CODEWITHSUNDEEP
javascript
Winnipass
Winnipass

Reputation: 950

Javascript For Loop Causes Unexpected Behaviour

Code Sample A

var isPrime = function (num) {
    //for(i = 1; i <= num; i++) {}
    if(num === 2){
      return true;
    }
    return false;
};


var sumPrimesUpTo = function (num) {
    
    for(i = 1; i <= num; i++) {
       console.log(isPrime(i));
    }
    
};

sumPrimesUpTo(5);

So we have a function that supposedly checks for a prime number and a second function that sums prime numbers up to a provided number. When you run this it outputs.

false
true
false
false
false

Now we have the same code sample but with the for loop in isPrime uncommented.

var isPrime = function (num) {
    for(i = 1; i <= num; i++) {}
    if(num === 2){
      return true;
    }
    return false;
};


var sumPrimesUpTo = function (num) {

    for(i = 1; i <= num; i++) {
       console.log(isPrime(i));
    }
   
};

sumPrimesUpTo(5);

This outputs

false
false
false

I don't seem to get why the for loop changes the output?

Upvotes: 0

Views: 54

Answers (1)

zmf
zmf

Reputation: 9303

i is scoped globally, you should use let to keep i scoped to the for loop.

You could use var as well but let is a better choice because it will give you the scope you want and also not allow it to be re-assigned in this block.

var isPrime = function (num) {
    for(let i = 1; i <= num; i++) {}
    if(num === 2){
      return true;
    }
    return false;
};


var sumPrimesUpTo = function (num) {

    for(let i = 1; i <= num; i++) {
       console.log(isPrime(i));
    }

};

sumPrimesUpTo(5);

Upvotes: 5

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