Merge remote-tracking branch 'upstream/master'

snippets/URL-parameters.md

@@ -1,11 +1,12 @@
 ### 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 (`...`).
+Use `match()` with an appropriate regular expression to get all key-value pairs, `Array.reduce()` to map and combine them into a single object.
 Pass `location.search` as the argument to apply to the current `url`.
 
 ```js
 const getUrlParameters = url =>
-  Object.assign(...url.match(/([^?=&]+)(=([^&]*))?/g).map(m => {[f,v] = m.split('='); return {[f]:v}}));
+  url.match(/([^?=&]+)(=([^&]*))/g).reduce(
+    (a, v) => (a[v.slice(0, v.indexOf('='))] = v.slice(v.indexOf('=') + 1), a), {}
+  );
 // getUrlParameters('http://url.com/page?name=Adam&surname=Smith') -> {name: 'Adam', surname: 'Smith'}
 ```

snippets/UUID-generator.md

@@ -4,7 +4,7 @@ Use `crypto` API to generate a UUID, compliant with [RFC4122](https://www.ietf.o
 
 ```js
 const uuid = _ =>
-  ( [1e7]+-1e3+-4e3+-8e3+-1e11 ).replace( /[018]/g, c =>
+  ([1e7] + -1e3 + -4e3 + -8e3 + -1e11).replace(/[018]/g, c =>
     (c ^ crypto.getRandomValues(new Uint8Array(1))[0] & 15 >> c / 4).toString(16)
   );
 // uuid() -> '7982fcfe-5721-4632-bede-6000885be57d'

snippets/anagrams-of-string-(with-duplicates).md

@@ -2,16 +2,14 @@
 
 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.
+Use `Array.map()` to combine the letter with each partial anagram, then `Array.reduce()` to combine all anagrams in one array.
 Base cases are for string `length` equal to `2` or `1`.
 
 ```js
 const anagrams = str => {
-  if(str.length <= 2)  return str.length === 2 ? [str, str[1] + str[0]] : [str];
-  return str.split('').reduce( (acc, letter, i) => {
-    anagrams(str.slice(0, i) + str.slice(i + 1)).map( val => acc.push(letter + val) );
-    return acc;
-  }, []);
-}
+  if (str.length <= 2) return str.length === 2 ? [str, str[1] + str[0]] : [str];
+  return str.split('').reduce((acc, letter, i) =>
+    acc.concat(anagrams(str.slice(0, i) + str.slice(i + 1)).map(val => letter + val)), []);
+};
 // anagrams('abc') -> ['abc','acb','bac','bca','cab','cba']
 ```

snippets/array-concatenation.md

@@ -0,0 +1,8 @@
+### Array concatenation
+
+Use `Array.concat()` to concatenate an array with any additional arrays and/or values, specified in `args`.
+
+```js
+const arrayConcat = (arr, ...args) => arr.concat(...args);
+// arrayConcat([1], 2, [3], [[4]]) -> [1,2,3,[4]]
+```

snippets/array-difference.md

@@ -0,0 +1,8 @@
+### Array difference
+
+Create a `Set` from `b`, then use `Array.filter()` on `a` to only keep values not contained in `b`.
+
+```js
+const difference = (a, b) => { const s = new Set(b); return a.filter(x => !s.has(x)); };
+// difference([1,2,3], [1,2]) -> [3]
+```

snippets/array-intersection.md

@@ -0,0 +1,8 @@
+### Array intersection
+
+Create a `Set` from `b`, then use `Array.filter()` on `a` to only keep values contained in `b`.
+
+```js
+const intersection = (a, b) => { const s = new Set(b); return a.filter(x => s.has(x)); };
+// intersection([1,2,3], [4,3,2]) -> [2,3]
+```

snippets/array-union.md

@@ -0,0 +1,8 @@
+### Array union
+
+Create a `Set` with all values of `a` and `b` and convert to an array.
+
+```js
+const union = (a, b) => Array.from(new Set([...a, ...b]));
+// union([1,2,3], [4,3,2]) -> [1,2,3,4]
+```

snippets/average-of-array-of-numbers.md

@@ -1,9 +1,8 @@
 ### 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.
+Use `Array.reduce()` to add each value to an accumulator, initialized with a value of `0`, divide by the `length` of the array.
 
