eslint/valid-typeof Correctness

✅ This rule is turned on by default.

🛠️ An auto-fix is available for this rule for some violations.

What it does

Enforce comparing typeof expressions against valid strings.

Why is this bad?

For a vast majority of use cases, the result of the typeof operator is one of the following string literals: "undefined", "object", "boolean", "number", "string", "function", "symbol", and "bigint". It is usually a typing mistake to compare the result of a typeof operator to other string literals.

Examples

Examples of incorrect code for this rule:

typeof foo === "strnig"
typeof foo == "undefimed"
typeof bar != "nunber"     // spellchecker:disable-line
typeof bar !== "fucntion"     // spellchecker:disable-line

Examples of correct code for this rule:

typeof foo === "string"
typeof bar == "undefined"
typeof foo === baz
typeof bar === typeof qux

Configuration

This rule accepts a configuration object with the following properties:

requireStringLiterals

type: boolean

default: false

The requireStringLiterals option when set to true, allows the comparison of typeof expressions with only string literals or other typeof expressions, and disallows comparisons to any other value. Default is false.

With requireStringLiterals set to true, the following are examples of incorrect code:

typeof foo === undefined
typeof bar == Object
typeof baz === "strnig"
typeof qux === "some invalid type"
typeof baz === anotherVariable
typeof foo == 5

With requireStringLiterals set to true, the following are examples of correct code:

typeof foo === "undefined"
typeof bar == "object"
typeof baz === "string"
typeof bar === typeof qux

How to use

To enable this rule using the config file or in the CLI, you can use:

Config (.oxlintrc.json)

{
    "rules": {
        "valid-typeof": "error"
    }
}

CLI

oxlint --deny valid-typeof

References

• Rule Source