no-constant-binary-expression

禁止操作不影响值的表达式

将始终评估为真或假的比较以及始终短路或从不短路的逻辑表达式（||，&&，??）都可能表明程序员错误。

这些错误在运算符优先级容易误判的复杂表达式中尤为常见。例如：

// One might think this would evaluate as `a + (b ?? c)`:
const x = a + b ?? c;

// But it actually evaluates as `(a + b) ?? c`. Since `a + b` can never be null,
// the `?? c` has no effect.

此外，此规则检测与新构造的对象/数组/函数/等的比较。在 JavaScript 中，对象通过引用进行比较，新构造的对象永远不能 === 任何其他值。对于来自按值比较对象的语言的程序员来说，这可能会令人惊讶。

// Programmers coming from a language where objects are compared by value might expect this to work:
const isEmpty = x === [];

// However, this will always result in `isEmpty` being `false`.

规则详情

此规则识别 == 和 === 比较，基于 JavaScript 语言的语义，它们将始终评估为 true 或 false。

它还标识了总是或从不短路的 ||、&& 和 ?? 逻辑表达式。

此规则的错误代码示例：

/*eslint no-constant-binary-expression: "error"*/

const value1 = +x == null;

const value2 = condition ? x : {} || DEFAULT;

const value3 = !foo == null;

const value4 = new Boolean(foo) === true;

const objIsEmpty = someObj === {};

const arrIsEmpty = someArr === [];

此规则的正确代码示例：

/*eslint no-constant-binary-expression: "error"*/

const value1 = x == null;

const value2 = (condition ? x : {}) || DEFAULT;

const value3 = !(foo == null);

const value4 = Boolean(foo) === true;

const objIsEmpty = Object.keys(someObj).length === 0;

const arrIsEmpty = someArr.length === 0;