 ```js
-const average = arr =>
-  arr.reduce( (acc , val) => acc + val, 0) / arr.length;
+const average = arr => arr.reduce((acc, val) => acc + val, 0) / arr.length;
 // average([1,2,3]) -> 2
 ```

snippets/bottom-visible.md

@@ -0,0 +1,9 @@
+### Bottom visible
+
+Use `scrollY`, `scrollHeight` and `clientHeight` to determine if the bottom of the page is visible.
+
+```js
+const bottomVisible = _ =>
+  document.documentElement.clientHeight + window.scrollY >= document.documentElement.scrollHeight || document.documentElement.clientHeight;
+// bottomVisible() -> true
+```

snippets/capitalize-first-letter.md

@@ -5,6 +5,6 @@ Omit the `lowerRest` parameter to keep the rest of the string intact, or set it
 
 ```js
 const capitalize = (str, lowerRest = false) =>
-  str.slice(0, 1).toUpperCase() + (lowerRest? str.slice(1).toLowerCase() : str.slice(1));
+  str.slice(0, 1).toUpperCase() + (lowerRest ? str.slice(1).toLowerCase() : str.slice(1));
 // capitalize('myName', true) -> 'Myname'
 ```

snippets/chain-asynchronous-functions.md

@@ -0,0 +1,14 @@
+### Chain asynchronous functions
+
+Loop through an array of functions containing asynchronous events, calling `next` when each asynchronous event has completed.
+
+```js
+const chainAsync = fns => { let curr = 0; const next = () => fns[curr++](next); next(); };
+/*
+chainAsync([
+  next => { console.log('0 seconds'); setTimeout(next, 1000); },
+  next => { console.log('1 second');  setTimeout(next, 1000); },
+  next => { console.log('2 seconds'); }
+])
+*/
+```

snippets/check-for-palindrome.md

@@ -4,7 +4,9 @@ Convert string `toLowerCase()` and use `replace()` to remove non-alphanumeric ch
 Then, `split('')` into individual characters, `reverse()`, `join('')` and compare to the original, unreversed string, after converting it `tolowerCase()`.
 
 ```js
-const palindrome = str =>
-  str.toLowerCase().replace(/[\W_]/g,'').split('').reverse().join('') === str.toLowerCase().replace(/[\W_]/g,'');
+const palindrome = str => {
+  const s = str.toLowerCase().replace(/[\W_]/g,'');
+  return s === s.split('').reverse().join('');
+}
 // palindrome('taco cat') -> true
  ```

snippets/chunk-array.md

@@ -0,0 +1,11 @@
+### Chunk array
+
+Use `Array.from()` to create a new array, that fits the number of chunks that will be produced.
+Use `Array.slice()` to map each element of the new array to a chunk the length of `size`.
+If the original array can't be split evenly, the final chunk will contain the remaining elements.
+
+```js
+const chunk = (arr, size) =>
+  Array.from({length: Math.ceil(arr.length / size)}, (v, i) => arr.slice(i * size, i * size + size));
+// chunk([1,2,3,4,5], 2) -> [[1,2],[3,4],5]
+```

snippets/collatz-algorithm.md

@@ -0,0 +1,9 @@
+### Collatz algorithm
+
+If `n` is even, return `n/2`. Otherwise  return `3n+1`.
+
+```js
+const collatz = n => (n % 2 == 0) ? (n / 2) : (3 * n + 1);
+// collatz(8) --> 4
+// collatz(5) --> 16
+```

snippets/compact.md

@@ -0,0 +1,8 @@
+### Compact
+
+Use `Array.filter()` to filter out falsey values (`false`, `null`, `0`, `""`, `undefined`, and `NaN`).
+
+```js
+const compact = (arr) => arr.filter(v => v);
+// compact([0, 1, false, 2, '', 3, 'a', 'e'*23, NaN, 's', 34]) -> [ 1, 2, 3, 'a', 's', 34 ]
+```

snippets/count-occurrences-of-a-value-in-array.md

@@ -1,6 +1,6 @@
 ### Count occurrences of a value in array
 
-Use `reduce()` to increment a counter each time you encounter the specific value inside the array.
+Use `Array.reduce()` to increment a counter each time you encounter the specific value inside the array.
 
 ```js
 const countOccurrences = (arr, value) => arr.reduce((a, v) => v === value ? a + 1 : a + 0, 0);

snippets/curry.md

@@ -3,10 +3,13 @@
 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.
+If you want to curry a function that accepts a variable number of arguments (a variadic function, e.g. `Math.min()`), you can optionally pass the number of arguments to the second parameter `arity`.
 
 ```js
-const curry = f =>
-  (...args) =>
-    args.length >= f.length ? f(...args) : (...otherArgs) => curry(f)(...args, ...otherArgs);
+const curry = (fn, arity = fn.length, ...args) =>
+  arity <= args.length
+    ? fn(...args)
+    : curry.bind(null, fn, arity, ...args);
 // curry(Math.pow)(2)(10) -> 1024
+// curry(Math.min, 3)(10)(50)(2) -> 2
 ```

