no-new-wrappers.md

Pattern: Use of new with primitive wrapper objects

Issue: -

Description

Creating primitive wrapper objects with new String(), new Number(), or new Boolean() leads to unexpected behavior. The resulting objects have different typeof results than primitives and boolean objects are always truthy, even when wrapping false.

Examples

Example of incorrect code:

const str = new String("hello");
const num = new Number(123);
const bool = new Boolean(false);

if (str === "hello") {  // false, because str is an object
  console.log("never reached");
}

if (new Boolean(false)) {  // always true!
  console.log("always reached");
}

Example of correct code:

const str = String("hello");
const num = Number(123);
const bool = Boolean(false);

// Or use literals
const str2 = "hello";
const num2 = 123;
const bool2 = false;