Why is this an issue?

The typeof operator returns a string indicating the type of its argument, and the set of returned values is limited:

• "undefined"
• "boolean"
• "number"
• "string"
• "symbol" (since ECMAScript 2015)
• "function"
• "object" (for null and any other object)
• "bigint" (since ECMAScript 2020)

Compare a typeof expression to anything else, and the result will always be false.

function isNumber(x) {
  return typeof x === "Number"; // Noncompliant: the function always returns 'false'
}

Instead, make sure you are always comparing the expression against one of the seven possible values.

function isNumber(x) {
  return typeof x === "number";
}

Resources

Documentation

• eslint - Rule valid-typeof
• MDN web docs - typeof
• MDN web docs - Primitive values