JS Fundamentals - while

Question 1

example

Answer 1

let i = 3;
while (i) { // when i becomes 0, the condition becomes falsy, and the loop stops
  alert( i );
  i--;
}

Question 2

Inline variable declaration

Answer 2

Here, the “counter” variable i is declared right in the loop. This is called an “inline” variable declaration. Such variables are visible only inside the loop.

for (let i = 0; i < 3; i++) {
  alert(i); // 0, 1, 2
}
alert(i); // error, no such variable

Question 3

Skipping parts

Answer 3

Any part of for can be skipped.

For example, we can omit begin if we don’t need to do anything at the loop start.

let i = 0; // we have i already declared and assigned

for (; i < 3; i++) { // no need for “begin”
alert( i ); // 0, 1, 2
}

Question 4

We can also remove the step part

Answer 4

We can also remove the step part:

let i = 0;

for (; i < 3;) {
alert( i++ );
}

Question 5

We can actually remove everything, creating an infinite loop:

Answer 5

for (;;) {
  // repeats without limits
}

Question 6

when do we need break

Answer 6

The combination “infinite loop + break as needed” is great for situations when a loop’s condition must be checked not in the beginning or end of the loop, but in the middle or even in several places of its body.

Question 7

continue

Answer 7

The continue directive is a “lighter version” of break. It doesn’t stop the whole loop. Instead, it stops the current iteration and forces the loop to start a new one (if the condition allows).

Question 8

No break/continue to the right side of ‘?’
Please note that syntax constructs that are not expressions cannot be used with the ternary operator ?. In particular, directives such as break/continue aren’t allowed there.

Answer 8

(i > 5) ? alert(i) : continue; // continue isn’t allowed here
…it stops working: there’s a syntax error.

This is just another reason not to use the question mark operator ? instead of if

Question 9

we can use break and continue with labels

Answer 9

to break a particular loop or continue with the next iteration of a particular loop

Question 10

labels can not be used to jump anywhere

Answer 10

Labels do not allow us to jump into an arbitrary place in the code.

For example, it is impossible to do this:

break label; // doesn’t jumps to the label below

label: for (…)
A call to break/continue is only possible from inside a loop and the label must be somewhere above the directive.