What does the term "mutability" mean in programming?
"Mutability" refers to an object's ability to undergo change. Mutable objects can be modified, while immutable objects cannot be changed after they are created.
** Identify which of the following JavaScript data types are mutable: number, string, boolean, array**
Numbers, strings, and booleans are all immutable.
.concat
no mutation
Description
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.
Copy Array.prototype.concat ( [ item1 [ , item2 [ , … ] ] ] )
Example
Copy let array1 = ['a', 'b', 'c'];
let array2 = ['d', 'e', 'f'];
console.log(array1.concat(array2));
// expected output: Array ["a", "b", "c", "d", "e", "f"] .copyWithin()
mutates
Description
The copyWithin() method shallow copies part of an array to another location in the same array and returns it, without modifying its size.
Copy arr.copyWithin(target)
arr.copyWithin(target, start)
arr.copyWithin(target, start, end) Example
Copy let array1 = ['a', 'b', 'c', 'd', 'e'];
// copy to index 0 the element at index 3
console.log(array1.copyWithin(0, 3, 4));
// expected output: Array ["d", "b", "c", "d", "e"]
// copy to index 1 all elements from index 3 to the end
console.log(array1.copyWithin(1, 3));
// expected output: Array ["d", "d", "e", "d", "e"] .entries()
no mutation
Description
The entries() method returns a new Array Iterator object that contains the key/value pairs for each index in the array.
Example
Copy let array1 = ['a', 'b', 'c'];
let iterator1 = array1.entries();
console.log(iterator1.next().value);
// expected output: Array [0, "a"]
console.log(iterator1.next().value);
// expected output: Array [1, "b"] .every
no mutation
Description
The every() method tests whether all elements in the array pass the test implemented by the provided function.
Copy Array.prototype.every ( callbackfn [ , thisArg ] ) Example
Copy function isBelowThreshold(currentValue) {
return currentValue < 40;
}
let array1 = [1, 30, 39, 29, 10, 13];
console.log(array1.every(isBelowThreshold));
// expected output: true .fill()
mutates
Description
The fill() method fills all the elements of an array from a start index to an end index with a static value.
Copy arr.fill(value)
arr.fill(value, start)
arr.fill(value, start, end) Example
Copy let array1 = [1, 2, 3, 4];
// fill with 0 from position 2 until position 4
console.log(array1.fill(0, 2, 4));
// expected output: [1, 2, 0, 0]
// fill with 5 from position 1
console.log(array1.fill(5, 1));
// expected output: [1, 5, 5, 5]
console.log(array1.fill(6));
// expected output: [6, 6, 6, 6] .filter
no mutation
Description
The filter() method creates a new array with all elements that pass the test implemented by the provided function.
Copy Array.prototype.filter ( callbackfn [ , thisArg ] ) Example
Copy let words = ['spray', 'limit', 'elite', 'exuberant', 'destruction', 'present'];
const result = words.filter(word => word.length > 6);
console.log(result);
// expected output: Array ["exuberant", "destruction", "present"] .find()
no mutation
Description
The find() method returns a value of the first element in the array that satisfies the provided testing function. Otherwise undefined is returned.
Copy arr.find(callback[, thisArg]) Example
Copy let array1 = [5, 12, 8, 130, 44];
let found = array1.find(function(element) {
return element > 10;
});
console.log(found);
// expected output: 12 .findIndex()
no mutation
Description
The findIndex() method returns an index of the first element in the array that satisfies the provided testing function. Otherwise -1 is returned.
Copy arr.findIndex(callback[, thisArg]) Example
Copy let array1 = [5, 12, 8, 130, 44];
function isLargeNumber(element) {
return element > 13;
}
console.log(array1.findIndex(isLargeNumber));
// expected output: 3 .flat
no mutation
Description
The flat() method creates a new array with all sub-array elements concatenated into it recursively up to the specified depth.
Copy let newArray = arr.flat([depth]); Example
Copy let arr1 = [1, 2, [3, 4]];
arr1.flat();
// [1, 2, 3, 4]
let arr2 = [1, 2, [3, 4, [5, 6]]];
arr2.flat();
// [1, 2, 3, 4, [5, 6]]
let arr3 = [1, 2, [3, 4, [5, 6]]];
arr3.flat(2);
// [1, 2, 3, 4, 5, 6]
let arr4 = [1, 2, [3, 4, [5, 6, [7, 8, [9, 10]]]]];
arr4.flat(Infinity);
// [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] .forEach
no mutation
Description
The forEach() method executes a provided function once per array element.