snippets/deep-flatten-array.md

@@ -1,10 +1,10 @@
 ### Deep flatten array
 
 Use recursion.
-Use `reduce()` to get all elements that are not arrays, flatten each element that is an array.
+Use `Array.reduce()` to get all elements that are not arrays, flatten each element that is an array.
 
 ```js
 const deepFlatten = arr =>
-  arr.reduce( (a, v) => a.concat( Array.isArray(v) ? flatten(v) : v ), []);
+  arr.reduce((a, v) => a.concat(Array.isArray(v) ? deepFlatten(v) : v), []);
 // deepFlatten([1,[2],[[3],4],5]) -> [1,2,3,4,5]
 ```

snippets/difference-between-arrays.md

@@ -1,8 +0,0 @@
-### Difference between arrays
-
-Use `filter()` to remove values that are part of `values`, determined using `includes()`.
-
-```js
-const difference = (arr, values) => arr.filter(v => !values.includes(v));
-// difference([1,2,3], [1,2]) -> [3]
-```

snippets/drop-elements-in-array.md

@@ -0,0 +1,12 @@
+### Drop elements in array
+
+Loop through the array, using `Array.shift()` to drop the first element of the array until the returned value from the function is `true`. 
+Returns the remaining elements.
+
+```js
+const dropElements = (arr, func) => {
+  while (arr.length > 0 && !func(arr[0])) arr.shift();
+  return arr;
+};
+// dropElements([1, 2, 3, 4], n => n >= 3) -> [3,4]
+```

snippets/element-is-visible-in-viewport.md

@@ -0,0 +1,19 @@
+### Element is visible in viewport
+
+Use `Element.getBoundingClientRect()` and the `window.inner(Width|Height)` values
+to determine if a given element is visible in the viewport.
+Omit the second argument to determine if the element is entirely visible, or specify `true` to determine if
+it is partially visible.
+
+```js
+const elementIsVisibleInViewport = (el, partiallyVisible = false) => {
+  const { top, left, bottom, right } = el.getBoundingClientRect();
+  return partiallyVisible
+    ? ((top > 0 && top < innerHeight) || (bottom > 0 && bottom < innerHeight)) &&
+      ((left > 0 && left < innerWidth) || (right > 0 && right < innerWidth))
+    : top >= 0 && left >= 0 && bottom <= innerHeight && right <= innerWidth;
+};
+// e.g. 100x100 viewport and a 10x10px element at position {top: -1, left: 0, bottom: 9, right: 10}
+// elementIsVisibleInViewport(el) -> false (not fully visible)
+// elementIsVisibleInViewport(el, true) -> true (partially visible)
+```

snippets/even-or-odd-number.md

@@ -1,9 +1,9 @@
 ### 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.
+Checks whether a number is odd or even using the modulo (`%`) operator.
+Returns `true` if the number is even, `false` if the number is odd.
 
 ```js
-const isEven = num => Math.abs(num) % 2 === 0;
+const isEven = num => num % 2 === 0;
 // isEven(3) -> false
 ```

snippets/fibonacci-array-generator.md

@@ -4,7 +4,7 @@ Create an empty array of the specific length, initializing the first two values
 Use `Array.reduce()` to add values into the array, using the sum of the last two values, except for the first two.
 
 ```js
-const fibonacci = n => 
-  Array(n).fill(0).reduce((acc, val, i) => acc.concat(i > 1 ? acc[i - 1] + acc[i - 2] : i),[]);
+const fibonacci = n =>
+  Array(n).fill(0).reduce((acc, val, i) => acc.concat(i > 1 ? acc[i - 1] + acc[i - 2] : i), []);
 // fibonacci(5) -> [0,1,1,2,3]
 ```

snippets/fill-array.md

@@ -0,0 +1,10 @@
+### Fill array
+
+Use `Array.map()` to map values between `start` (inclusive) and `end` (exclusive) to `value`.
+Omit `start` to start at the first element and/or `end` to finish at the last.
+
+```js
+const fillArray = (arr, value, start = 0, end = arr.length) =>
+  arr.map((v, i) => i >= start && i < end ? value : v);
+// fillArray([1,2,3,4],'8',1,3) -> [1,'8','8',4]
+```

snippets/filter-out-non-uniqe-values-in-an-array.md

@@ -1,8 +0,0 @@
-### Filter out non-unique values in an array
-
-Use `Array.filter()` for an array containing only the unique values.
-
-```js
-const unique = arr => arr.filter(i => arr.indexOf(i) === arr.lastIndexOf(i));
-// unique([1,2,2,3,4,4,5]) -> [1,3,5]
-```

snippets/filter-out-non-unique-values-in-an-array.md

@@ -0,0 +1,8 @@
+### Filter out non-unique values in an array
+
+Use `Array.filter()` for an array containing only the unique values.
+
+```js
+const filterNonUnique = arr => arr.filter(i => arr.indexOf(i) === arr.lastIndexOf(i));
+// filterNonUnique([1,2,2,3,4,4,5]) -> [1,3,5]
+```

