30 seconds of code

Math

approximatelyEqual

Checks if two numbers are approximately equal to each other.

Use Math.abs() to compare the absolute difference of the two values to epsilon. Omit the third parameter, epsilon, to use a default value of 0.001.

检查两个数是否近似相等。

使用 Math.abs() 将两个值的绝对差值与 epsilon 进行比较。省略第三个参数 epsilon，使用默认值0.001。

const approximatelyEqual = (v1, v2, epsilon = 0.001) => Math.abs(v1 - v2) < epsilon;

示例代码

approximatelyEqual(Math.PI / 2.0, 1.5708); // true

average

返回两个或更多数字的平均值。

使用 array. prototype.reduce() 将每个值累加，初始值为 0，然后除以数组的 length。

const average = (...nums) => nums.reduce((acc, val) => acc + val, 0) / nums.length;

示例代码

average(...[1, 2, 3]); // 2
average(1, 2, 3); // 2

averageBy

使用提供的函数将每个元素映射到一个值后，返回数组的平均值。

使用 Array.prototype.map() 将每个元素映射到 fn 函数的返回值，使用 array. prototype.reduce() 将每个值累加，初始值为 0，然后除以数组的 length。

const averageBy = (arr, fn) =>
  arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val) => acc + val, 0) /
  arr.length;

示例代码

averageBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], o => o.n); // 5
averageBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 5

binomialCoefficient

Evaluates the binomial coefficient of two integers n and k.

Use Number.isNaN() to check if any of the two values is NaN. Check if k is less than 0, greater than or equal to n, equal to 1 or n - 1 and return the appropriate result. Check if n - k is less than k and switch their values accordingly. Loop from 2 through k and calculate the binomial coefficient. Use Math.round() to account for rounding errors in the calculation.

const binomialCoefficient = (n, k) => {
  if (Number.isNaN(n) || Number.isNaN(k)) return NaN;
  if (k < 0 || k > n) return 0;
  if (k === 0 || k === n) return 1;
  if (k === 1 || k === n - 1) return n;
  if (n - k < k) k = n - k;
  let res = n;
  for (let j = 2; j <= k; j++) res *= (n - j + 1) / j;
  return Math.round(res);
};

示例代码

binomialCoefficient(8, 2); // 28

clampNumber

Clamps num within the inclusive range specified by the boundary values a and b.

If num falls within the range, return num. Otherwise, return the nearest number in the range.

将num夹在由边值a和b指定的包含范围内。

在边界值“a”和“b”指定的包含范围内钳位num。

返回由边界值 a 和 b 限定范围内的 num 。

如果 num 在限定范围内，则返回 num。否则，返回范围内最接近 num 的数字。

const clampNumber = (num, a, b) => Math.max(Math.min(num, Math.max(a, b)), Math.min(a, b));

示例代码

clampNumber(2, 3, 5); // 3
clampNumber(1, -1, -5); // -1

degreesToRads

Converts an angle from degrees to radians.

Use Math.PI and the degree to radian formula to convert the angle from degrees to radians.

const degreesToRads = deg => (deg * Math.PI) / 180.0;

示例代码

degreesToRads(90.0); // ~1.5708

digitize

Converts a number to an array of digits.

Convert the number to a string, using the spread operator (...) to build an array. Use Array.prototype.map() and parseInt() to transform each value to an integer.

const digitize = n => [...`${n}`].map(i => parseInt(i));

示例代码

digitize(123); // [1, 2, 3]

distance

Returns the 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);

示例代码

distance(1, 1, 2, 3); // 2.23606797749979

elo

Computes the new ratings between two or more opponents using the Elo rating system. It takes an array of pre-ratings and returns an array containing post-ratings. The array should be ordered from best performer to worst performer (winner -> loser).

Use the exponent ** operator and math operators to compute the expected score (chance of winning). of each opponent and compute the new rating for each. Loop through the ratings, using each permutation to compute the post-Elo rating for each player in a pairwise fashion. Omit the second argument to use the default kFactor of 32.

const elo = ([...ratings], kFactor = 32, selfRating) => {
  const [a, b] = ratings;
  const expectedScore = (self, opponent) => 1 / (1 + 10 ** ((opponent - self) / 400));
  const newRating = (rating, i) =>
    (selfRating || rating) + kFactor * (i - expectedScore(i ? a : b, i ? b : a));
  if (ratings.length === 2) return [newRating(a, 1), newRating(b, 0)];

  for (let i = 0, len = ratings.length; i < len; i++) {
    let j = i;
    while (j < len - 1) {
      j++;
      [ratings[i], ratings[j]] = elo([ratings[i], ratings[j]], kFactor);
    }
  }
  return ratings;
};

示例代码

// Standard 1v1s
elo([1200, 1200]); // [1216, 1184]
elo([1200, 1200], 64); // [1232, 1168]
// 4 player FFA, all same rank
elo([1200, 1200, 1200, 1200]).map(Math.round); // [1246, 1215, 1185, 1154]
/*
For teams, each rating can adjusted based on own team's average rating vs.
average rating of opposing team, with the score being added to their
own individual rating by supplying it as the third argument.
*/

factorial

Calculates the factorial of a number.

