eslint/for-direction Correctness

✅ This rule is turned on by default.

⚠️🛠️️ A dangerous auto-fix is available for this rule.

What it does

Disallow for loop update causing the counter to move in the wrong direction.

Why is this bad?

A for loop with a stop condition that can never be reached, such as one with a counter that moves in the wrong direction, will run infinitely. While there are occasions when an infinite loop is intended, the convention is to construct such loops as while loops. More typically, an infinite for loop is a bug.

This rule forbids for loops where the counter variable changes in such a way that the stop condition will never be met. For example, if the counter variable is increasing (i.e. i++``) and the stop condition tests that the counter is greater than zero (i >= 0``) then the loop will never exit.

Example

Examples of incorrect code for this rule:

for (var i = 0; i < 10; i--) {}

for (var i = 10; i >= 0; i++) {}

for (var i = 0; i > 10; i++) {}

for (var i = 0; 10 > i; i--) {}

const n = -2;
for (let i = 0; i < 10; i += n) {}

Examples of correct code for this rule:

for (var i = 0; i < 10; i++) {}

for (var i = 0; 10 > i; i++) {
  // with counter "i" on the right
}

for (let i = 10; i >= 0; i += this.step) {
  // direction unknown
}

for (let i = MIN; i <= MAX; i -= 0) {
  // not increasing or decreasing
}

References

• Rule Source