snippets/flatten-array-up-to-depth.md

@@ -0,0 +1,13 @@
+### Flatten array up to depth
+
+Use recursion, decrementing `depth` by 1 for each level of depth.
+Use `Array.reduce()` and `Array.concat()` to merge elements or arrays.
+Base case, for `depth` equal to `1` stops recursion.
+Omit the second element, `depth` to flatten only to a depth of `1` (single flatten).
+
+```js
+const flattenDepth = (arr, depth = 1) =>
+  depth != 1 ? arr.reduce((a, v) => a.concat(Array.isArray(v) ? flattenDepth(v, depth - 1) : v), [])
+  : arr.reduce((a, v) => a.concat(v), []);
+// flattenDepth([1,[2],[[[3],4],5]], 2) -> [1,2,[3],4,5]
+```

snippets/flatten-array.md

@@ -1,8 +1,8 @@
 ### Flatten array
 
-Use `reduce()` to get all elements inside the array and `concat()` to flatten them.
+Use `Array.reduce()` to get all elements inside the array and `concat()` to flatten them.
 
 ```js
-const flatten = arr => arr.reduce( (a, v) => a.concat(v), []);
+const flatten = arr => arr.reduce((a, v) => a.concat(v), []);
 // flatten([1,[2],3,4]) -> [1,2,3,4]
 ```

snippets/get-days-difference-between-dates.md

@@ -0,0 +1,8 @@
+### Get days difference between dates
+
+Calculate the difference (in days) between to `Date` objects.
+
+```js
+const getDaysDiffBetweenDates = (dateInitial, dateFinal) => (dateFinal - dateInitial) / (1000 * 3600 * 24);
+// getDaysDiffBetweenDates(new Date("2017-12-13"), new Date("2017-12-22")) -> 9
+```

snippets/get-max-value-from-array.md

@@ -1,6 +1,6 @@
 ### Get max value from array
 
-Use `Math.max()` combined with the spread operator (`...`) to get the minimum value in the array.
+Use `Math.max()` combined with the spread operator (`...`) to get the maximum value in the array.
 
 ```js
 const arrayMax = arr => Math.max(...arr);

snippets/get-native-type-of-value.md

@@ -0,0 +1,9 @@
+### Get native type of value
+
+Returns lower-cased constructor name of value, "undefined" or "null" if value is undefined or null
+
+```js
+const getType = v =>
+  v === undefined ? 'undefined' : v === null ? 'null' : v.constructor.name.toLowerCase();
+// getType(new Set([1,2,3])) -> "set"
+```

snippets/get-scroll-position.md

@@ -5,7 +5,7 @@ You can omit `el` to use a default value of `window`.
 
 ```js
 const getScrollPos = (el = window) =>
-  ( {x: (el.pageXOffset !== undefined) ? el.pageXOffset : el.scrollLeft,
-   y: (el.pageYOffset !== undefined) ? el.pageYOffset : el.scrollTop} );
+  ({x: (el.pageXOffset !== undefined) ? el.pageXOffset : el.scrollLeft,
+    y: (el.pageYOffset !== undefined) ? el.pageYOffset : el.scrollTop});
 // getScrollPos() -> {x: 0, y: 200}
 ```

snippets/greatest-common-divisor-(GCD).md

@@ -5,6 +5,6 @@ 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`.
 
 ```js
-const gcd = (x , y) => !y ? x : gcd(y, x % y);
+const gcd = (x, y) => !y ? x : gcd(y, x % y);
 // gcd (8, 36) -> 4
 ```

snippets/group-by.md

@@ -0,0 +1,12 @@
+### Group by
+
+Use `Array.map()` to map the values of an array to a function or property name.
+Use `Array.reduce()` to create an object, where the keys are produced from the mapped results.
+
+```js
+const groupBy = (arr, func) =>
+  arr.map(typeof func === 'function' ? func : val => val[func])
+    .reduce((acc, val, i) => { acc[val] = (acc[val] || []).concat(arr[i]); return acc; }, {});
+// groupBy([6.1, 4.2, 6.3], Math.floor) -> {4: [4.2], 6: [6.1, 6.3]}
+// groupBy(['one', 'two', 'three'], 'length') -> {3: ['one', 'two'], 5: ['three']}
+```

snippets/hamming-distance.md

@@ -0,0 +1,10 @@
+### Hamming distance
+
+Use XOR operator (`^`) to find the bit difference between the two numbers, convert to binary string using `toString(2)`.
+Count and return the number of `1`s in the string, using `match(/1/g)`.
+
+```js
+const hammingDistance = (num1, num2) =>
+  ((num1 ^ num2).toString(2).match(/1/g) || '').length;
+// hammingDistance(2,3) -> 1
+```

snippets/head-of-list.md

@@ -1,6 +1,6 @@
 ### Head of list
 
-Return `arr[0]`.
+Use `arr[0]` to return the first element of the passed array.
 
