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What is this handleChange method doing? [closed]

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handleChange(e) {
const index = Number(e.target.id.substring(e.target.id.length - 1, 19));
const copyFormArray = JSON.parse(JSON.stringify(this.state.educationData));
copyFormArray[index][e.target.name] = e.target.value;
this.setState({
educationData: copyFormArray,
});
};
What does this handleChange method do, specifically in the index variable? I understand what substring and the Number constructor do but the second argument I cannot find any answers for this specific question although it works as intended.

Substring works in a tricky way, if second argument (in your case 19) is less than first argument - then they will be "swapped", and if not - it will just take text between indexes (argument1 and argument2).
So say for example i have a string:
var x = "aaaaa-5";
if i will write:
x.substring(x.length-1)
It will give me "5".
Then if i will write:
x.substring(3,6)
It will give me "aa-"
But then if i will write:
x.substring(6,3)
It will be exactly the same as previous call - "aa-"
So internally logic of substring is kind of like this:
function substring(start,end) {
if(start > end) {
var buf = end;
end = start;
start = buf;
}
//... other logic
}

How can i find the index of the element of an Array? [closed]

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Here is my code, it gives me undefined i have also tried indexof() method
let Numbers = [2,3,1,5,6,7,8 ];`
console.log("Unsorted array " + Numbers);
for(var i=0 ;i<Numbers.length;i++){`
alert(Numbers.findIndex[i]);
}

The below code would work after some changes in your code -
let Numbers = [2, 3, 1, 5, 6, 7, 8];
console.log("Unsorted array " + Numbers);
for (var i = 0; i < Numbers.length; i++) {
console.log(Numbers.indexOf(Numbers[i]));
}
In your case, you were using findIndex() which takes a function and executes it for each element of the array. You were passing it a number which is not correct. Also, the invocation of function you were doing was not correct - use () brackets and not [] brackets for function call.
Also, the i itself is the index. I don't know why you would need to use indexOf to get the index of element which you already know is present at a particular index. This method wouldn't be practical unless your array has duplicates and you need to find the first occurring index number for each element of array.
As a side tip, avoid using alert for such purposes. Stick with console log.

Handling undefined in a for ... of loop? [closed]

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Suppose my code is as follows:
const days = ['Monday', 'Tuesday'];
for (const day of days) {
console.log(day);
}
It prints out 'Monday' and 'Tuesday'.
If for whatever reason, days is undefined, then I get a runtime error.
I could handle this by putting in code like the following:
if (undefined == days) {
// logic here
}
To me, that seems a little clunky. Is there a way to have the for ... of loop ignore undefined values? Basically, treat it like a zero-length array and so the logic inside the for loop won't execute, but it won't throw an error either.

You could always use a forEach, and a logical or || within a parenthesis block.
(days||[]).forEach(day=>{console.log(day)});
I suppose you could also use this in your for loop.
for(let day of (days||[])) {
console.log(day);
}

The best way to check for a empth value is to initialize a variable before using it, Either is a string, object or array. This way the programm understand the variable and can easily suggest function for you.
//Initializing Varable
let stringVariable = '' ;
let objectVariable = {};
let arrayVariable = [] ;
In your case
let days = []
for(let day of (days)) {
console.log(day);
}

using map to check for true or false doesn't work [closed]

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I am trying no to use a loop as Im practicing map, filter etc. and wonder why my code doesn't work?
function checking(array,item){
var temporary=true;
array.map(function(x){
if (x!=item){
temporary=false;
}
});
return temporary;
}
checking([1, 2, 3], 2);

Why not use Array#some, it is perfect for checking and returnig a boolean value. And the best is the short circuit if the callback return is true.
function checking(array, item) {
return array.some(function (x) {
return x === item;
});
}
document.write(checking([1, 2, 3], 2));

Because you are not checking if 2 is in array, but you are checking if array has a different value than 2.
You should wether check if it there eg :
let temporary=false;
array.map(function(x){
if (x==item){
temporary=true;
}

I want to put into variables multiple value [closed]

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I want to do this
if ($(this).val() in (1,2,3) ) {
...
}
I have tried this and it works but It's not what I want
if ($(this).val()==1 || $(this).val()==2 || $(this).val()==3 ) {
...
}

I notice OP is using jQuery but this should work for anyone who isn't
if([1,2,3].indexOf(yourValue) > -1) {
...
}
Edit, Gah, beaten by Blazemonger by 20 seconds

Use an array and $.inArray():
if ($.inArray(parseInt($(this).val()), [1, 2, 3]) > -1) {
}

Attempting to synthesize information that is currently scattered in a number of different answers and comments... Your code is not working because of two things:
You are using in which checks for keys, not values in an array. Thus, for example, 3 in [1,2,3] will be false, since [1,2,3] has keys (indices) 0,1,2.
Quite possibly, your value is a string
The solution is to make sure that your value is of the right type, and to use the correct function. There are two obvious choices:
indexOf - returns the index of the (first) element that matches
inArray - is available if you are using jQuery
Sample code:
var v = parseInt($(this).val()); // radix 10
var r = [1,2,3]; // the values to check against
if(r.indexOf(v) > -1) { // method 1
if($.inArray(v, r)) { // method 2

if ([1,2,3].indexOf($(this).val()) > -1) {
or
for (i in [1,2,3])
if (i==$(this).val()) {
...
}