array-callback-return

Enforce return statements in callbacks of array methods

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}
var indexMap = myArray.reduce(function(memo, item, index) {
  memo[item] = index;
}, {}); // Error: cannot set property 'b' of undefined

Rule Details

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.from
• Array.prototype.every
• Array.prototype.filter
• Array.prototype.find
• Array.prototype.findIndex
• Array.prototype.findLast
• Array.prototype.findLastIndex
• Array.prototype.flatMap
• Array.prototype.forEach (optional, based on checkForEach parameter)
• Array.prototype.map
• Array.prototype.reduce
• Array.prototype.reduceRight
• Array.prototype.some
• Array.prototype.sort
• And above of typed arrays.

Examples of incorrect code for this rule:

/*eslint array-callback-return: "error"*/

var indexMap = myArray.reduce(function(memo, item, index) {
    memo[item] = index;
}, {});

var foo = Array.from(nodes, function(node) {
    if (node.tagName === "DIV") {
        return true;
    }
});

var bar = foo.filter(function(x) {
    if (x) {
        return true;
    } else {
        return;
    }
});

Examples of correct code for this rule:

/*eslint array-callback-return: "error"*/

var indexMap = myArray.reduce(function(memo, item, index) {
    memo[item] = index;
    return memo;
}, {});

var foo = Array.from(nodes, function(node) {
    if (node.tagName === "DIV") {
        return true;
    }
    return false;
});

var bar = foo.map(node => node.getAttribute("id"));

Options

This rule accepts a configuration object with two 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.

allowImplicit

Examples of correct code for the { "allowImplicit": true } option:

/*eslint array-callback-return: ["error", { allowImplicit: true }]*/
var undefAllTheThings = myArray.map(function(item) {
    return;
});

checkForEach

Examples of incorrect code for the { "checkForEach": true } option:

/*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(item => handleItem(item));

myArray.forEach(item => {
    return handleItem(item);
});

Examples of correct code for the { "checkForEach": true } option:

/*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);
});

Known Limitations

This rule checks callback functions of methods with the given names, even if the object which has the method is not an array.

When Not To Use It

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.

Version

This rule was introduced in ESLint v2.0.0-alpha-1.

Resources

• Rule source
• Tests source