 ```js
 const head = arr => arr[0];

snippets/initial-of-list.md

@@ -1,8 +1,8 @@
 ### Initial of list
 
-Return `arr.slice(0,-1)`.
+Use `arr.slice(0,-1)`to return all but the last element of the array.
 
 ```js
-const initial = arr => arr.slice(0,-1);
+const initial = arr => arr.slice(0, -1);
 // initial([1,2,3]) -> [1,2]
 ```

snippets/initialize-array-with-range.md

@@ -1,10 +1,10 @@
 ### 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.
+Use `Array(end-start)` to create an array of the desired length, `Array.map()` to fill with the desired values in a range.
 You can omit `start` to use a default value of `0`.
 
 ```js
 const initializeArrayRange = (end, start = 0) =>
-  Array.apply(null, Array(end-start)).map( (v,i) => i + start );
+  Array.apply(null, Array(end - start)).map((v, i) => i + start);
 // initializeArrayRange(5) -> [0,1,2,3,4]
 ```

snippets/is-array.md

@@ -0,0 +1,9 @@
+### Is array
+
+Use `Array.isArray()` to check if a value is classified as an array.
+
+```js
+const isArray = val => !!val && Array.isArray(val);
+// isArray(null) -> false
+// isArray([1]) -> true
+```

snippets/is-boolean.md

@@ -0,0 +1,9 @@
+### Is boolean
+
+Use `typeof` to check if a value is classified as a boolean primitive.
+
+```js
+const isBoolean = val => typeof val === 'boolean';
+// isBoolean(null) -> false
+// isBoolean(false) -> true
+```

snippets/is-function.md

@@ -0,0 +1,9 @@
+### Is function
+
+Use `typeof` to check if a value is classified as a function primitive.
+
+```js
+const isFunction = val => val && typeof val === 'function';
+// isFunction('x') -> false
+// isFunction(x => x) -> true
+```

snippets/is-number.md

@@ -0,0 +1,9 @@
+### Is number
+
+Use `typeof` to check if a value is classified as a number primitive.
+
+```js
+const isNumber = val => typeof val === 'number';
+// isNumber('1') -> false
+// isNumber(1) -> true
+```

snippets/is-string.md

@@ -0,0 +1,9 @@
+### Is string
+
+Use `typeof` to check if a value is classified as a string primitive.
+
+```js
+const isString = val => typeof val === 'string';
+// isString(10) -> false
+// isString('10') -> true
+```

snippets/is-symbol.md

@@ -0,0 +1,9 @@
+### Is symbol
+
+Use `typeof` to check if a value is classified as a symbol primitive.
+
+```js
+const isSymbol = val => typeof val === 'symbol';
+// isSymbol('x') -> false
+// isSymbol(Symbol('x')) -> true
+```

snippets/last-of-list.md

@@ -1,6 +1,6 @@
 ### Last of list
 
-Return `arr.slice(-1)[0]`.
+Use `arr.slice(-1)[0]` to get the last element of the given array.
 
 ```js
 const last = arr => arr.slice(-1)[0];

snippets/measure-time-taken-by-function.md

@@ -1,13 +1,14 @@
 ### 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.
+Use `console.time()` and `console.timeEnd()` to measure the difference between the start and end times to determine how long the callback took to execute.
 
 ```js
-const timeTaken = (func,...args) => {
-  var t0 = performance.now(), r = func(...args);
-  console.log(performance.now() - t0);
+const timeTaken = callback => {
+  console.time('timeTaken');
+  const r = callback();
+  console.timeEnd('timeTaken');
   return r;
-}
-// timeTaken(Math.pow, 2, 10) -> 1024 (0.010000000009313226 logged in console)
+};
+// timeTaken(() => Math.pow(2, 10)) -> 1024
+// (logged): timeTaken: 0.02099609375ms
 ```

snippets/median-of-array-of-numbers.md

@@ -0,0 +1,13 @@
+### Median of array of numbers
+
+Find the middle of the array, use `Array.sort()` to sort the values.
+Return the number at the midpoint if `length` is odd, otherwise the average of the two middle numbers.
+
+```js
+const median = arr => {
+  const mid = Math.floor(arr.length / 2), nums = arr.sort((a, b) => a - b);
+  return arr.length % 2 !== 0 ? nums[mid] : (nums[mid - 1] + nums[mid]) / 2;
+};
+// median([5,6,50,1,-5]) -> 5
+// median([0,10,-2,7]) -> 3.5
+```

snippets/nth-element-of-array.md

@@ -0,0 +1,11 @@
+### Nth element of array
+
+Use `Array.slice()` to get an array containing the nth element at the first place. 
+If the index is out of bounds, return `[]`.
+Omit the second argument, `n`, to get the first element of the array.
+
+```js
+const nth = (arr, n=0) => (n>0? arr.slice(n,n+1) : arr.slice(n))[0];
+// nth(['a','b','c'],1) -> 'b'
+// nth(['a','b','b']-2) -> 'a'
+```

snippets/number-to-array-of-digits.md

@@ -0,0 +1,9 @@
+### Number to array of digits
+
+Convert the number to a string, use `split()` to convert build an array.
+Use `Array.map()` and `parseInt()` to transform each value to an integer. 
+
+```js
+const digitize = n => (''+n).split('').map(i => parseInt(i));
+// digitize(2334) -> [2, 3, 3, 4]
+```

snippets/object-from-key-value-pairs.md

@@ -3,6 +3,6 @@
 Use `Array.reduce()` to create and combine key-value pairs.
 
