Essential Vanilla JavaScript Methods Cheat Sheet

for (key in obj)	Iterates over keys
for (value of obj)	Iterates over values

arr.forEach(function(val, I, arr))	Executes a function for each value.
arr.every(function(val, i, arr))	Returns true if every value passes the function test
arr.reverse()	Reverses the original array
Searching
arr.includes(val)	Returns true if array contains a value
arr.indexOf(val)	Returns the index of a value in an array or -1 if not found
arr.find(function)	Returns the value of the first index to satisfy the function
arr.findIndex(function)	Returns the index of the first value to satisfy the function
Add Values
arr.splice(start, 0, items...)	Adds items to array beginning at starting index
arr.unshift(val)	Adds val to front of array
arr.push(val)	Adds val to back of array
Delete Values
arr.splice(start, n)	Delete n items from array beginning at starting index
arr.shift()	Removes arr[0] and returns the value
arr.pop()	Removes last value in a array and returns it
Copy Values
arr.slice(beg, end)	Returns a shallow copy of arr from beg to end (non-inclusive)
To String
arr.join(separator)	Returns a string from the array values, with an optional separator string
JSON.stringify(arr)	Returns a JSON string representation of array. Deep string.
arr.toString()	Returns a comma-delimited string of scalar array values. Shallow string.
Sorting
arr.sort()	Sorts lexicographically
arr.sort((a, b) => a - b)	Sorts numbers

num.toFixed(n)	Returns a number rounded to n significant figures after the decimal
num.toPrecision(n)	Returns a string rounded to n significant figures
num.toString()	Returns a string representation of a number

str.slice(beg, end)	returns a substring from beginning to end index
str.toLowerCase()	Returns a lowercase version of str
String to Array
str.split(separator)	Returns an array of the string, split by the separator
Searching Strings
str.includes(str)	Returns true if str is in string
str.indexOf(str)	Returns index of str if within string, or -1 otherwise
str.startsWith(str, beg)	Returns true if string starts with str, optionally from beg index.
str.endsWith(str, len)	Returns true if string starts with str, optionally limited to the first len characters
Combine Strings
str.concat(strings...)	Returns new string concatenation

Math.abs(num)	Returns the absolute value of num
Random Numbers
Math.round(Math.random() * (max - min)) + min)	Returns a random number between two ints
Math.random()	Returns a random number between 0 and 1.
Rounding
Math.floor(num)	Goes down to the nearest int <= num
Math.ceil(num)	Goes up to the nearest int >= num
Math.round(num)	Rounds to the nearest int

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Essential Vanilla JavaScript Methods Cheat Sheet by bytecut