String assignment behavior in JavaScript

Question

Does the following result in a memory copy operation?

1: var foo, bar;
2: foo = 'abc';
3: bar = foo;

Is the memory representation of foo copied into the memory pointed at by bar on line 3?

Does this change if foo is a string 1 MB in size (unlike 6 bytes in this example)?

Finally, is this behavior defined by the ECMAScript specification or left to implementors?

that is just a reference of a var, so both points to same primitive. So, IMHO there would be one primitive while two vars are having reference to it. I wish i could say it better. — Jai
– Jai, Commented Apr 10, 2017 at 10:38
But strings have pass-by-value semantics which would seem to contradict you. — Ben Aston
– Ben Aston, Commented Apr 10, 2017 at 10:39
@BenAston Where are you observing pass-by-value semantics? JavaScript has "pass-reference-by-value" semantics for non-numeric values - also known as "call by sharing": en.wikipedia.org/wiki/Evaluation_strategy#Call_by_sharing — Dai
– Dai, Commented Apr 10, 2017 at 10:40
OK, so JavaScript has "pass-reference-by-value" semantics for strings - it just doesn't expose a mechanism for changing that value because all "modifier" operations result in a new string? — Ben Aston
– Ben Aston, Commented Apr 10, 2017 at 10:51

Community · Accepted Answer · 2017-05-23 12:17:45Z

2

Strings in JavaScript are immutable and can be considered as "reference types" (a-la Java and C#): Are JavaScript strings immutable? Do I need a "string builder" in JavaScript?

I had a quick look in the ECMAScript 7 specification but I can't find a single, succinct authoritative reference that simply says "strings are immutable" - you'll have to infer that from the rest of the specification: https://www.ecma-international.org/ecma-262/7.0/index.html

Regarding the individual questions:

No, the contents of the string are not copied on assignment, a reference to the string is.
No, if foo pointed to a 1MB-sized string then as before, a reference would be passed.
See second paragraph.

edited May 23, 2017 at 12:17

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answered Apr 10, 2017 at 10:39

Dai

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