JS arrays - advanced assignment

Question

This is related to the arrays in Javascript, which I am trying to use in a complex logic.

Consider the following code:

a['a1'] = 'AJA'

We know that, this is same as a.a1 = 'AJA' (provided proper definitions were given).

So, if we go ahead and interpret this:

console.log(a.a1[0])
console.log(a.a1[1])
console.log(a.a1[2])
console.log(a.a1)

It logs :

A
J
A
AJA

Now, all I need is to assign a new character at the 4th position.

When I try a[a1][3] = 'Y' or a.a1[3] = 'Y' and then try console.log(a.a1), It still displays AJA instead of AJAY.

I know that we can do this using string concatenation, i.e. a['a1'] = a['a1'] + 'Y' and get this accomplished.

But why wasn't the first method working? By what other ways can do this?

please don't use arrays with custom properties, btw, it is not an array problem, but a string problem. — Nina Scholz, Jan 11 '17 at 08:04
If you want to achieve this behaviour you need to declare the string as an array of chars. `var str = "AJA"; --> var strArr = ["A","J","A"];`, when you finish editing the char-array you need to transform it back to the string. — DomeTune, Jan 11 '17 at 08:05
`var str = "AJA"; var strArr = str.split(''); strArr[3] = "Y"; str = strArr.join('');` — , Jan 11 '17 at 08:23
@LearnHowToBeTransparent This could have worked for a simple array. But not here.. — , Jan 11 '17 at 08:27

score 3 · Answer 1 · edited Jan 11 '17 at 08:12

3

Strings are immutable. It means that if you create a string, you can't modify it anymore. So your a1 doesn't know anything about 4th character.

You can see this example. I try to change the second char of the already created string, but it will not be changed anymore.

let a  = {};
a['a1'] = 'AJA';

a.a1[1] = 'A';

console.log(a.a1);

For more you can see MDN Documentation

edited Jan 11 '17 at 08:12

Sebastian Simon

answered Jan 11 '17 at 08:03

Suren Srapyan

Yes they are immutable. But does it mean that string concatenation is the only way to get this done? – Jan 11 '17 at 08:18
@A.J. there are many ways to do it. But the string concatenation is the simple one – Suren Srapyan Jan 11 '17 at 08:22
"By what other ways can do this?" – Jan 11 '17 at 08:23
You can create a function to get parameters char and index, but under the hood you need to use string concatenation :) – Suren Srapyan Jan 11 '17 at 08:24
You're basically confirming what the OP already figured out, without providing an answer. – Cerbrus Jan 11 '17 at 08:25

score -2 · Answer 2 · answered Jan 11 '17 at 08:14

-2

As I know a[a1][3] or a.a1[3] is a string variable, you can treat it as:

var s = 'ss';

When you evaluate s[0] you'll get a string value. So when you assign any string value to s, you'll not get 'ss' + anyvalue but anyvalue instead. :)

answered Jan 11 '17 at 08:14

yupengzhang

There’s no assignment to the string itself, only to a property (not `s = …` but `s[2] = …`). How does this relate to the question? – Sebastian Simon Jan 11 '17 at 08:33
Just an example, when you try to evaluate a[a1][3] or a.a1[3] you'll get the same type as the example I referred above. try typeof(a[a1][3]) – yupengzhang Jan 11 '17 at 08:41

2 Answers2