no-constant-binary-expression
Disallow expressions where the operation doesn't affect the value
总是评估为真或假的比较和总是短路或从不短路的逻辑表达式 (||, &&, ??) 都可能是程序员错误的迹象。
这些错误在复杂的表达式中特别常见，因为在这些表达式中，运算符的优先级很容易被误判。比如：
// 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，这个规则还可以检测到与新构建的对象/数组/函数等的比较。在 JavaScript 中，对象是通过引用进行比较的，一个新构造的对象永远不可能 === 任何其他值。这对于那些来自于对象通过值进行比较的语言的程序员来说，可能会感到惊讶。
// 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`.
规则细节
这条规则确定了 == 和 === 的比较，根据 JavaScript 语言的语义，这些比较将总是评估为 true 或 false。
它还识别了 ||、&& 和 ?? 逻辑表达式，这些表达式将始终或永远不会短路。
使用此规则的错误示例：
Open in Playground
/*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 === [];
使用此规则的正确示例：
Open in Playground
/*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;
const shortCircuit1 = condition1 && false && condition2;
const shortCircuit2 = condition1 || true || condition2;
const shortCircuit3 = condition1 ?? "non-nullish" ?? condition2;
Related Rules
Version
This rule was introduced in ESLint v8.14.0.
Further Reading