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. Throws an exception if n is a negative number.

const factorial = n =>
  n < 0
    ? (() => {
      throw new TypeError('Negative numbers are not allowed!');
    })()
    : n <= 1
      ? 1
      : n * factorial(n - 1);

示例代码

factorial(6); // 720

fibonacci

Generates an array, containing the Fibonacci sequence, up until the nth term.

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

const fibonacci = n =>
  Array.from({ length: n }).reduce(
    (acc, val, i) => acc.concat(i > 1 ? acc[i - 1] + acc[i - 2] : i),
    []
  );

示例代码

fibonacci(6); // [0, 1, 1, 2, 3, 5]

gcd

Calculates the greatest common divisor between two or more numbers/arrays.

The inner _gcd function uses 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.

const gcd = (...arr) => {
  const _gcd = (x, y) => (!y ? x : gcd(y, x % y));
  return [...arr].reduce((a, b) => _gcd(a, b));
};

示例代码

gcd(8, 36); // 4
gcd(...[12, 8, 32]); // 4

geometricProgression

Initializes an array containing the numbers in the specified range where start and end are inclusive and the ratio between two terms is step. Returns an error if step equals 1.

Use Array.from(), Math.log() and Math.floor() to create an array of the desired length, Array.prototype.map() to fill with the desired values in a range. Omit the second argument, start, to use a default value of 1. Omit the third argument, step, to use a default value of 2.

const geometricProgression = (end, start = 1, step = 2) =>
  Array.from({ length: Math.floor(Math.log(end / start) / Math.log(step)) + 1 }).map(
    (v, i) => start * step ** i
  );

示例代码

geometricProgression(256); // [1, 2, 4, 8, 16, 32, 64, 128, 256]
geometricProgression(256, 3); // [3, 6, 12, 24, 48, 96, 192]
geometricProgression(256, 1, 4); // [1, 4, 16, 64, 256]

hammingDistance

Calculates the Hamming distance between two values.

Use XOR operator (^) to find the bit difference between the two numbers, convert to a binary string using toString(2). Count and return the number of 1s in the string, using match(/1/g).

const hammingDistance = (num1, num2) => ((num1 ^ num2).toString(2).match(/1/g) || '').length;

示例代码

hammingDistance(2, 3); // 1

inRange

Checks if the given number falls within the given range.

Use arithmetic comparison to check if the given number is in the specified range. If the second parameter, end, is not specified, the range is considered to be from 0 to start.

const inRange = (n, start, end = null) => {
  if (end && start > end) [end, start] = [start, end];
  return end == null ? n >= 0 && n < start : n >= start && n < end;
};

示例代码

inRange(3, 2, 5); // true
inRange(3, 4); // true
inRange(2, 3, 5); // false
inRange(3, 2); // false

isDivisible

Checks if the first numeric argument is divisible by the second one.

Use the modulo operator (%) to check if the remainder is equal to 0.

const isDivisible = (dividend, divisor) => dividend % divisor === 0;

示例代码

isDivisible(6, 3); // true

isEven

Returns true if the given number is even, false otherwise.

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.

const isEven = num => num % 2 === 0;

示例代码

isEven(3); // false

isNegativeZero

Checks if the given value is equal to negative zero (-0).

Checks whether a passed value is equal to 0 and if 1 divided by the value equals -Infinity.

const isNegativeZero = val => val === 0 && 1 / val === -Infinity;

示例代码

isNegativeZero(-0); // true
isNegativeZero(0); // false

isPrime

Checks if the provided integer is a prime number.

Check numbers from 2 to the square root of the given number. Return false if any of them divides the given number, else return true, unless the number is less than 2.

const isPrime = num => {
  const boundary = Math.floor(Math.sqrt(num));
  for (var i = 2; i <= boundary; i++) if (num % i === 0) return false;
  return num >= 2;
};

示例代码

isPrime(11); // true

lcm

Returns the least common multiple of two or more numbers.