Copy Array.prototype.forEach ( callbackfn [ , thisArg ] ) Example
Copy let array1 = ['a', 'b', 'c'];
array1.forEach(function(element) {
console.log(element);
});
// expected output: "a"
// expected output: "b"
// expected output: "c" .includes()
no mutation
Description
The includes() method determines whether an array includes a certain element, returning true or false as appropriate.
Copy arr.includes(searchElement)
arr.includes(searchElement, fromIndex) Example
Copy let array1 = [1, 2, 3];
console.log(array1.includes(2));
// expected output: true
let pets = ['cat', 'dog', 'bat'];
console.log(pets.includes('cat'));
// expected output: true
console.log(pets.includes('at'));
// expected output: false .indexOf
no mutation
Description
The indexOf() method returns the first index at which a given element can be found in the array, or -1 if it is not present.
Copy Array.prototype.indexOf ( searchElement [ , fromIndex ] ) Example
Copy let beasts = ['ant', 'bison', 'camel', 'duck', 'bison'];
console.log(beasts.indexOf('bison'));
// expected output: 1
// start from index 2
console.log(beasts.indexOf('bison', 2));
// expected output: 4
console.log(beasts.indexOf('giraffe'));
// expected output: -1 .join
no mutation
Description
The join() method joins all elements of an array into a string.
Copy Array.prototype.join (separator) Example
Copy let elements = ['Fire', 'Air', 'Water'];
console.log(elements.join());
// expected output: "Fire,Air,Water"
console.log(elements.join(''));
// expected output: "FireAirWater"
console.log(elements.join('-'));
// expected output: "Fire-Air-Water" .keys()
no mutation
Description
The keys() method returns a new Array Iterator that contains the keys for each index in the array.
Example
Copy let array1 = ['a', 'b', 'c'];
let iterator = array1.keys();
for (let key of iterator) {
console.log(key); // expected output: 0 1 2
} .lastIndexOf
no mutation
Description
The lastIndexOf() method returns the last index at which a given element can be found in the array, or -1 if it is not present. The array is searched backwards, starting at fromIndex.
Copy Array.prototype.lastIndexOf ( searchElement [ , fromIndex ] ) Example
Copy let animals = ['Dodo', 'Tiger', 'Penguin', 'Dodo'];
console.log(animals.lastIndexOf('Dodo'));
// expected output: 3
console.log(animals.lastIndexOf('Tiger'));
// expected output: 1 .map
no mutation
Description
The map() method creates a new array with the results of calling a provided function on every element in this array.
Copy Array.prototype.map ( callbackfn [ , thisArg ] ) Example
Copy let array1 = [1, 4, 9, 16];
// pass a function to map
const map1 = array1.map(x => x * 2);
console.log(map1);
// expected output: Array [2, 8, 18, 32] .pop
mutates
Description
The pop() method removes the last element from an array and returns that element. This method changes the length of the array.
Copy Array.prototype.pop ( ) Example
Copy let plants = ['broccoli', 'cauliflower', 'cabbage', 'kale', 'tomato'];
console.log(plants.pop());
// expected output: "tomato"
console.log(plants);
// expected output: Array ["broccoli", "cauliflower", "cabbage", "kale"]
plants.pop();
console.log(plants);
// expected output: Array ["broccoli", "cauliflower", "cabbage"] .push
mutates
Description
The push() method adds one or more elements to the end of an array and returns the new length of the array.
Copy Array.prototype.push ( [ item1 [ , item2 [ , … ] ] ] ) Example
Copy let animals = ['pigs', 'goats', 'sheep'];
console.log(animals.push('cows'));
// expected output: 4
console.log(animals);
// expected output: Array ["pigs", "goats", "sheep", "cows"]
animals.push('chickens');
console.log(animals);
// expected output: Array ["pigs", "goats", "sheep", "cows", "chickens"] .reduce
no mutation
Description
The reduce() method applies a function against an accumulator and each value of the array (from left-to-right) to reduce it to a single value.
Copy Array.prototype.reduce ( callbackfn [ , initialValue ] ) Example
Copy const array1 = [1, 2, 3, 4];
const reducer = (accumulator, currentValue) => accumulator + currentValue;
// 1 + 2 + 3 + 4
console.log(array1.reduce(reducer));
// expected output: 10
// 5 + 1 + 2 + 3 + 4
console.log(array1.reduce(reducer, 5));
// expected output: 15 .reduceRight
no mutation
Description
The reduceRight() method applies a function against an accumulator and each value of the array (from right-to-left) has to reduce it to a single value.
