valid-typeof

Enforce comparing typeof expressions against valid strings

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.

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

Options

This rule has an object option:

• "requireStringLiterals": 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.

requireStringLiterals

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

/*eslint valid-typeof: ["error", { "requireStringLiterals": true }]*/

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:

/*eslint valid-typeof: ["error", { "requireStringLiterals": true }]*/

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.

Version

This rule was introduced in ESLint v0.5.0.

Further Reading

Resources

• Rule source
• Tests source