disallow use of optional chaining in contexts where the undefined value is not allowed (no-unsafe-optional-chaining)

The "extends": "eslint:recommended" property in a configuration file enables this rule.

The optional chaining (?.) expression can short-circuit with a return value of undefined. Therefore, treating an evaluated optional chaining expression as a function, object, number, etc., can cause TypeError or unexpected results. For example:

var obj = undefined;

1 in obj?.foo;  // TypeError
with (obj?.foo);  // TypeError
for (bar of obj?.foo);  // TypeError
bar instanceof obj?.foo;  // TypeError
const { bar } = obj?.foo;  // TypeError

Also, parentheses limit the scope of short-circuiting in chains. For example:

var obj = undefined;

(obj?.foo)(); // TypeError
(obj?.foo).bar; // TypeError

Rule Details

This rule aims to detect some cases where the use of optional chaining doesn't prevent runtime errors. In particular, it flags optional chaining expressions in positions where short-circuiting to undefined causes throwing a TypeError afterward.

Examples of incorrect code for this rule:

/*eslint no-unsafe-optional-chaining: "error"*/

(obj?.foo)();

(obj?.foo).bar;

(foo?.()).bar;

(foo?.()).bar();

(obj?.foo ?? obj?.bar)();

(foo || obj?.foo)();

(obj?.foo && foo)();

(foo ? obj?.foo : bar)();

(foo, obj?.bar).baz;

(obj?.foo)`template`;

new (obj?.foo)();

[...obj?.foo];

bar(...obj?.foo);

1 in obj?.foo;

bar instanceof obj?.foo;

for (bar of obj?.foo);

const { bar } = obj?.foo;

[{ bar } = obj?.foo] = [];

with (obj?.foo);

class A extends obj?.foo {}

var a = class A extends obj?.foo {};

async function foo () {
    const { bar } = await obj?.foo;
   (await obj?.foo)();
   (await obj?.foo).bar;
}

Examples of correct code for this rule:

/*eslint no-unsafe-optional-chaining: "error"*/

(obj?.foo)?.();

obj?.foo();

(obj?.foo ?? bar)();

obj?.foo.bar;

obj.foo?.bar;

foo?.()?.bar;

(obj?.foo ?? bar)`template`;

new (obj?.foo ?? bar)();

var baz = {...obj?.foo};

const { bar } = obj?.foo || baz;

async function foo () {
  const { bar } = await obj?.foo || baz;
   (await obj?.foo)?.();
   (await obj?.foo)?.bar;
}

Options

This rule has an object option:

• disallowArithmeticOperators: Disallow arithmetic operations on optional chaining expressions (Default false). If this is true, this rule warns arithmetic operations on optional chaining expressions, which possibly result in NaN.

disallowArithmeticOperators

With this option set to true the rule is enforced for:

• Unary operators: -, +
• Arithmetic operators: +, -, /, *, %, **
• Assignment operators: +=, -=, /=, *=, %=, **=

Examples of additional incorrect code for this rule with the { "disallowArithmeticOperators": true } option:

/*eslint no-unsafe-optional-chaining: ["error", { "disallowArithmeticOperators": true }]*/

+obj?.foo;
-obj?.foo;

obj?.foo + bar;
obj?.foo - bar;
obj?.foo / bar;
obj?.foo * bar;
obj?.foo % bar;
obj?.foo ** bar;

baz += obj?.foo;
baz -= obj?.foo;
baz /= obj?.foo;
baz *= obj?.foo;
baz %= obj?.foo;
baz **= obj?.foo;

async function foo () {
  +await obj?.foo;
  await obj?.foo + bar;
  baz += await obj?.foo;
}

Version

This rule was introduced in ESLint 7.15.0.

Resources

• Rule source
• Documentation source