Converting an Array to a String
.toString() : converts an array to a string of (comma separated) array values.
Looping Array Elements
.forEach(Function);
Adding Array Elements
.push(value) : add a new element to an array or array[array.length] = ""
new Array()
ex)new Array( value , value); new Array(position);
How to Recognize an Array
Array.isArray(fruits); or (fruits instanceof Array);
Array Methods
.length : returns the length (size) of an array
.toString() : converts an array to a string of (comma separated) array values
.at(position); eturns an indexed element from an array. == array[position]
.join(" * ") : like toString() but in addition you can specify the separator
.pop() : removes the last element from an array
.push(value) : adds a new element to an array (at the end)
.shift() : returns the value that was "shifted out"
.unshift(value) : adds a new element to an array (at the beginning), and "unshifts" older elements. like array[0] = value
Array delete()
Using delete() leaves undefined holes in the array.
Use pop() or shift() instead.
Merging Arrays (Concatenating)
.concat(array) : creates a new array by merging (concatenating) existing arrays
Array copyWithin()
.copyWithin(position, copyValuePosition) : copies array elements to another position in an array
.copyWithin(position , copyValuePositionStartIndex , copyValuePositionEndIndex ) : does not change the length of the array.
Array flat()
const myArr = [[1,2],[3,4],[5,6]]; : creates a new array with sub-array elements concatenated to a specified depth.
const newArr = myArr.flat();
const myArr = [1, 2, 3, 4, 5, 6]; : first maps all elements of an array and then creates a new array by flattening the array.
const newArr = myArr.flatMap(x => [x, x * 10]);
Splicing and Slicing Arrays
.splice(addPosition, removecount, addvalue, addvalue1......); : can be used to add new items to an array
.toSpliced(removePosition, removecount) : safe way to splice an array without altering the original array.
new toSpliced() method and the old splice() method is that the new method creates a new array, keeping the original array unchanged, while the old method altered the original array
.slice(removeIndex) : slices out a piece of an array into a new array ( does not remove any elements from the source array. )
Array Search
Array indexOf()
.indexOf("value") : searches an array for an element value and returns its position.
.indexOf(value, start) : start at the given position counting from the end, and search to the end
returns -1 if the item is not found.
.lastIndexOf(value) : returns the position of the last occurrence of the specified element
.lastIndexOf(item, start) : start at the given position counting from the end, and search to the beginning
.includes(value) : to check if an element is present in an array( including NaN, unlike indexOf )
Array find()
.find(myFunction) : returns the value of the first array element that passes a test function
.findIndex(myFunction); : returns the index of the first array element that passes a test function
.findLast(x => x > 40); : start from the end of an array and return the value of the first element that satisfies a condition
.findLastIndex(x => x > 40) : finds the index of the last element that satisfies a condition.
Sorting an Array
.sort(); : sorts an array alphabetically
.toSorted(); : as a safe way to sort an array without altering the original array
.reverse() : reverses the elements in an array
.toReversed() : as a safe way to reverse an array without altering the original array.
Numeric Sort
sort() function sorts values as strings.
.sort(function(a, b){return a - b});
.sort(function(a, b){return b - a});
.sort(function(){return 0.5 - Math.random()});
Using Math.min() or Math.max() on an Array
Math.max.apply(null, arr) : == Math.min(arr)
Math.min.apply(null, arr) : == Math.max(arr)
Array forEach()
.forEach( function) : calls a function (a callback function) once for each array element
Array map()
.map(function)
.flatMap(function) :
Array filter()
.filter(function) : creates a new array with array elements that pass a test.
ex ) function parameter : value, index, array.
- The item value
- The item index
- The array itself
Array reduce()
.reduce(myFunction) : a function on each array element to produce (reduce it to) a single value.
.reduce(myFunction, number);
.reduceRight(myFunction) : from right-to-left in the array.
- The total (the initial value / previously returned value)
- The item value
- The item index
- The array itself
Array every() , some()
.every(myFunction); : checks if all array values pass a test.
.some(myFunction); : checks if some array values pass a test.
- The item value
- The item index
- The array itself
Array.from()
Array.from(String) : returns an Array object from any object with a length property or any iterable object.
Array keys()
.keys(); : returns an Array Iterator object with the keys of an array.
Array entries()
.entries(); : returns an Array Iterator object with key/value pairs
Array with() Method
with() method as a safe way to update elements in an array without altering the original array.
ex) = months.with(2, "March"); : replace the index 2 value
Array Spread (...)
ex) const year = [...q1, ...q2, ...q3, ...q4]; : ... operator expands an iterable (like an array) into more elements:
