To reverse array in JavaScript without mutating original array, we copy the array before calling reverse.
For instance, we write
const newArray = [...array].reverse();
to copy the array with the spread operator.
And then we call reverse to reverse the copied array.
To sum a property value in an array with JavaScript, we use the map and reduce methods.
For instance, we write
const traveler = [
{ description: "Senior", amount: 50 },
{ description: "Senior", amount: 50 },
{ description: "Adult", amount: 75 },
{ description: "Child", amount: 35 },
{ description: "Infant", amount: 25 },
];
const sum = traveler
.map((item) => item.amount)
.reduce((prev, next) => prev + next);
to call traveler.map with a callback to get the amount property from each entry and put it in the returned array.
And then we call reduce with a callback to sum up the values from the array returned by map.
To count the occurrences / frequency of array elements with JavaScript, we use a for-of loop.
For instance, we write
const arr = [5, 5, 5, 2, 2, 2, 2, 2, 9, 4];
const counts = {};
for (const num of arr) {
counts[num] = counts[num] ? counts[num] + 1 : 1;
}
to define the counts object.
Then we use a for-of loop to loop through the arr array and set counts[num] to counts[num] + 1 if it’s already set and 1 otherwise.
To convert JavaScript NodeList to an array, we use the Array.from method.
For instance, we write
const myArray = Array.from(nl);
to call Array.from with nl to convery the nl node list into an array.
To prepend a value to an array with JavaScript, we call the unshift method.
For instance, we write
a.slice().unshift(0);
to call a.slice to make copy of the a array.
And then we call unshift on the copied array to prepend 0 into the array.