30 seconds of code

Curated collection of useful Javascript snippets that you can understand in 30 seconds or less.

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Contents

• Anagrams of string (with duplicates)
• Average of array of numbers
• Capitalize first letter
• Count occurrences of a value in array
• Current URL
• Curry
• Difference between arrays
• Distance between two points
• Escape regular expression
• Even or odd number
• Factorial
• Fibonacci array generator
• Filter out non uniqe values in an array
• Flatten array
• Get scroll position
• Greatest common divisor (GCD)
• Head of list
• Initial of list
• Initialize array with range
• Initialize array with values
• Last of list
• Measure time taken by function
• Object from key value pairs
• Powerset
• Random number in range
• Randomize order of array
• Redirect to url
• Reverse a string
• RGB to hexadecimal
• Scroll to top
• Similarity between arrays
• Sort characters in string (alphabetical)
• Sum of array of numbers
• Swap values of two variables
• Tail of list
• Unique values of array
• URL parameters
• UUID generator
• Validate number

Anagrams of string (with duplicates)

Use recursion. For each letter in the given string, create all the partial anagrams for the rest of its letters. Use map() to combine the letter with each partial anagram, then reduce() to combine all anagrams in one array. Base cases are for string length equal to 2 or 1.

var anagrams = s => {
  if(s.length <= 2)  return s.length === 2 ? [s, s[1] + s[0]] : [s];
  return s.split('').reduce( (a,l,i) => {
    anagrams(s.slice(0,i) + s.slice(i+1)).map( v => a.push(l+v) );
    return a;
  }, []);
}

Average of array of numbers

Use reduce() to add each value to an accumulator, initialized with a value of 0, divide by the length of the array.

var average = arr =>
  arr.reduce( (acc , val) => acc + val, 0) / arr.length;

Capitalize first letter

Use slice(0,1) and toUpperCase() to capitalize first letter, slice(1) to get the rest of the string.

var capitalize = str => str.slice(0, 1).toUpperCase() + str.slice(1);

Count occurrences of a value in array

Use reduce() to increment a counter each time you encounter the specific value inside the array.

var countOccurrences = (arr, value) => arr.reduce((a, v) => v===value ? a + 1 : a + 0, 0);

Current URL

Use window.location.href to get current URL.

var currentUrl = _ => window.location.href;

Curry

Use recursion. If the number of provided arguments (args) is sufficient, call the passed function f. Otherwise return a curried function f that expects the rest of the arguments.

var curry = f =>
  (...args) =>
    args.length >= f.length ? f(...args) : (...otherArgs) => curry(f)(...args, ...otherArgs)

Difference between arrays

Use filter() to remove values that are part of values, determined using includes().

var difference = (arr, values) => arr.filter(v => !values.includes(v));

Distance between two points

Use Math.hypot() to calculate the Euclidean distance between two points.

const distance = (x0, y0, x1, y1) => Math.hypot(x1 - x0, y1 - y0);

Escape regular expression

Use replace() to escape special characters.

var escapeRegExp = s =>
  s.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
}

Even or odd number

Use Math.abs() to extend logic to negative numbers, check using the modulo (%) operator. Return true if the number is even, false if the number is odd.

var isEven = num => Math.abs(num) % 2 === 0;

Factorial

Use recursion. If n is less than or equal to 1, return 1. Otherwise, return the product of n and the factorial of n - 1.

const factorial = n => n <= 1 ? 1 : n * factorial(n - 1)

Fibonacci array generator

Create an empty array of the specific length, initializing the first two values (0 and 1). Use reduce() to add values into the array, using the sum of the last two values, except for the first two.

var fibonacci = n =>
  Array.apply(null, [0,1].concat(Array(n-2))).reduce(
    (acc, val, i) => {
      acc.push( i>1 ? acc[i-1]+acc[i-2] : val);
      return acc;
    },[]);

Filter out non-unique values in an array

Use Array.filter() for an array containing only the unique values.

const unique = arr => arr.filter(i => arr.indexOf(i) === arr.lastIndexOf(i));
// unique([1,2,2,3,4,4,5]) -> [1,3,5]

Flatten array

Use recursion. Use reduce() to get all elements that are not arrays, flatten each element that is an array.

var flatten = arr =>
  arr.reduce( (a, v) => a.concat( Array.isArray(v) ? flatten(v) : v ), []);

Get Scroll Position

Get the current distance scrolled by window or HTMLElement as an {x,y} object

const getScrollPos = (scroller = window) => {
  let x = (scroller.pageXOffset !== undefined) ? scroller.pageXOffset : scroller.scrollLeft;
  let y = (scroller.pageYOffset !== undefined) ? scroller.pageYOffset : scroller.scrollTop;

  return {x, y}
}

// getScrollPos() -> {x: number, y: number}

Greatest common divisor (GCD)

Use recursion. Base case is when y equals 0. In this case, return x. Otherwise, return the GCD of y and the remainder of the division x/y.

var gcd = (x , y) => !y ? x : gcd(y, x % y);

