Disallow loops with a body that allows only one iteration (no-unreachable-loop) #1556

feross · 2020-10-21T22:10:07Z

https://eslint.org/docs/rules/no-unreachable-loop

A loop that can never reach the second iteration is a possible error in the code.

for (let i = 0; i < arr.length; i++) {
    if (arr[i].name === myName) {
        doSomething(arr[i]);
        // break was supposed to be here
    }
    break;
}

In rare cases where only one iteration (or at most one iteration) is intended behavior, the code should be refactored to use if conditionals instead of while, do-while and for loops. It's considered a best practice to avoid using loop constructs for such cases.

Rule Details

This rule aims to detect and disallow loops that can have at most one iteration, by performing static code path analysis on loop bodies.

In particular, this rule will disallow a loop with a body that exits the loop in all code paths. If all code paths in the loop's body will end with either a break, return or a throw statement, the second iteration of such loop is certainly unreachable, regardless of the loop's condition.

This rule checks while, do-while, for, for-in and for-of loops. You can optionally disable checks for each of these constructs.

Examples of incorrect code for this rule:

/*eslint no-unreachable-loop: "error"*/

while (foo) {
    doSomething(foo);
    foo = foo.parent;
    break;
}

function verifyList(head) {
    let item = head;
    do {
        if (verify(item)) {
            return true;
        } else {
            return false;
        }
    } while (item);
}

function findSomething(arr) {
    for (var i = 0; i < arr.length; i++) {
        if (isSomething(arr[i])) {
            return arr[i];
        } else {
            throw new Error("Doesn't exist.");
        }
    }
}

for (key in obj) {
    if (key.startsWith("_")) {
        break;
    }
    firstKey = key;
    firstValue = obj[key];
    break;
}

for (foo of bar) {
    if (foo.id === id) {
        doSomething(foo);
    }
    break;
}

Examples of correct code for this rule:

/*eslint no-unreachable-loop: "error"*/

while (foo) {
    doSomething(foo);
    foo = foo.parent;
}

function verifyList(head) {
    let item = head;
    do {
        if (verify(item)) {
            item = item.next;
        } else {
            return false;
        }
    } while (item);

    return true;
}

function findSomething(arr) {
    for (var i = 0; i < arr.length; i++) {
        if (isSomething(arr[i])) {
            return arr[i];
        }
    }
    throw new Error("Doesn't exist.");
}

for (key in obj) {
    if (key.startsWith("_")) {
        continue;
    }
    firstKey = key;
    firstValue = obj[key];
    break;
}

for (foo of bar) {
    if (foo.id === id) {
        doSomething(foo);
        break;
    }
}

Please note that this rule is not designed to check loop conditions, and will not warn in cases such as the following examples.

Examples of additional correct code for this rule:

/*eslint no-unreachable-loop: "error"*/

do {
    doSomething();
} while (false)

for (let i = 0; i < 1; i++) {
    doSomething(i);
}

for (const a of [1]) {
    doSomething(a);
}

The text was updated successfully, but these errors were encountered:

feross · 2020-10-21T22:25:37Z

Only 1 ecosystem failure:

    while ((entry = queue.shift()) != null) {
      setImmediate(next, null, entry)
      return
    }

IMO, this is confusing code since there is never any actual looping.

So let's enable this rule. Shipping in standard v15.

feross added this to the standard 15 milestone Oct 21, 2020

feross added the enhancement label Oct 21, 2020

feross added the accepted label Oct 21, 2020

feross closed this as completed in standard/eslint-config-standard@b964206 Oct 21, 2020

feross mentioned this issue Oct 21, 2020

Add new rules from ESLint 7.0.0 standard/eslint-config-standard#170

Closed

github-actions bot locked as resolved and limited conversation to collaborators Oct 26, 2021

mightyiam added this to The Standard Project Sep 16, 2023

mightyiam moved this to Done in The Standard Project Sep 16, 2023

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Disallow loops with a body that allows only one iteration (no-unreachable-loop) #1556

Disallow loops with a body that allows only one iteration (no-unreachable-loop) #1556

feross commented Oct 21, 2020

feross commented Oct 21, 2020

Disallow loops with a body that allows only one iteration (no-unreachable-loop) #1556

Disallow loops with a body that allows only one iteration (no-unreachable-loop) #1556

Comments

feross commented Oct 21, 2020

Rule Details

feross commented Oct 21, 2020