enforce comparing typeof expressions against valid strings (valid-typeof)

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

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.

Rule Details

This rule enforces comparing typeof expressions to valid string literals.

Options

This rule has an object option:

• "requireStringLiterals": true requires typeof expressions to only be compared to string literals or other typeof expressions, and disallows comparisons to any other value.

Examples of incorrect code for this rule:

/*eslint valid-typeof: "error"*/

typeof foo === "strnig"
typeof foo == "undefimed"
typeof bar != "nunber"
typeof bar !== "fucntion"

Examples of correct code for this rule:

/*eslint valid-typeof: "error"*/

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

Examples of incorrect code with the { "requireStringLiterals": true } option:

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

Examples of correct code with the { "requireStringLiterals": true } option:

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

When Not To Use It

You may want to turn this rule off if you will be using the typeof operator on host objects.

Further Reading

• MDN: typeof documentation

Version

This rule was introduced in ESLint 0.5.0.

Resources

• Rule source
• Documentation source