Copy Array.prototype.reduceRight ( callbackfn [ , initialValue ] ) Example
Copy const array1 = [[0, 1], [2, 3], [4, 5]].reduceRight(
(accumulator, currentValue) => accumulator.concat(currentValue)
);
console.log(array1);
// expected output: Array [4, 5, 2, 3, 0, 1] .reverse
mutates
Description
The reverse() method reverses an array in place. The first array element becomes the last, and the last array element becomes the first.
Copy Array.prototype.reverse ( ) Example
Copy let array1 = ['one', 'two', 'three'];
console.log('array1: ', array1);
// expected output: Array ['one', 'two', 'three']
let reversed = array1.reverse();
console.log('reversed: ', reversed);
// expected output: Array ['three', 'two', 'one']
/* Careful: reverse is destructive. It also changes
the original array */
console.log('array1: ', array1);
// expected output: Array ['three', 'two', 'one'] .shift
mutates
Description
The shift() method removes the first element from an array and returns that element. This method changes the length of the array.
Copy Array.prototype.shift ( ) Example
Copy let array1 = [1, 2, 3];
let firstElement = array1.shift();
console.log(array1);
// expected output: Array [2, 3]
console.log(firstElement);
// expected output: 1 .slice
no mutation
Description
The slice() method returns a shallow copy of a portion of an array into a new array object selected from begin to end (end not included). The original array will not be modified.
Copy Array.prototype.slice (start, end) Example
Copy let animals = ['ant', 'bison', 'camel', 'duck', 'elephant'];
console.log(animals.slice(2));
// expected output: Array ["camel", "duck", "elephant"]
console.log(animals.slice(2, 4));
// expected output: Array ["camel", "duck"]
console.log(animals.slice(1, 5));
// expected output: Array ["bison", "camel", "duck", "elephant"] .some
no mutation
Description
The some() method tests whether some element in the array passes the test implemented by the provided function.
Copy Array.prototype.some ( callbackfn [ , thisArg ] ) Example
Copy let array = [1, 2, 3, 4, 5];
let even = function(element) {
// checks whether an element is even
return element % 2 === 0;
};
console.log(array.some(even));
// expected output: true .sort
mutates
Description
The sort() method sorts the elements of an array in place and returns the array. The sort is not necessarily stable. The default sort order is according to string Unicode code points.
Copy Array.prototype.sort (comparefn) Example
Copy let months = ['March', 'Jan', 'Feb', 'Dec'];
months.sort();
console.log(months);
// expected output: Array ["Dec", "Feb", "Jan", "March"]
let array1 = [1, 30, 4, 21, 100000];
array1.sort();
console.log(array1);
// expected output: Array [1, 100000, 21, 30, 4] .splice
mutates
Description
The splice() method changes the content of an array by removing existing elements and/or adding new elements.
Copy Array.prototype.splice (start, deleteCount [ , item1 [ , item2 [ , … ] ] ] ) Example
Copy let months = ['Jan', 'March', 'April', 'June'];
months.splice(1, 0, 'Feb');
// inserts at 1st index position
console.log(months);
// expected output: Array ['Jan', 'Feb', 'March', 'April', 'June']
months.splice(4, 1, 'May');
// replaces 1 element at 4th index
console.log(months);
// expected output: Array ['Jan', 'Feb', 'March', 'April', 'May']
.toString
no mutation
Description
The toString() method returns a string representing the specified array and its elements.
Copy Array.prototype.toString ( ) Example
Copy let array1 = [1, 2, 'a', '1a'];
console.log(array1.toString());
// expected output: "1,2,a,1a" .unshift
mutates
Description
The unshift() method adds one or more elements to the beginning of an array and returns the new length of the array.
Copy Array.prototype.unshift ( [ item1 [ , item2 [ , … ] ] ] ) Example
Copy let array1 = [1, 2, 3];
console.log(array1.unshift(4, 5));
// expected output: 5
console.log(array1);
// expected output: Array [4, 5, 1, 2, 3] .values()
no mutation
Description
The values() method returns a new Array Iterator object that contains the values for each index in the array.
Example
Copy const array1 = ['a', 'b', 'c'];
const iterator = array1.values();
for (const value of iterator) {
console.log(value); // expected output: "a" "b" "c"
} \