Array Methods Flashcards
How would I obtain the length of an array?
array.length
const myArray = [1, 2, 3, 4]
console.log(myArray.length) // prints 4
How would I add one or more elements to the end of an array?
array.push()
const myArray = [1, 2, 3, 4]
console.log(myArray.push(5)) // prints [1, 2, 3, 4, 5]
Returns the new length of the array.
How would I remove and return the last element from an array?
array.pop()
const myArray = [1, 2, 3, 4] const lastElement = myArray.pop()
console.log(myArray) // prints [1, 2, 3]console.log(lastElement) // prints 4
Mutates the original array.
How would I remove and return the first element of an array?
array.shift()
const myArray = [1, 2, 3, 4]; const firstElement = myArray.shift();
console.log(myArray); // prints [2, 3, 4]console.log(firstElement); // prints 1
Mutates the original array.
How would I add one or more elements to the beginning of an array?
array.unshift()
~~~
const myArray = [1, 2, 3, 4];
myArray.unshift(0);
console.log(myArray);
``` // prints [0, 1, 2, 3, 4]
How would I return a portion of an array as a new array?
array.slice(a, b)
Parameter a is starting index.
Parameter b is ending ending index, which is not included.
const myArray = [1, 2, 3, 4]; const newArray = myArray.slice(1, 3)
console.log(newArray) // prints [2, 3];console.log(myArray) // prints [1, 2, 3, 4];
The original array is not mutated.
How would I remove and/or replace existing elements at any location in an array?
array.splice(a, b, c)
Parameter a is starting index (required)
Parameter b is number of elements to remove (required)
Paremeter c is one or more elements to be inserted (optional)
~~~
const myArray = [1, 2, 3, 4];
my array.splice(2, 1, “a”, “b”);
console.log(myArray)
``` // prints [1, 2, “a”, “b”, 4]
How would I combine two or more arrays?
Method 1:
array.concat()
const array1 = [1, 2] const array2 = [3, 4] const array3 = [5, 6] const combinedArray = array1.concat(array2, array3)
Method 2:
Spread operator
const combinedArray = [...array1, ...array2, ...array3]
How would I check whether all elements in an array pass a certain condition?
array.every(function)
~~~
const numbers = [2, 4, 6, 8];
const areAllEven = numbers.every(num => num % 2 === 0);
console.log(areAllEven);
``` // true
How would I create a new array with all elements that pass a provided test?
array.filter(function)
~~~
function isBigEnough(value) {
return value >= 10;
}
const filtered = [12, 5, 8, 130, 44].filter(isBigEnough);
``` // filtered is [12, 130, 44]
How would I return the value of the first element in an array that satisfies a provided test?
array.find(function)
~~~
const numbers = [2, 4, 6, 8, 10];
const foundNumber = numbers.find(function (num) {
return num > 5;
})
console.log(foundNumber)
``` // 6
Returns undefined if no element satisfies condition.
How would I return the index of the first element in an array that satisfies a provided test?
array.findIndex(function)
~~~
const numbers = [2, 4, 6, 8, 10];
const firstEvenIndex = numbers.findIndex(num => num % 2 === 0);
console.log(firstEvenIndex)
``` // 0
Returns -1 if no elements pass the test.
How would I create a new array with sub-array elements flatted by a specified depth?
array.flat()
~~~
const arr = [1, 2, [3, 4]];
const flattened = arr.flat();
console.log(flattened);
``` // [1, 2, 3, 4]
How would I loop through an array and perform a function on each element using the .forEach() method?
array.forEach(element, index, array)
element parameter is current value of element being processed
index parameter is index of current element being processed
array parameter is array that forEach() is being called on
Example : Looping through array and logging each element
const myArray = ['apple', 'banana', 'orange'];
myArray.forEach((element) => {
console.log(element); //apple banana orange
});
Example 2: Modifying the elements of an array
~~~
const myArray = [1, 2, 3];
myArray.forEach((element, index, array) => {
array[index] = element * 2;
});
console.log(myArray);
``` //[2, 4, 6]
How would I check whether an array contains a certain value?
array.includes(value)const arr = ["apple", "banana", "orange"];
console.log(arr.includes("banana")); // trueconsole.log(arr.includes("BANANA")); // falseconsole.log(arr.includes("grape")); // false
Returns boolean value.
How would I find the index of a certain element in an array?
array.indexOf(a, b)
Parameter a is the element sought
Parameter b is the starting index from which to search (optional)
const animals = [“cat”, “dog”, “fish”];
const index = animals.indexOf(“bird”);
console.log(index); // Output: -1
Returns -1 if the element is not found
How would I concatenate all elements of an array into a single string?
array.join(separator)
The separator parameter is optional and will default to a comma if nothing else is provided. Otherwise, joins the elements of the array using the separator.
const fruits = [‘apple’, ‘banana’, ‘kiwi’];
const fruitsString = fruits.join(); // ‘apple,banana,kiwi’
How would I locate the index of the last occurance of a specified value in an array?
array.lastIndexOf(a, b)
Parameter a is the value
Parameter b is index from which to start the search (optional)
Searches array from right to left.
How would I reverse the order of the elements in an array?
array.reverse()
Mutates the original array
How would I determine whether at least one element in an array meets a certain condition?
array.some(function(currentValue, index, array))
const numbers = [1, 5, 8, 12, 3];
const result = numbers.some(function(num, index) {
return num > index;
});
console.log(result); // true
Returns boolean
How would I sort the elements of an array in place?
array.sort()
const numbers = [4, 2, 7, 1, 3];
numbers.sort();
console.log(numbers); // Output: [1, 2, 3, 4, 7]
Using .sort() with a compare function:
const numbers = [4, 2, 7, 1, 3];
numbers.sort((a, b) => a - b);
console.log(numbers); // Output: [1, 2, 3, 4, 7]
How would I return a string representation of an array?
array.toString()
Returns elements of the array separated by commas, with no spaces beteween the elements.
const fruits = [‘apple’, ‘banana’, ‘cherry’];
const fruitsString = fruits.toString();
console.log(fruitsString); // “apple,banana,cherry”
How would I create a new array from an interable object?
array.from()
console.log(Array.from(‘foo’)) // [‘f’, ‘o’, ‘o’]
console.log(Array.from([1, 2, 3], x => x + x)) // [2, 4, 6]
How would I change all elements in an array to a static value?
array.fill()
let a = [1, 2, 3, 4]
let b = a.fill(0, 2, 4)
console.log(b) // [1, 2, 0, 0]
How would I test whether a variable is an array?
array.isArray()
Returns boolean
