How does one replace all elements of an array without losing references?

var arr = [1, 2, 3];
var b = arr;
b == arr; // true
magic(arr, [4, 5, 6]);
b == arr; // should return true

One way of doing it is by popping and pushing. Is there a clean way?

How does one replace all elements of an array without losing references?

var arr = [1, 2, 3];
var b = arr;
b == arr; // true
magic(arr, [4, 5, 6]);
b == arr; // should return true

One way of doing it is by popping and pushing. Is there a clean way?

• javascript

5 Answers 5

You could splice the old values and append the new values.

function magic(reference, array) {
    [].splice.apply(reference, [0, reference.length].concat(array));
}

var arr = [1, 2, 3],
    b = arr;

console.log(b === arr); // true
magic(arr, [4, 5, 6]);
console.log(b === arr); // should return true

console.log(arr);

Another way, is to use Object.assign. This requires to set the length of the array, if it is smaller than the original array.

function magic(reference, array) {
    Object.assign(reference, array, { length: array.length });
}

var arr = [1, 2, 3],
    b = arr;

console.log(b === arr); // true
magic(arr, [4, 5, 6, 7]);
console.log(b === arr); // should return true

console.log(arr);

The magic part could be:

arr.splice(0, arr.length, 4, 5, 6);

var arr = [1, 2, 3];
var b = arr;
b == arr; // true
arr.splice(0, arr.length, 4, 5, 6);
console.log(b);
console.log(arr);
console.log(arr === b);

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If you already have the replacing array in a variable (let's say repl = [4, 5, 6]), then use the rest parameters syntax:

arr.splice(0, arr.length, ...repl);

var arr = [1, 2, 3];
var b = arr;
var repl = [4, 5, 6];
b == arr; // true

arr.splice(0, arr.length, ...repl);
console.log(b);
console.log(arr);
console.log(arr === b);

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Here's one way:

var arr = [1, 2, 3];
var b = arr;
console.log(`b == arr, b
`, b == arr, b.join());
var c = magic(arr, [4, 5, 6]);
console.log(`b == arr, b
`, b == arr, b.join());
console.log(`c == arr, c
`, c == arr, c.join());

function magic(to, from) {
  // remove elements from existing array
  var old = to.splice(0);
  
  for (var i = 0; i < from.length; i++) {
    to[i] = from[i];
  }
  
  return old;
}

This implementation returns a copy of the old elements that were originally in the array.

Copy the new values over the old ones.

function magic(arr, newvals) {
  for (let i = 0; i < newvals.length; i++) arr[i] = newvals[i];
  arr.length = newvals.length;
}

function replaceArrValues(arrRef, newValues)
{
  arrRef.length = 0; // clear the array without losing reference
  newValues.forEach(x => arrRef.push(x));
}