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;

Options

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 in the CLI or using the config file, you can use:

CLI

oxlint --deny valid-typeof

Config (.oxlintrc.json)

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

References

• Rule Source