if statements only allow boolean and nullish expressions (not falsey ones)

[coolaj86]

This rule is taken from Go.

❌ Bad Example

if (!arr.length) {
    throw new Error("no items in array");
}

✅ Good Example

if (0 === arr.length) {
    throw new Error("no items in array");
}

Rationale

It's more difficult to do proper checking of type-constrained conditions if 0 and "" are considered valid boolean expressions.

For example:

/** @type {Number|String} foo */

if (!foo) {
  // if we constrain the boolean expression then we can detect
  // that there's a conflict between the code and the type:
  // `foo` is not optional according to the type, but perhaps the user thinks it can be
}