guard-for-in
Require for-in loops to include an if statement
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);
}
For codebases that do not support ES2022, Object.prototype.hasOwnProperty.call(foo, key) can be used as a check that the property is not inherited.
For codebases that do support ES2022, Object.hasOwn(foo, key) can be used as a shorter alternative; see prefer-object-has-own.
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:
Open in Playground
/*eslint guard-for-in: "error"*/
for (key in foo) {
doSomething(key);
}
Examples of correct code for this rule:
Open in Playground
/*eslint guard-for-in: "error"*/
for (key in foo) {
if (Object.hasOwn(foo, key)) {
doSomething(key);
}
}
for (key in foo) {
if (Object.prototype.hasOwnProperty.call(foo, key)) {
doSomething(key);
}
}
for (key in foo) {
if ({}.hasOwnProperty.call(foo, key)) {
doSomething(key);
}
}
Related Rules
Version
This rule was introduced in ESLint v0.0.6.
Further Reading
