no-unnecessary-condition
)Any expression being used as a condition must be able to evaluate as truthy or falsy in order to be considered “necessary”. Conversely, any expression that always evaluates to truthy or always evaluates to falsy, as determined by the type of the expression, is considered unnecessary and will be flagged by this rule.
The following expressions are checked:
&&
, ||
and ?:
(ternary) operatorsif
, for
, while
, and do-while
statementsExamples of incorrect code for this rule:
function head<T>(items: T[]) { // items can never be nullable, so this is unnecessary if (items) { return items[0].toUpperCase(); } } function foo(arg: 'bar' | 'baz') { // arg is never nullable or empty string, so this is unnecessary if (arg) { } } function bar<T>(arg: string) { // arg can never be nullish, so ?. is unnecessary return arg?.length; }
Examples of correct code for this rule:
function head<T>(items: T[]) { // Necessary, since items.length might be 0 if (items.length) { return items[0].toUpperCase(); } } function foo(arg: string) { // Necessary, since foo might be ''. if (arg) { } } function bar(arg?: string | null) { // Necessary, since arg might be nullish return arg?.length; }
Accepts an object with the following options:
ignoreRhs
(default false
) - doesn't check if the right-hand side of &&
and ||
is a necessary condition. For example, the following code is valid with this option on:function head<T>(items: T[]) { return items.length && items[0].toUpperCase(); }
allowConstantLoopConditions
(default false
) - allows constant expressions in loops.Example of correct code for when allowConstantLoopConditions
is true
:
while (true) {} for (; true; ) {} do {} while (true);
checkArrayPredicates
(default: false
) - if set checks that the return value from certain array method callbacks (filter
, find
, some
, every
) is necessarily conditional.// Valid: numbers can be truthy or falsy. [0, 1, 2, 3].filter(t => t); // Invalid: arrays are always falsy. [ [1, 2], [3, 4], ].filter(t => t);
The main downside to using this rule is the need for type information.
ESLint: no-constant-condition - no-unnecessary-condition
is essentially a stronger version of no-constant-condition
, but requires type information.
strict-boolean-expressions - a more opinionated version of no-unnecessary-condition
. strict-boolean-expressions
enforces a specific code style, while no-unnecessary-condition
is about correctness.