docs/src/rules/array-callback-return.md
Array has several methods for filtering, mapping, and folding.
If we forget to write return statement in a callback of those, it's probably a mistake. If you don't want to use a return or don't need the returned results, consider using .forEach instead.
// example: convert ['a', 'b', 'c'] --> {a: 0, b: 1, c: 2}
const indexMap = myArray.reduce(function(memo, item, index) {
memo[item] = index;
}, {}); // Error: cannot set property 'b' of undefined
This rule enforces usage of return statement in callbacks of array's methods.
Additionally, it may also enforce the forEach array method callback to not return a value by using the checkForEach option.
This rule finds callback functions of the following methods, then checks usage of return statement.
Array.fromArray.fromAsyncArray.prototype.everyArray.prototype.filterArray.prototype.findArray.prototype.findIndexArray.prototype.findLastArray.prototype.findLastIndexArray.prototype.flatMapArray.prototype.forEach (optional, based on checkForEach parameter)Array.prototype.mapArray.prototype.reduceArray.prototype.reduceRightArray.prototype.someArray.prototype.sortArray.prototype.toSortedExamples of incorrect code for this rule:
:::incorrect
/*eslint array-callback-return: "error"*/
const indexMap = myArray.reduce(function(memo, item, index) {
memo[item] = index;
}, {});
const foo = Array.from(nodes, function(node) {
if (node.tagName === "DIV") {
return true;
}
});
const bar = foo.filter(function(x) {
if (x) {
return true;
} else {
return;
}
});
:::
Examples of correct code for this rule:
:::correct
/*eslint array-callback-return: "error"*/
const indexMap = myArray.reduce(function(memo, item, index) {
memo[item] = index;
return memo;
}, {});
const foo = Array.from(nodes, function(node) {
if (node.tagName === "DIV") {
return true;
}
return false;
});
const bar = foo.map(node => node.getAttribute("id"));
:::
This rule accepts a configuration object with three options:
"allowImplicit": false (default) When set to true, allows callbacks of methods that require a return value to implicitly return undefined with a return statement containing no expression."checkForEach": false (default) When set to true, rule will also report forEach callbacks that return a value."allowVoid": false (default) When set to true, allows void in forEach callbacks, so rule will not report the return value with a void operator.Note: { "allowVoid": true } works only if checkForEach option is set to true.
Examples of correct code for the { "allowImplicit": true } option:
:::correct
/*eslint array-callback-return: ["error", { allowImplicit: true }]*/
const undefAllTheThings = myArray.map(function(item) {
return;
});
:::
Examples of incorrect code for the { "checkForEach": true } option:
:::incorrect
/*eslint array-callback-return: ["error", { checkForEach: true }]*/
myArray.forEach(function(item) {
return handleItem(item);
});
myArray.forEach(function(item) {
if (item < 0) {
return x;
}
handleItem(item);
});
myArray.forEach(function(item) {
if (item < 0) {
return void x;
}
handleItem(item);
});
myArray.forEach(item => handleItem(item));
myArray.forEach(item => void handleItem(item));
myArray.forEach(item => {
return handleItem(item);
});
myArray.forEach(item => {
return void handleItem(item);
});
:::
Examples of correct code for the { "checkForEach": true } option:
:::correct
/*eslint array-callback-return: ["error", { checkForEach: true }]*/
myArray.forEach(function(item) {
handleItem(item)
});
myArray.forEach(function(item) {
if (item < 0) {
return;
}
handleItem(item);
});
myArray.forEach(function(item) {
handleItem(item);
return;
});
myArray.forEach(item => {
handleItem(item);
});
:::
Examples of correct code for the { "allowVoid": true } option:
:::correct
/*eslint array-callback-return: ["error", { checkForEach: true, allowVoid: true }]*/
myArray.forEach(item => void handleItem(item));
myArray.forEach(item => {
return void handleItem(item);
});
myArray.forEach(item => {
if (item < 0) {
return void x;
}
handleItem(item);
});
:::
This rule checks callback functions of methods with the given names, even if the object which has the method is not an array.
If you don't want to warn about usage of return statement in callbacks of array's methods, then it's safe to disable this rule.