for-direction

Enforce for loop update clause moving the counter in the right direction

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.

Rule Details

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.

Examples of incorrect code for this rule:

/*eslint for-direction: "error"*/
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) {
}
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Examples of correct code for this rule:

/*eslint for-direction: "error"*/
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
}
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Version

This rule was introduced in ESLint v4.0.0-beta.0.

Resources

• Rule source
• Tests source