 ```js
-const objectFromPairs = arr => arr.reduce((a,v) => (a[v[0]] = v[1], a), {});
+const objectFromPairs = arr => arr.reduce((a, v) => (a[v[0]] = v[1], a), {});
 // objectFromPairs([['a',1],['b',2]]) -> {a: 1, b: 2}
 ```

snippets/object-to-key-value-pairs.md

@@ -0,0 +1,8 @@
+### Object to key-value pairs
+
+Use `Object.keys()` and `Array.map()` to iterate over the object's keys and produce an array with key-value pairs.
+
+```js
+const objectToPairs = obj => Object.keys(obj).map(k => [k, obj[k]]);
+// objectToPairs({a: 1, b: 2}) -> [['a',1],['b',2]])
+```

snippets/ordinal-suffix-of-number.md

@@ -0,0 +1,15 @@
+### Ordinal suffix of number
+
+Use the modulo operator (`%`) to find values of single and tens digits.
+Find which ordinal pattern digits match.
+If digit is found in teens pattern, use teens ordinal.
+
+```js
+const toOrdinalSuffix = num => {
+  const int = parseInt(num), digits = [(int % 10), (int % 100)],
+    ordinals = ['st', 'nd', 'rd', 'th'], oPattern = [1, 2, 3, 4],
+    tPattern = [11, 12, 13, 14, 15, 16, 17, 18, 19];
+  return oPattern.includes(digits[0]) && !tPattern.includes(digits[1]) ? int + ordinals[digits[0] - 1] : int + ordinals[3];
+};
+// toOrdinalSuffix("123") -> "123rd"
+```

snippets/percentile.md

@@ -0,0 +1,10 @@
+### Percentile
+
+Use `Array.reduce()` to calculate how many numbers are below the value and how many are the same value and
+apply the percentile formula.
+
+```js
+const percentile = (arr, val) => 
+  100 * arr.reduce((acc,v) => acc + (v < val ? 1 : 0) + (v === val ? 0.5 : 0), 0) / arr.length;
+// percentile([1,2,3,4,5,6,7,8,9,10], 6) -> 55
+ ```

snippets/pick.md

@@ -0,0 +1,10 @@
+### Pick
+
+Use `Array.reduce()` to convert the filtered/picked keys back to a object with the corresponding key:value pair if the key exist in the obj.
+
+```js
+const pick = (obj, arr) =>
+  arr.reduce((acc, curr) => (curr in obj && (acc[curr] = obj[curr]), acc), {});
+// pick({ 'a': 1, 'b': '2', 'c': 3 }, ['a', 'c']) -> { 'a': 1, 'c': 3 }
+// pick(object, ['a', 'c'])['a'] -> 1
+```

snippets/powerset.md

@@ -1,9 +1,9 @@
 ### Powerset
 
-Use `reduce()` combined with `map()` to iterate over elements and combine into an array containing all combinations.
+Use `Array.reduce()` combined with `Array.map()` to iterate over elements and combine into an array containing all combinations.
 
 ```js
 const powerset = arr =>
-  arr.reduce( (a,v) => a.concat(a.map( r => [v].concat(r) )), [[]]);
+  arr.reduce((a, v) => a.concat(a.map(r => [v].concat(r))), [[]]);
 // powerset([1,2]) -> [[], [1], [2], [2,1]]
 ```

snippets/promisify.md

@@ -0,0 +1,17 @@
+### Promisify
+
+Use currying to return a function returning a `Promise` that calls the original function. 
+Use the `...rest` operator to pass in all the parameters. 
+
+*In Node 8+, you can use [`util.promisify`](https://nodejs.org/api/util.html#util_util_promisify_original)*
+
+```js
+const promisify = func =>
+  (...args) =>
+    new Promise((resolve, reject) =>
+      func(...args, (err, result) =>
+        err ? reject(err) : resolve(result))
+    );
+// const delay = promisify((d, cb) => setTimeout(cb, d))
+// delay(2000).then(() => console.log('Hi!')) -> Promise resolves after 2s
+```

