JavaScript Backend Flashcards
Read a JSON file
fetch(‘path/to/JSON.json’)
.then(res => res.json())
.then(data => {
console.log(data);
})
.catch(err => console.error(‘Error’: err));
Add to end of array
arr.push(4)
Remove starting element from array
arr.shift()
Remove last element in array
arr.pop()
Remove element/s in middle of array
arr.splice(1, 2)
Add element/s to middle of array
arr.splice(1, 0, 2, 3) //adds 2, 3 at index 1
Convert string to array
arr.split(‘’)
Extract substring
str.slice(0, 5)
Convert string to upper case
str.toUpperCase()
Convert string to lower case
str.toLowerCase
Does string include value
str.includes(‘a’)
Does a certain key exist in object
obj.hasOwnProperty(‘a’)
‘a’ in obj
Add key value pair to object
obj[‘c’] = 3
Get array of keys from object
Object.keys(obj)
Delete key value pair
delete obj.a
Loop through keys in an object
for (let key in obj) {
}
Initiate a set
mySet = new Set()
= new Set([1, 2, 3])
Add value to set
mySet.add(4)
How to see if value in set
mySet.has(3)
Delete value from set
mySet.delete(2)
Remove duplicates from an array
let newArr = [… new Set(array)]
Find length of set
mySet.size()
Iterate over values in a set
for (let value of mySet) {
}
Clear set
mySet.clear()
Create a map
let myMap = new Map()
Add to a map
myMap.set(‘a’, 1)
Get a value with key
myMap.get(‘a’)
Does key exist in map
myMap.has(‘b’)
Delete key value pair from map
myMap.delete(‘a’)
Round down number
Math.floor(4.8) // 4
Round up number
Math.ceil(4.2) // 5
Find remainder of division
10 % 3 // 1
Sort elements in place
arr.sort((a, b) => a -b)
Pass filter on array
arr.filter(num => num > 2)
Pass function on every element in array and return array
arr.map(num => num * 2)
Execute function on each array element
arr.forEach(num => console.log(num))
Write a normal function
function greet() {
console.log(‘Hello World’)
}
Write an arrow function
let greet = () => {
console.log(‘Hello World’)
}
Find character at index for string
str.charAt(7)
Find index of substring in string
str.indexOf(‘World’)
Replace substring with other string
str.replace(‘World’, ‘Universe’)
