no-extra-boolean-cast.md

no-extra-boolean-cast

disallow double-negation boolean casts in a boolean context

Do not cast variables as to booleans inside of an expression where they will already be case, such as inside of an if statement, or in a ternary operator.

Example:

Good

if (foo) {

}
while (foo) {

}
for(; foo; ) {

}
var bar = foo ? 'bar' : 'baz';

Bad, coercing to a boolean that will already be coerced

if (!!foo) {

}
while (!!foo) {

}
for(; !!foo; ) {

}
var bar = !!foo ? 'bar' : 'baz';

Justification

To cast these to booleans is a pointless operation, as they will already be cast as such by virtue of being inside of a boolean context.

Note this rule doesn't stop you using !! outside of areas that aren't implicitly cast to booleans - the rule no-implicit-coercion does this.

How to disable

There's little point to disabling this rule, but if you want to, it is best to do it project wide:

package.json (project-wide)

"eslintConfig": {
  "extends": ["strict/es5"],
  "rules": {
    "no-extra-boolean-cast": 0
  }
}