods to a string?

Community · Accepted Answer · 2017-05-23 10:32:11Z

0

You are defining str as a string, not an object. Maybe you meant to do something like this:

var o = {};
o.str = "abc";
o.foo = 123;

See the selected answer to this question for lots of great detail:

Why can't I add properties to a string object in javascript?

edited May 23, 2017 at 10:32

CommunityBot

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answered Apr 2, 2013 at 15:41

lbstr

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3 Comments

dyurkavets Over a year ago

str = new String("abc");

Ryan Over a year ago

This question is on the way to being closed as a duplicate. see here meta.stackexchange.com/questions/82420/… for why you shouldn't answer these types of questions.

lbstr Over a year ago

@frontendmagic I just deleted a comment questioning why you wrote that, but after a quick test, its quite clear. typeof new String("abc") is object whereas typeof "abc" is string. Thanks for the comment!

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