The following initialization of b seems to copy array data from a instead of referencing it, as intended:
let a = [0,1];
let b = [a[0], 2];
a[0]=3;
console.log(b);The output is 0,2.
3,2?b[0] be initialized with a reference to a[0] so that it reflects changes to a?You need to assign by reference. It means that variable b stores reference to variable a and variable a stores reference to array [0, 1]. It means any time you are editing any of variable a or b, then the array [0, 1] will be changed :
let a = [0,1];
let b = a;
a[0] = 3;
b[1] = 2;
An example:
let a = [0,1];
let b = a;
a[0]=3;
b[1] = 2;
document.querySelector("div").innerText = b;<div></div>UPDATE:
You can get indexOf array to be edited and then just edit number by index:
let a = [0,1];
let b = [a[0], 2];
let index = b.indexOf(a[0]);
let valueToSet = 3;
a[0] = valueToSet;
b[index] = valueToSet;
console.log(`a is`, a);
console.log(`b is`, b);
a[0]? According to your link, that might not be possible ("non-primitive-value"). Then, what are the alternatives? The duplicate reference b === a does not solve my problem; a needs to be a "child" of b...Primitive values, such as numbers, booleans, and strings are copied by value, because they're immutable; conversely, objects, arrays, and functions are copied by reference.
See also: Primitive
This can be demonstrated with a simple example:
let a = [{foo: 0},{bar: 1}];
let b = [a[0], 2];
a[0].foo = 3;
console.log(b);If you want to copy by reference, you need to copy the whole array, instead of individual values.
It is copying the value only, not the reference.
Technically once you add into array b the value of a[0], you are not copying its reference, just the value. So later if you modify elements in array a then it won't affect the values of array b.
It works exactly like using a variable - using elem below:
let a = [0,1];
let elem = a[0]; // value type, not copying the reference
let b = [elem, 2];
a[0]=3;
document.querySelector("div").innerText = b;<div></div>I hope this explains!
As suspected,
let b = [a[0], 2];
initializes b with a copy of the Number in a instead of referencing it, so later changes to the item are not (automatically) reflected in b.
Unfortunately I was not able to find this behavior in the MDN JavaScript reference docs at https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures. See StepUp's answer for a link to some information.
I am still hoping there might be a way (maybe Pointers in JavaScript??).
In the meantime, simply defining the variable(s) again ("update", "redefine") works around my problem - it's "better" than copying to both arrays only in that it's the simplest way to update a number of dependent variables:
let a = [0,1];
let b = [a[0], 2];
a[0]=3;
b = [a[0], 2];
console.log(b); // [3,2]This is the minimal case, it can of course be wrapped in a function (return b;) to simplify the (re-)definition of many variables.