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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:

/*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.

Further Reading

Version

This rule was introduced in ESLint 0.5.0.

Resources

© JS Foundation and other contributors
Licensed under the MIT License.
https://eslint.org/docs/rules/valid-typeof