snippets/random-integer-in-range.md

@@ -0,0 +1,8 @@
+### Random integer in range
+
+Use `Math.random()` to generate a random number and map it to the desired range, using `Math.floor()` to make it an integer.
+
+```js
+const randomIntegerInRange = (min, max) => Math.floor(Math.random() * (max - min + 1)) + min;
+// randomIntegerInRange(0, 5) -> 2
+```

snippets/randomize-order-of-array.md

@@ -1,8 +0,0 @@
-### Randomize order of array
-
-Use `sort()` to reorder elements, utilizing `Math.random()` to randomize the sorting.
-
-```js
-const randomizeOrder = arr => arr.sort( (a,b) => Math.random() >= 0.5 ? -1 : 1);
-// randomizeOrder([1,2,3]) -> [1,3,2]
-```

snippets/round-number-to-n-digits.md

@@ -0,0 +1,9 @@
+### Round number to n digits
+
+Use `Math.round()` and template literals to round the number to the specified number of digits.
+Omit the second argument, `decimals` to round to an integer.
+
+```js
+const round = (n, decimals=0) => Number(`${Math.round(`${n}e${decimals}`)}e-${decimals}`);
+// round(1.005, 2) -> 1.01
+```

snippets/run-promises-in-series.md

@@ -0,0 +1,9 @@
+### Run promises in series
+
+Run an array of promises in series using `Array.reduce()` by creating a promise chain, where each promise returns the next promise when resolved.
+
+```js
+const series = ps => ps.reduce((p, next) => p.then(next), Promise.resolve());
+// const delay = (d) => new Promise(r => setTimeout(r, d))
+// series([() => delay(1000), () => delay(2000)]) -> executes each promise sequentially, taking a total of 3 seconds to complete
+```

snippets/scroll-to-top.md

@@ -6,10 +6,10 @@ Scroll by a fraction of the distance from top. Use `window.requestAnimationFrame
 ```js
 const scrollToTop = _ => {
   const c = document.documentElement.scrollTop || document.body.scrollTop;
-  if(c > 0) {
+  if (c > 0) {
     window.requestAnimationFrame(scrollToTop);
-    window.scrollTo(0, c - c/8);
+    window.scrollTo(0, c - c / 8);
   }
-}
+};
 // scrollToTop()
 ```

snippets/shallow-clone-object.md

@@ -0,0 +1,12 @@
+### Shallow clone object
+
+Use the object `...spread` operator to spread the properties of the target object into the clone.
+
+```js
+const shallowClone = obj => ({ ...obj });
+/*
+const a = { x: true, y: 1 };
+const b = shallowClone(a);
+a === b -> false
+*/
+```

snippets/shuffle-array-values.md

@@ -1,12 +0,0 @@
-### Shuffle array values
-
-Create an array of random values by using `Array.map()` and `Math.random()`.
-Use `Array.sort()` to sort the elements of the original array based on the random values.
-
-```js
-const shuffle = arr => {
-  let r = arr.map(Math.random);
-  return arr.sort((a,b) => r[a] - r[b]);
-}
-// shuffle([1,2,3]) -> [2, 1, 3]
-```

snippets/shuffle-array.md

@@ -0,0 +1,8 @@
+### Shuffle array
+
+Use `Array.sort()` to reorder elements, using `Math.random()` in the comparator.
+
+```js
+const shuffle = arr => arr.sort(() => Math.random() - 0.5);
+// shuffle([1,2,3]) -> [2,3,1]
+```

snippets/sleep.md

@@ -4,9 +4,11 @@ Delay executing part of an `async` function, by putting it to sleep, returning a
 
 ```js
 const sleep = ms => new Promise(resolve => setTimeout(resolve, ms));
-// async function sleepyWork() {
-//   console.log('I\'m going to sleep for 1 second.');
-//   await sleep(1000);
-//   console.log('I woke up after 1 second.');
-// }
+/*
+async function sleepyWork() {
+  console.log('I\'m going to sleep for 1 second.');
+  await sleep(1000);
+  console.log('I woke up after 1 second.');
+}
+*/
 ```

snippets/sort-characters-in-string-(alphabetical).md

@@ -1,9 +1,9 @@
 ### Sort characters in string (alphabetical)
 