Head of list

Return arr[0].

var head = arr => arr[0];

Initial of list

Return arr.slice(0,-1).

var initial = arr => arr.slice(0,-1);

Initialize array with range

Use Array(end-start) to create an array of the desired length, map() to fill with the desired values in a range. You can omit start to use a default value of 0.

var initializeArrayRange = (end, start = 0) =>
  Array.apply(null, Array(end-start)).map( (v,i) => i + start );

Initialize array with values

Use Array(n) to create an array of the desired length, fill(v) to fill it with the desired values. You can omit v to use a default value of 0.

var initializeArray = (n, v = 0) =>
  Array(n).fill(v);

Last of list

Return arr.slice(-1)[0].

var last = arr => arr.slice(-1)[0];

Measure time taken by function

Use performance.now() to get start and end time for the function, console.log() the time taken. First argument is the function name, subsequent arguments are passed to the function.

var timeTaken = (f,...args) => {
  var t0 = performance.now(), r = f(...args);
  console.log(performance.now() - t0);
  return r;
}

Object from key-value pairs

Use map() to create objects for each key-value pair, combine with Object.assign().

var objectFromPairs = arr =>
  Object.assign(...arr.map( v => {return {[v[0]] : v[1]};} ));

Powerset

Use reduce() combined with map() to iterate over elements and combine into an array containing all combinations.

var powerset = arr =>
  arr.reduce( (a,v) => a.concat(a.map( r => [v].concat(r) )), [[]]);

Random number in range

Use Math.random() to generate a random value, map it to the desired range using multiplication.

var randomInRange = (min, max) => Math.random() * (max - min) + min;

Randomize order of array

Use sort() to reorder elements, utilizing Math.random() to randomize the sorting.

var randomizeOrder = arr => arr.sort( (a,b) => Math.random() >= 0.5 ? -1 : 1)

Redirect to URL

Use window.location.href or window.location.replace() to redirect to url. Pass a second argument to simulate a link click (true - default) or an HTTP redirect (false).

var redirect = (url, asLink = true) =>
  asLink ? window.location.href = url : window.location.replace(url);

Reverse a string

Use array destructuring and Array.reverse() to reverse the order of the characters in the string. Combine characters to get a string using join('').

var reverseString = str => [...str].reverse().join(''); 

RGB to hexadecimal

Convert each value to a hexadecimal string, using toString(16), then padStart(2,'0') to get a 2-digit hexadecimal value. Combine values using join('').

var rgbToHex = (r, g, b) =>
  [r,g,b].map( v => v.toString(16).padStart(2,'0')).join('');

Scroll to top

Get distance from top using document.documentElement.scrollTop or document.body.scrollTop. Scroll by a fraction of the distance from top. Use window.requestAnimationFrame() to animate the scrolling.

var scrollToTop = _ => {
  var c = document.documentElement.scrollTop || document.body.scrollTop;
  if(c > 0) {
    window.requestAnimationFrame(scrollToTop);
    window.scrollTo(0, c - c/8);
  }
}

Similarity between arrays

Use filter() to remove values that are not part of values, determined using includes().

var difference = (arr, values) => arr.filter(v => values.includes(v));

Sort characters in string (alphabetical)

Split the string using split(''), sort() utilizing localeCompare(), recombine using join('').

var sortCharactersInString = str =>
  str.split('').sort( (a,b) => a.localeCompare(b) ).join('');

Sum of array of numbers

Use reduce() to add each value to an accumulator, initialized with a value of 0.

var sum = arr =>
  arr.reduce( (acc , val) => acc + val, 0);

Swap values of two variables

Use array destructuring to swap values between two variables.

[varA, varB] = [varB, varA];

Tail of list

Return arr.slice(1).

var tail = arr => arr.slice(1);

Unique values of array

Use ES6 Set and the ...rest operator to discard all duplicated values.

const unique = arr => [...new Set(arr)];
// unique([1,2,2,3,4,4,5]) -> [1,2,3,4,5]

URL parameters

Use match() with an appropriate regular expression to get all key-value pairs, map() them appropriately. Combine all key-value pairs into a single object using Object.assign() and the spread operator (...). Pass location.search as the argument to apply to the current url.

var getUrlParameters = url =>
  Object.assign(...url.match(/([^?=&]+)(=([^&]*))?/g).map(m => {[f,v] = m.split('='); return {[f]:v}}));

UUID generator

Use crypto API to generate a UUID, compliant with RFC4122 version 4.

var uuid = _ =>
  ( [1e7]+-1e3+-4e3+-8e3+-1e11 ).replace( /[018]/g, c =>
    (c ^ crypto.getRandomValues(new Uint8Array(1))[0] & 15 >> c / 4).toString(16)
  )

Validate number

Use !isNaN in combination with parseFloat() to check if the argument is a number. Use isFinite() to check if the number is finite.

var validateNumber = n => !isNaN(parseFloat(n)) && isFinite(n);

Credits

Icons made by Smashicons from www.flaticon.com is licensed by CC 3.0 BY.