guard-for-in.md

title	layout	edit_link	rule_type	related_rules	further_reading
guard-for-in	doc	https://github.com/eslint/eslint/edit/main/docs/src/rules/guard-for-in.md	suggestion	no-prototype-builtins	https://javascriptweblog.wordpress.com/2011/01/04/exploring-javascript-for-in-loops/ https://2ality.com/2012/01/objects-as-maps.html

Looping over objects with a for in loop will include properties that are inherited through the prototype chain. This behavior can lead to unexpected items in your for loop.

for (key in foo) {
    doSomething(key);
}

Note that simply checking foo.hasOwnProperty(key) is likely to cause an error in some cases; see no-prototype-builtins.

Rule Details

This rule is aimed at preventing unexpected behavior that could arise from using a for in loop without filtering the results in the loop. As such, it will warn when for in loops do not filter their results with an if statement.

Examples of incorrect code for this rule:

::: incorrect

/*eslint guard-for-in: "error"*/

for (key in foo) {
    doSomething(key);
}

:::

Examples of correct code for this rule:

::: correct

/*eslint guard-for-in: "error"*/

for (key in foo) {
    if (Object.prototype.hasOwnProperty.call(foo, key)) {
        doSomething(key);
    }
}

for (key in foo) {
    if ({}.hasOwnProperty.call(foo, key)) {
        doSomething(key);
    }
}

:::