Avoid duplication in JavaScript way

I would like to prevent for adding duplicate values.

if (this.sQuestions.findIndex((item) => 
    item.question.replace(/\s/g, "").toLowerCase() ===  
        this.quest.replace(/\s/g, "").toLowerCase()) > 0) 
{
  this.isQuestionExist = true;
}
else {
  //save function
}

It's working except the sQuestions[0] element, Why?.

I suggest to use Set for this purpose. As the documentation states:

> The Set object lets you store unique values of any type, whether primitive values or object references.

Example from the documentation:

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

const set1 = new Set([1, 2, 3, 4, 5]);

console.log(set1.has(1));
// expected output: true

console.log(set1.has(5));
// expected output: true

console.log(set1.has(6));
// expected output: false

<!-- end snippet -->

I hope this helps!

You are comparing against > 0 if the item is not found in the array, the function will return -1 so it should be compared against -1

This will be working.

if (this.sQuestions.findIndex((item) => item.question.replace(/\s/g, "").toLowerCase() === this.quest.replace(/\s/g, "").toLowerCase()) !== -1) {
  this.isQuestionExist = true;
}
else {
  //save function
}

According to your question, you can try

let uniqueQuestions = new Set(this.sQuestions)

For performance and clarity reasons you should index your questions in a dedicated structure like a Map:

function getKey(question)
{
	return question.replace(/\s/g, '').toLowerCase()
}

var question1 = {
	ID: 1,
	text: 'What time is it?'
}

var question2 = {
	ID: 2,
	text: 'Where is it?'
}

var question3 = {
	ID: 3,
	text: 'What is it?'
}

var questions = new Map()
questions.set(getKey(question1.text), question1)
questions.set(getKey(question2.text), question2)
questions.set(getKey(question3.text), question3)

var quest = 'Where is it?'
var match = questions.get(getKey(quest))
console.log(match)

quest = 'WTF?'
match = questions.get(getKey(quest))
console.log(match)

Result:

{ ID: 2, text: 'Where is it?' }
undefined

You can use some method to check whether the data is existing in your array:

if (!this.sQuestions.some(q => q.question == newQuestion)) 
{
  this.isQuestionExist = true;
}
else {
  //save function
}

An example:

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

let arr = [1, 2, 3, 4, 5];
console.log(`arr has 1`, arr.some(s=> s == 1))
console.log(`arr has 10`, arr.some(s=> s == 10))
console.log(`arr has 1`, arr.some(s=> s == 6))

<!-- end snippet -->