This question already has answers here:
When is the comma operator useful?
(15 answers)
Closed 1 year ago.
I have the following:
let currentLocalStorage = [];
currentLocalStorage = (initialLoad) ? JSON.parse(localStorage.getItem('tasks')): (currentLocalStorage.push(taskInput.value),currentLocalStorage)
which works but I would like to get a reference or documentation for the following:
: (currentLocalStorage.push(taskInput.value),currentLocalStorage)
so basically we are pushing on to the array and then defaulting to the array. I was surprised that we can do this and was wondering where one we look for the documentation
This is using the comma operator. Because .push() returns the new length of the array, you want to make sure you don't assign that to currentLocalStorage, so you use the comma operator to have that expression evaluate to currentLocalStorage.
So it effectively becomes currentLocalStorage = currentLocalStorage in that case, except now the array has one more item thanks to .push().
Related
This question already has answers here:
Check if array of objects contain certain key
(6 answers)
Closed 5 months ago.
I need to find out if an array of a JS object includes an object with a specific key.
I am using,
my_array.filter((e) => Object.keys(e).includes(my_key)).length > 0
but there must be a way to do it simpler.
Any suggestions in addition to Check if array of objects contain certain key ?
Instead of .filter(…).length > 0, use .some(…).
Instead of Object.keys(…).includes(…), use … in … (or Object.hasOwn(…, …) if you need to ignore inherited properties).
So
my_array.some(e => my_key in e)
instead of filter use some, already returns true or false and it shortcircuits as soon as it finds the first
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/some
This question already has answers here:
Javascript indexOf for an array of arrays not finding array
(6 answers)
Closed 1 year ago.
const a = [[1,1], [2,1], [3,1]];
console.log(a);
console.log(a.includes([1,1]));
> false
Can someone explain to me why the above outputs false? For what its worth, this occurs in any of the search functions (.find, .indexof, etc), as well as if I try [1,1] in a. I'm clearly missing something about how multidimensional array searching works in javascript.
This is basically how equal to operator works in JavaScript.
If we try to compare two arrays
[1]=== [1] // false
equality operator can compare primitive values(number,string Boolean). But in case of arrays and objects it compares for reference. So here we are comparing two different references so it is coming as false.
For same reason you are getting false. You have to write customer logic to compare arrays.
This question already has answers here:
Determine whether an array contains a value [duplicate]
(18 answers)
What is the difference between "in operator" and "includes()" for JavaScript arrays
(4 answers)
Closed 1 year ago.
I need to execute some code. But my code is not working properly. I have made an simple example to demonstrate the malfunction.
When I run
console.log("val" in Object.values({key:"val"})); //returns false
It gives me false. But if I run
console.log(Object.values({key:"val"}))
outputs => ['val']
I don't understand if it is supposed to work like this. If yes. Why ?
Thanks in Advance.....:)
MDN says "the in operator returns true if the specified property is in the specified object or its prototype chain.", Object.values returns an array. To check if an item exists in an array, use the Array.includes method.
console.log(
Object.values({key:"val"}).includes("val")
); // Returns true
This question already has answers here:
Why Doesn't Array Concatenation Work in Javascript? [closed]
(3 answers)
Closed 2 years ago.
I am getting below like values at runtime. Below is for only example. I will have same array structure at runtime. I want to merge them.
let finalSearchResult =[];
data [{"a":1000,"a":1000001,"a":10000002,"D":5000000}]
data [{"P":1000,"Q":1000001,"R":10000002,"S":5000000}]
finalSearchResult.concat(finalSearchResult,data);
but its not working. When I am printing finalSearchResult its coming as null.
The concat() method is used to merge two or more arrays. This method does not change the existing arrays, but instead returns a new array.
please refer to:
Array.prototype.concat documentation
Therefore:
finalSearchResult = finalSearchResult.concat(data);
Will be the correct way to concat the arrays.
This question already has answers here:
What "..." means in Javascript (ES6)? [duplicate]
(1 answer)
Spread Syntax vs Rest Parameter in ES2015 / ES6
(11 answers)
Closed 4 years ago.
I’m new to coding and slef teaching JavaScript.
My task I was set was to identify the largest value in an array.
My soloition works, but I needed the ‘...’ for it to work properly.
My question is, what does the ‘...’ in the last line actually mean/do?
function createAnArr(){
let myArr = prompt("Enter your numbers separated by commas:").split(",");
return myArr;
}
console.log(Math.max(...createAnArr()));
'...' it means it will be able to take any number of arguments of the respective scope
'...': The spread operator is used for array construction and destructuring, and to fill function arguments from an array on invocation. A case when the operator spreads the array (or iterable object) elements.
more details you can see here