How to update an array after splice in Svelte?

As mentioned, Svelte's reactivity is triggered by assignments. The current Svelte tutorial uses JavaScript's (ES6) spread syntax (three dots) to add the next-higher number to an array, providing a more idiomatic solution than a redundant assignment using push:

function pushNumber() {     
    numbers = [...numbers, lastnumber]; // 1, 2, 3, 4, 5
}

You could use spread syntax to replace pop, shift, unshift and splicethough it might increase the time and complexity of the operation in some cases:

function unshiftNumber() {  
    numbers = [firstnumber, ...numbers]; // 0, 1, 2, 3, 4
}

function popNumber() {
    numbers = [...numbers.slice(0,numbers.length - 1)]; // 1, 2, 3
}

function shiftNumber() {
    numbers = [...numbers.slice(1,numbers.length)]; // 2, 3, 4
}

function spliceNumber() {
    numbers = [firstnumber, ...numbers.slice(0,numbers.length-1)];// 0, 1, 2, 3
}   

Spread is just one way to do it, though. The purpose behind not using pop/push etc is to encourage immutability. So any removal can just be a filter, for example.

You could e.g. use the filter array method to create a new array without the element 'hello':

messages = messages.filter(m => m !== 'hello');