# Use native JS to judge prime numbers and find all prime numbers between 100 and 1000

Time：2021-3-25

### Prime number:It is also called prime number, that is, a number greater than 1 and divisible only by 1 and itself

Step idea:
1. For traverses the loop to find other divisors
2. If the setting satisfies the condition (the counter starts from 2, excluding itself), the output will be interrupted

## Demand 1: judge prime (quality) number

``````Function primenum (Num) {// declare named functions (formal parameters)
Var flag = true; // variable storage = condition true
For (VaR I = 2; I < num; I + +) {// for sets the loop body and iterates over whether there are other divisors (Prime: starting from 2, it can only be divided by 1 and itself)
if (num % i === 0) {
flag = false
If break // exists, the execution is terminated and the output is returned
}
}
Return flag // returns the true or false value of the variable
}
Var values = primenum (9) // test with the number 9
console.log(values); // false``````

## Requirement 2: find out all prime numbers between 100 and 1000

``````Var primenum = [] // the set of all prime numbers in the interval is an array
var i = 0;
Var count = 0; // calculates the number of primes
for (i = 100; i <= 1000; i++) {
var j = 0;
For (J = 2; J < I; j + +) {// prime numbers need the condition "numbers greater than 1 and divisible only by 1 and itself"
If (I% J = = = 0) {// can be divided by other numbers, then it is a non prime number and the traversal is terminated
break;
}
}
if (j >= i) { // ！ JavaScript has no "block level scope", and the variable J still exists in the "external environment" after the assignment of the for loop!
Count + +; // calculates the number of primes
primeNum.push (i) // cumulative prime number appended
}
}
console.log ('total prime '+ count +'number');
console.log (primenum); // output target result array``````

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