no-implicit-coercion

!!value, +value and "" + value convert a value as a side effect of an operator. You have to know the trick to see that a conversion happens. Boolean(value), Number(value) and String(value) say it by name.

--fix rewrites !!value to Boolean(value). For the conversions to a number or a string, the rewrite is offered as a suggestion.

Configure

Off by default. To turn it on:

[lint.rules]
"no-implicit-coercion" = "error" # or "warn"

Every option, set to its default:

[lint.rules]
"no-implicit-coercion" = ["error", {
  boolean = true,
  number = true,
  string = true,
  disallowTemplateShorthand = false,
  allow = [], # list of "~", "!!", "+", "- -", "-" or "*"
}]

Examples

Reported:

const hasItems = !!items.length;
const isMember = ~names.indexOf(user.name);
const quantity = +input.value;
const price = 1 * row.price;
const id = "" + user.id;
label += "";

Not reported:

const hasItems = Boolean(items.length);
const isMember = names.indexOf(user.name) !== -1;
const quantity = Number(input.value);
const price = Number(row.price);
const id = String(user.id);
label = String(label);

Options

Option	Default	What it does
boolean	true	Report !!value and ~list.indexOf(value).
number	true	Report +value, - -value, 1 * value and value - 0.
string	true	Report "" + value and value += "".
disallowTemplateShorthand	false	Also report a template that holds one expression and nothing else.
allow	[]	Operators to leave alone: any of "~", "!!", "+", "- -", "-", "*".

const id = `${user.id}`;

allow keeps one trick while the rest of its group is still reported:

const hasItems = !!items.length;

const isMember = ~names.indexOf(user.name);

"+" stands for both +value and "" + value:

const quantity = +input.value;
const id = "" + user.id;