for loop without condition checking

Question

I was reading through this question, I am not able to grasp the concept used for the 'for loop'

Generally, syntax of for loop is for(assign value, check condition, increment){} They have used the for loop but there is no condition checking, how does this work?

for(var i = arr1.length; i--;) {
 if(arr1[i] !== arr2[i])
 return false;
}

• Rajesh Dixit: Above code is checking if any 2 values are different, the array is not identical and breaks the loop using return false
• Harsh Patel: because there is explicitly condition checking inside the for loop. If the condition gets false then it will return false.
• prasanth: you could use array function like Array#filter it's helpful to find the match
• Rajesh Dixit: @prasad No. Filter will loop till end. This is the best solution
• Deepika Rao: What I meant was, is it okay to use for loop without the second condtion - should we not write - for(var i = arr1.length;" leave this blank";i--;){}

Answer 1

Actually, it's

for ([initialization]; [condition]; [final-expression])

where all three expressions are optional.

In this case, i-- is a condition, when it reaches 0 it's falsy, and the loop stops.
The "final-expression" is the one not used here.

var arr = [1, 2, 3, 4];
for (var i = arr.length; i--;) {
 console.log(arr[i]);
}

The for statement sets the value of i to a positive integer, then on each iteration it evaluates the condition, effectively decrementing i until it reaches a number that is falsy, which happens when i reaches zero.

---

Here's some other examples of how to skip expressions in a for loop

Skipping the initialization

var i = 0;
for (; i < 4; i++) {
 console.log(i);
}

Skipping the condition

for (var i = 0;; i++) {
 console.log(i);
 if (i > 3) break;
}

Skipping everything

var i = 0;
for (;;) {
 if (i > 4) break;
 console.log(i);
 i++;
}

• flakes: @DeepikaRao They're all technically optional depending on what you want to do. for (;;); will compile, but probably won't be something you want

Answer 2

See the code carefully

for(var i = arr1.length; i--;) {
 if(arr1[i] !== arr2[i])
 return false;
}

Whenever the browser encounters the return statement the control is moved outside the for loop and even outside the function containing this for loop. So, the loops breaks without having a condition when the condition arr1[i] !== arr2[i] is satisfied.

Answer 3

All the three arguments in a for loop is optional.

Here the loop iterates with i value changing from arr1.length -> 1. Decrementing i is done in the second argument itself. Since the second argument is for condition checking it will return a falsy value when i becomes zero, and the iteration stops. If the arrays don't match the loop will return false in the middle.