-Split the string using `split('')`, `sort()` utilizing `localeCompare()`, recombine using `join('')`.
+Split the string using `split('')`, `Array.sort()` utilizing `localeCompare()`, recombine using `join('')`.
 
 ```js
 const sortCharactersInString = str =>
-  str.split('').sort( (a,b) => a.localeCompare(b) ).join('');
+  str.split('').sort((a, b) => a.localeCompare(b)).join('');
 // sortCharactersInString('cabbage') -> 'aabbceg'
 ```

snippets/speech-synthesis-(experimental).md

@@ -0,0 +1,15 @@
+### Speech synthesis (experimental)
+
+Use `SpeechSynthesisUtterance.voice` and `indow.speechSynthesis.getVoices()` to convert a message to speech.
+Use `window.speechSynthesis.speak()` to play the message.
+
+Learn more about the [SpeechSynthesisUtterance interface of the Web Speech API](https://developer.mozilla.org/en-US/docs/Web/API/SpeechSynthesisUtterance).
+
+```js
+const speak = message => {
+  const msg = new SpeechSynthesisUtterance(message);
+  msg.voice = window.speechSynthesis.getVoices()[0];
+  window.speechSynthesis.speak(msg);
+};
+// speak('Hello, World') -> plays the message
+```

snippets/standard-deviation.md

@@ -0,0 +1,17 @@
+### Standard deviation
+
+Use `Array.reduce()` to calculate the mean, variance and the sum of the variance of the values, the variance of the values, then
+determine the standard deviation.
+You can omit the second argument to get the sample standard deviation or set it to `true` to get the population standard deviation.
+
+```js
+const standardDeviation = (arr, usePopulation = false) => {
+  const mean = arr.reduce((acc, val) => acc + val, 0) / arr.length;
+  return Math.sqrt(
+    arr.reduce((acc, val) => acc.concat(Math.pow(val - mean, 2)), [])
+       .reduce((acc, val) => acc + val, 0) / (arr.length - (usePopulation ? 0 : 1))
+  );
+};
+// standardDeviation([10,2,38,23,38,23,21]) -> 13.284434142114991 (sample)
+// standardDeviation([10,2,38,23,38,23,21], true) -> 12.29899614287479 (population)
+```

snippets/sum-of-array-of-numbers.md

@@ -1,8 +1,8 @@
 ### Sum of array of numbers
 
-Use `reduce()` to add each value to an accumulator, initialized with a value of `0`.
+Use `Array.reduce()` to add each value to an accumulator, initialized with a value of `0`.
 
 ```js
-const sum = arr => arr.reduce( (acc , val) => acc + val, 0);
+const sum = arr => arr.reduce((acc, val) => acc + val, 0);
 // sum([1,2,3,4]) -> 10
 ```

snippets/tail-of-list.md

@@ -1,8 +1,9 @@
 ### Tail of list
 
-Return `arr.slice(1)`.
+Return `arr.slice(1)` if the array's `length` is more than `1`, otherwise return the whole array.
 
 ```js
-const tail = arr => arr.slice(1);
+const tail = arr => arr.length > 1 ? arr.slice(1) : arr;
 // tail([1,2,3]) -> [2,3]
+// tail([1]) -> [1]
 ```

snippets/take.md

@@ -0,0 +1,9 @@
+### Take
+
+Use `Array.slice()` to create a slice of the array with `n` elements taken from the beginning.
+
+```js
+const take = (arr, n = 1) => arr.slice(0, n);
+// take([1, 2, 3], 5) -> [1, 2, 3]
+// take([1, 2, 3], 0) -> []
+```

snippets/truncate-a-string.md

@@ -0,0 +1,10 @@
+### Truncate a String
+
+Determine if the string's `length` is greater than `num`.
+Return the string truncated to the desired length, with `...` appended to the end or the original string.
+
+```js
+const truncate = (str, num) =>
+  str.length > num ? str.slice(0, num > 3 ? num - 3 : num) + '...' : str;
+// truncate('boomerang', 7) -> 'boom...'
+```

snippets/validate-email.md

@@ -0,0 +1,10 @@
+### Validate email
+
+Use a regular experssion to check if the email is valid.
+Returns `true` if email is valid, `false` if not.
+
+```js
+const validateEmail = str =>
+  /^(([^<>()\[\]\\.,;:\s@"]+(\.[^<>()\[\]\\.,;:\s@"]+)*)|(".+"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/.test(str);
+// validateEmail(mymail@gmail.com) -> true
+```

snippets/validate-number.md

@@ -2,8 +2,9 @@
 
 Use `!isNaN` in combination with `parseFloat()` to check if the argument is a number.
 Use `isFinite()` to check if the number is finite.
+Use `Number()` to check if the coercion holds.
 
 ```js
-const validateNumber = n => !isNaN(parseFloat(n)) && isFinite(n);
+const validateNumber = n => !isNaN(parseFloat(n)) && isFinite(n) && Number(n) == n;
 // validateNumber('10') -> true
 ```

snippets/value-or-default.md

@@ -0,0 +1,8 @@
+### Value or default
+
+Returns value, or default value if passed value is `falsy`.
+
+```js
+const valueOrDefault = (value, d) => value || d;
+// valueOrDefault(NaN, 30) -> 30
+```