Use the greatest common divisor (GCD) formula and the fact that lcm(x,y) = x * y / gcd(x,y) to determine the least common multiple. The GCD formula uses recursion.

const lcm = (...arr) => {
  const gcd = (x, y) => (!y ? x : gcd(y, x % y));
  const _lcm = (x, y) => (x * y) / gcd(x, y);
  return [...arr].reduce((a, b) => _lcm(a, b));
};

示例代码

lcm(12, 7); // 84
lcm(...[1, 3, 4, 5]); // 60

luhnCheck

Implementation of the Luhn Algorithm used to validate a variety of identification numbers, such as credit card numbers, IMEI numbers, National Provider Identifier numbers etc.

Use String.prototype.split(''), Array.prototype.reverse() and Array.prototype.map() in combination with parseInt() to obtain an array of digits. Use Array.prototype.splice(0,1) to obtain the last digit. Use Array.prototype.reduce() to implement the Luhn Algorithm. Return true if sum is divisible by 10, false otherwise.

const luhnCheck = num => {
  let arr = (num + '')
    .split('')
    .reverse()
    .map(x => parseInt(x));
  let lastDigit = arr.splice(0, 1)[0];
  let sum = arr.reduce((acc, val, i) => (i % 2 !== 0 ? acc + val : acc + ((val * 2) % 9) || 9), 0);
  sum += lastDigit;
  return sum % 10 === 0;
};

示例代码

luhnCheck('4485275742308327'); // true
luhnCheck(6011329933655299); //  false
luhnCheck(123456789); // false

mapNumRange

Maps a number from one range to another range.

Returns num mapped between outMin-outMax from inMin-inMax.

const mapNumRange = (num, inMin, inMax, outMin, outMax) =>
  ((num - inMin) * (outMax - outMin)) / (inMax - inMin) + outMin;

示例代码

mapNumRange(5, 0, 10, 0, 100); // 50

Recommended Resource - JavaScript: From Fundamentals to Functional JS

Learn higher-order functions, closures, scope, master key functional methods like map, reduce and filter and promises and ES6+ asynchronous JavaScript.

maxBy

Returns the maximum value of an array, after mapping each element to a value using the provided function.

Use Array.prototype.map() to map each element to the value returned by fn, Math.max() to get the maximum value.

const maxBy = (arr, fn) => Math.max(...arr.map(typeof fn === 'function' ? fn : val => val[fn]));

示例代码

maxBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], o => o.n); // 8
maxBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 8

median

Returns the median of an array of numbers.

Find the middle of the array, use Array.prototype.sort() to sort the values. Return the number at the midpoint if length is odd, otherwise the average of the two middle numbers.

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;
};

示例代码

median([5, 6, 50, 1, -5]); // 5

midpoint

Calculates the midpoint between two pairs of (x,y) points.

Destructure the array to get x1, y1, x2 and y2, calculate the midpoint for each dimension by dividing the sum of the two endpoints by 2.

const midpoint = ([x1, y1], [x2, y2]) => [(x1 + x2) / 2, (y1 + y2) / 2];

示例代码

midpoint([2, 2], [4, 4]); // [3, 3]
midpoint([4, 4], [6, 6]); // [5, 5]
midpoint([1, 3], [2, 4]); // [1.5, 3.5]

minBy

Returns the minimum value of an array, after mapping each element to a value using the provided function.

Use Array.prototype.map() to map each element to the value returned by fn, Math.min() to get the minimum value.

const minBy = (arr, fn) => Math.min(...arr.map(typeof fn === 'function' ? fn : val => val[fn]));

示例代码

minBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], o => o.n); // 2
minBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 2

percentile

Uses the percentile formula to calculate how many numbers in the given array are less or equal to the given value.

Use Array.prototype.reduce() to calculate how many numbers are below the value and how many are the same value and apply the percentile formula.

const percentile = (arr, val) =>
  (100 * arr.reduce((acc, v) => acc + (v < val ? 1 : 0) + (v === val ? 0.5 : 0), 0)) / arr.length;

示例代码

percentile([1, 2, 3, 4, 5, 6, 7, 8, 9, 10], 6); // 55

powerset

Returns the powerset of a given array of numbers.

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

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

示例代码

powerset([1, 2]); // [[], [1], [2], [2, 1]]

primes

Generates primes up to a given number, using the Sieve of Eratosthenes.

Generate an array from 2 to the given number. Use Array.prototype.filter() to filter out the values divisible by any number from 2 to the square root of the provided number.

const primes = num => {
  let arr = Array.from({ length: num - 1 }).map((x, i) => i + 2),
    sqroot = Math.floor(Math.sqrt(num)),
    numsTillSqroot = Array.from({ length: sqroot - 1 }).map((x, i) => i + 2);
  numsTillSqroot.forEach(x => (arr = arr.filter(y => y % x !== 0 || y === x)));
  return arr;
};

示例代码

primes(10); // [2,3,5,7]

radsToDegrees

Converts an angle from radians to degrees.

