What is the mechanism that guides a reverse while loop to determine its endpoint in JavaScript?

Question

What is the mechanism that guides a reverse while loop to determine its endpoint in JavaScript?

Lately in my exploration of JavaScript, I've discovered that reverse while loops are more efficient.

They are often written in this manner:

var i = someArray.length;

while (i--) {
  console.log(someArray[i]);
}

I decided to test it out and noticed that it stops once it reaches the end of the array.

But how does it actually know when to stop after going through the entire array?

javascript arrays loops while-loop

Answer 1

Answer №1

When using a while loop, the expression within the parentheses is evaluated each time the loop runs. Once this expression evaluates to a falsey value, the loop will terminate.

Some examples of falsey values include:

false 
0  
undefined  
NaN
null
""

In a specific case where the value of i is being decremented with each iteration, the loop will continue until i reaches the value of 0, at which point it will stop. Since it's a post-decrement operator, the comparison happens before the decrement operation. Thus, the inner loop will access values of i ranging from someArray.length - 1 down to 0, inclusive, which corresponds to all the indexes in that array.

Your code snippet:

var i = someArray.length;

while (i--) {
    console.log(someArray[i]);
}

produces the same result as this equivalent code:

for (var i = someArray.length - 1; i >= 0; i--) {
    console.log(someArray[i]);
}

Answer 2

When using a while loop, the expression within the parentheses is evaluated each time the loop runs. Once this expression evaluates to a falsey value, the loop will terminate.

Some examples of falsey values include:

false 
0  
undefined  
NaN
null
""

In a specific case where the value of i is being decremented with each iteration, the loop will continue until i reaches the value of 0, at which point it will stop. Since it's a post-decrement operator, the comparison happens before the decrement operation. Thus, the inner loop will access values of i ranging from someArray.length - 1 down to 0, inclusive, which corresponds to all the indexes in that array.

Your code snippet:

var i = someArray.length;

while (i--) {
    console.log(someArray[i]);
}

produces the same result as this equivalent code:

for (var i = someArray.length - 1; i >= 0; i--) {
    console.log(someArray[i]);
}

Answer 3

Answer №2

As the value of i-- approaches 0, the loop will come to an end.

Once the condition within the while ( ... ) statement is no longer true, the loop will terminate.
Given that i starts as a positive number and decreases with each iteration, it is inevitable that it will eventually reach zero.

In a boolean context, zero equates to false, ensuring that the loop will not continue indefinitely.
Since i-- decrements the value after returning it, the final value of i when the loop stops is actually -1.

Answer 4

As the value of i-- approaches 0, the loop will come to an end.

Once the condition within the while ( ... ) statement is no longer true, the loop will terminate.
Given that i starts as a positive number and decreases with each iteration, it is inevitable that it will eventually reach zero.

In a boolean context, zero equates to false, ensuring that the loop will not continue indefinitely.
Since i-- decrements the value after returning it, the final value of i when the loop stops is actually -1.

Answer 5

Answer №3

As the loop iterates, the condition i-- is evaluated. The operator -- instructs Javascript to decrease the value of the variable i by 1 each time. When i eventually reaches 0, it evaluates to false, prompting the loop to terminate.

Answer 6

As the loop iterates, the condition i-- is evaluated. The operator -- instructs Javascript to decrease the value of the variable i by 1 each time. When i eventually reaches 0, it evaluates to false, prompting the loop to terminate.

Answer 7

Answer №4

Guessing blindly, the reason may be that 0 is considered false in a boolean expression. 0 == true results in false 0 == false results in true

If you try starting at -1, the loop will continue indefinitely.

Answer 8

Guessing blindly, the reason may be that 0 is considered false in a boolean expression. 0 == true results in false 0 == false results in true

If you try starting at -1, the loop will continue indefinitely.

Answer 9

Answer №5

In a while loop, the evaluation stops once it encounters a falsy value. In this case, 0 is considered a falsy value and is the first index of the array. The reason why the for loop continues to iterate even when "i" is 0 is because the return value of the decrement expression actually reflects the value of the variable before the operation.

let i = 10;
let j = i--

console.log(i) // 9
console.log(j) // 10

Therefore, when your while loop reaches 1 and then the condition in the for loop decrements "--" the i variable, the return value remains 1 which is truthy. However, the actual value of i when accessed by the rest of your loop is 0.

Answer 10

In a while loop, the evaluation stops once it encounters a falsy value. In this case, 0 is considered a falsy value and is the first index of the array. The reason why the for loop continues to iterate even when "i" is 0 is because the return value of the decrement expression actually reflects the value of the variable before the operation.

let i = 10;
let j = i--

console.log(i) // 9
console.log(j) // 10

Therefore, when your while loop reaches 1 and then the condition in the for loop decrements "--" the i variable, the return value remains 1 which is truthy. However, the actual value of i when accessed by the rest of your loop is 0.

What is the mechanism that guides a reverse while loop to determine its endpoint in JavaScript?

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

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