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---
title: Updating arrays and objects
---
Because Svelte's reactivity is triggered by assignments, using array methods like `push` and `splice` won't automatically cause updates. For example, clicking the button doesn't do anything.
One way to fix that is to add an assignment that would otherwise be redundant:
```js
function addNumber() {
numbers.push(numbers.length + 1);
numbers = numbers;
}
```
But there's a more idiomatic solution:
```js
function addNumber() {
numbers = [...numbers, numbers.length + 1];
}
```
You can use similar patterns to replace `pop` , `shift` , `unshift` and `splice` .
Assignments to *properties* of arrays and objects — e.g. `obj.foo += 1` or `array[i] = x` — work the same way as assignments to the values themselves.
```js
function addNumber() {
numbers[numbers.length] = numbers.length + 1;
}
```
A simple rule of thumb: the name of the updated variable must appear on the left hand side of the assignment. For example this...
```js
const foo = obj.foo;
foo.bar = 'baz';
```
...won't update references to `obj.foo.bar` , unless you follow it up with `obj = obj` .
