https://www.freecodecamp.org/learn/javascript-algorithms-and-data-structures/intermediate-algorithm-scripting/sum-all-primes

Sum All Primes

A prime number is a whole number greater than 1 with exactly two divisors: 1 and itself. For example, 2 is a prime number because it is only divisible by 1 and 2. In contrast, 4 is not prime since it is divisible by 1, 2 and 4.

Rewrite sumPrimes so it returns the sum of all prime numbers that are less than or equal to num.

注意1不是质数, 2是质数

sumPrimes(10) should return a number.

Passed

sumPrimes(10) should return 17.

Passed

sumPrimes(977) should return 73156.

  1. function sumPrimes(num) {
  2.   let map = {}
  3.   function isPrime(x) {
  4.     for (let i = 2; i < x; i++) {
  5.       if (map.hasOwnProperty(x)) {
  6.         return map[x]
  7.       }
  8.       if (x % i === 0) {
  9.         map[x] = false
  10.         return map[x]
  11.       }
  12.     }
  13.     map[x] = true
  14.     return map[x]
  15.   }
  17.   let prime = []
  18.   for (let i = 2; i <= num; i++) {
  19.     if (isPrime(i)) {
  20.       prime.push(i)
  21.     }
  22.   }
  24.   console.log(prime)
  25.   return prime.reduce((x, y) => x + y);
  26. }
  28. console.log(
  30.   sumPrimes(10),
  31.   sumPrimes(11)
  32. )

hint

https://forum.freecodecamp.org/t/freecodecamp-challenge-guide-sum-all-primes/16085

函数return掉就不会继续执行

关于every

  1. console.log([].every(() => {})) //true
  1. function sumPrimes(num) {
  2.   // Check all numbers for primality
  3.   let primes = [];
  4.   for (let i = 2; i <= num; i++) {
  5.     if (primes.every((prime) => i % prime !== 0))
  6.       primes.push(i);
  7.   }
  8.   return primes.reduce((sum, prime) => sum + prime, 0);
  9. }