To filter an array from all elements of another array with JavaScript, we use the filter and includes methods.
For instance, we write
const arr1 = [1, 2, 3, 4];
const arr2 = [2, 4];
const res = arr1.filter((item) => !arr2.includes(item));
console.log(res);
to call filter with a callback that checks if the item in arr1 isn’t in arr2.
If it’s not, then it’s included in the array returned by filter.
To get the last 5 elements, excluding the first element from an array with JavaScript, we call the Math.max and slice methods.
For instance, we write
arr.slice(Math.max(arr.length - 5, 0));
to call Math.max with arr.length - 5 and 0 to return the max number between the 2 numbers.
And then we call slice to return an array with the arr entries between the index returned by max to the last element.
To search in array of object in JavaScript MongoDB, we use the $elemMatch property.
For instance, we write
db.users.find({
awards: { $elemMatch: { award: "National Medal", year: 1975 } },
});
to call find with an object that has awars.$elemMatch set to an object with the field values we’re looking for to get the values in the awards array field that has the property values listed.
To swap array elements with JavaScript, we use destructuring.
For instance, we write
[arr[0], arr[1]] = [arr[1], arr[0]];
to ass arr[1] to arr[0]and assignarr[0]toarr[1]` with destructuring.
To loop through an array containing objects and access their properties with JavaScript, we use the forEach method.
For instance, we write
yourArray.forEach((arrayItem) => {
const x = arrayItem.prop + 2;
console.log(x);
});
to call forEach with a callback that gets the prop property value from the arrayItem being looped through.
And then we add 2 to it assign the sum to x.