Use Math.PI and the radian to degree formula to convert the angle from radians to degrees.

const radsToDegrees = rad => (rad * 180.0) / Math.PI;

示例代码

radsToDegrees(Math.PI / 2); // 90

randomIntArrayInRange

Returns an array of n random integers in the specified range.

Use Array.from() to create an empty array of the specific length, Math.random() to generate a random number and map it to the desired range, using Math.floor() to make it an integer.

const randomIntArrayInRange = (min, max, n = 1) =>
  Array.from({ length: n }, () => Math.floor(Math.random() * (max - min + 1)) + min);

示例代码

randomIntArrayInRange(12, 35, 10); // [ 34, 14, 27, 17, 30, 27, 20, 26, 21, 14 ]

randomIntegerInRange

Returns a random integer in the specified range.

Use Math.random() to generate a random number and map it to the desired range, using Math.floor() to make it an integer.

const randomIntegerInRange = (min, max) => Math.floor(Math.random() * (max - min + 1)) + min;

示例代码

randomIntegerInRange(0, 5); // 2

randomNumberInRange

Returns a random number in the specified range.

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

const randomNumberInRange = (min, max) => Math.random() * (max - min) + min;

示例代码

randomNumberInRange(2, 10); // 6.0211363285087005

round

Rounds a number to a specified amount of 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.

const round = (n, decimals = 0) => Number(`${Math.round(`${n}e${decimals}`)}e-${decimals}`);

示例代码

round(1.005, 2); // 1.01

sdbm

Hashes the input string into a whole number.

Use String.prototype.split('') and Array.prototype.reduce() to create a hash of the input string, utilizing bit shifting.

const sdbm = str => {
  let arr = str.split('');
  return arr.reduce(
    (hashCode, currentVal) =>
      (hashCode = currentVal.charCodeAt(0) + (hashCode << 6) + (hashCode << 16) - hashCode),
    0
  );
};

示例代码

sdbm('name'); // -3521204949

standardDeviation

Returns the standard deviation of an array of numbers.

Use Array.prototype.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.

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((val - mean) ** 2), []).reduce((acc, val) => acc + val, 0) /
      (arr.length - (usePopulation ? 0 : 1))
  );
};

示例代码

standardDeviation([10, 2, 38, 23, 38, 23, 21]); // 13.284434142114991 (sample)
standardDeviation([10, 2, 38, 23, 38, 23, 21], true); // 12.29899614287479 (population)

sum

Returns the sum of two or more numbers/arrays.

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

const sum = (...arr) => [...arr].reduce((acc, val) => acc + val, 0);

示例代码

sum(1, 2, 3, 4); // 10
sum(...[1, 2, 3, 4]); // 10

sumBy

Returns the sum of an array, after mapping each element to a value using the provided function.

Use Array.prototype.map() to map each element to the value returned by fn, Array.prototype.reduce() to add each value to an accumulator, initialized with a value of 0.

const sumBy = (arr, fn) =>
  arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val) => acc + val, 0);

示例代码

sumBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], o => o.n); // 20
sumBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 20

sumPower

Returns the sum of the powers of all the numbers from start to end (both inclusive).

Use Array.prototype.fill() to create an array of all the numbers in the target range, Array.prototype.map() and the exponent operator (**) to raise them to power and Array.prototype.reduce() to add them together. Omit the second argument, power, to use a default power of 2. Omit the third argument, start, to use a default starting value of 1.

const sumPower = (end, power = 2, start = 1) =>
  Array(end + 1 - start)
    .fill(0)
    .map((x, i) => (i + start) ** power)
    .reduce((a, b) => a + b, 0);

示例代码

sumPower(10); // 385
sumPower(10, 3); // 3025
sumPower(10, 3, 5); // 2925

toSafeInteger

Converts a value to a safe integer.

Use Math.max() and Math.min() to find the closest safe value. Use Math.round() to convert to an integer.

const toSafeInteger = num =>
  Math.round(Math.max(Math.min(num, Number.MAX_SAFE_INTEGER), Number.MIN_SAFE_INTEGER));

示例代码

toSafeInteger('3.2'); // 3
toSafeInteger(Infinity); // 9007199254740991

vectorDistance

Returns the distance between two vectors.

Use Array.prototype.reduce(), Math.pow() and Math.sqrt() to calculate the Euclidean distance between two vectors.

const vectorDistance = (...coords) => {
  let pointLength = Math.trunc(coords.length / 2);
  let sum = coords
    .slice(0, pointLength)
    .reduce((acc, val, i) => acc + Math.pow(val - coords[pointLength + i], 2), 0);
  return Math.sqrt(sum);
};

示例代码

vectorDistance(10, 0, 5, 20, 0, 10); // 11.180339887498949