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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
}
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function takaAnumber() {
startHealth = parseInt(prompt("give a positive number "));
//here i am calling the same function inside the if()block if the input is not a number or lesser than 0 is this a good practice?
if (isNaN(startHealth) || startHealth <= 0) {
takaAnumber();
}
// checkItsnumber();
playerHealth = startHealth;
monsterHealth = startHealth;
adjustHealthBars(startHealth);
}
I am trying get a number as an input, by calling a function. Inside the function the I have wrote a if() statement to check its a number if isn't it will call the function again. My question is it safe(good practice) to call a function inside it's own function in a large project code.
Your approach needs a return statement, because if the function returns, it goes on with the code of the calling function.
function takaAnumber() {
const startHealth = parseInt(prompt("give a positive number ")); // declare variables
if (isNaN(startHealth) || startHealth <= 0) {
return takaAnumber();
}
// other code
}
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I have this line.
let x = "Categories=All&Search=hat&ListPage=15&Page=1";
How to use regular expressions to say what you need to take from the beginning of a line and up to 2 characters &.Categories=All&Search=hat&
Use split and pass the limit argument, then just reconstruct it back into a string (if you really need the & - if not, discard everything after the split):
let x = "Categories=All&Search=hat&ListPage=15&Page=1";
const res = x.split("&", 2).map(e => e + "&").join("");
console.log(res);
You can use /([^&]*&){2}/g regex to match what you want.
let x = "Categories=All&Search=hat&ListPage=15&Page=1";
let y = x.match(/([^&]*&){2}/g);
console.log(y);
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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);
}
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Var randomLetters = "ljhgfdza";
Var randomString = "";
Now I'll have to add elements in the first variable to the second Randomly
Using the while loop, and Math.floor(Math.random() * random letters.length)
I am having a problem at my
"While (condition)" what should it be?
To flat out answer your question, you can use:
while(randomLetters.length > 0){
Then when you use a letter from randomLetters, you delete the letter and the length is now 1 less.
This will be enough for you:
const randomLetters = "ljhgfdza";
const returnRandom = (randomString) => {
const arrString = [...randomString].sort((a, b) =>{
return 0.5 - Math.random()
}).join("");
console.log(typeof arrString,arrString);
}
returnRandom(randomLetters);
but... in this case sort method is not that random as you think. This link will tell you why. I would do this with reduce() or map(), both are described in link above.
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The book that I'm studying says about iterating over arrays with every that:
The function these methods use must follow one rule—it must accept
three arguments like the following code:
function functionName(value, index, array) {
// do something here
}
Does that mean that I must always use 3 arguments? If so then why does this code work?
var numbers = [ 1, 2, 2 ];
function isLessThan3(value) {
var returnValue = false;
if (value < 3) {
returnValue = true;
}
return returnValue; }
document.write(numbers.every(isLessThan3));
There is no limitation on how many arrguments you can put in a function with Javascript.
you have a very good explenation about this topic in the next answer by #Niet the Dark Absol
https://stackoverflow.com/a/22747272/1283672
i believe that the book was reffering to something more specific within it's scope.
And just to be clear you can put no arrgs in a function either.
It's a bit ugly, the code, you have, but there is help. You might use the following without a temporary variable. Just return the result of the comparison.
function allLessThan3(value) {
return value < 3;
}
var numbers = [1, 2, 2];
console.log(numbers.every(allLessThan3));
No, you can use from 0 to 3 arguments