类型错误：无法使用 'in' 运算符在 'y' 中搜索 'x'

¥Message

TypeError: Cannot use 'in' operator to search for 'x' in 'y' (V8-based & Firefox)
TypeError: right-hand side of 'in' should be an object, got null (Firefox)
TypeError: "y" is not an Object. (evaluating '"x" in "y"') (Safari)

¥Error type

¥What went wrong?

in 运算符 只能用于检查属性是否在对象中。你无法在字符串、数字或其他基本类型中进行搜索。

¥The in operator can only be used to check if a property is in an object. You can't search in strings, or in numbers, or other primitive types.

¥Examples

¥Searching in strings

与其他编程语言（例如 Python）不同，你无法使用 in 运算符。

¥Unlike in other programming languages (e.g. Python), you can't search in strings using the in operator.

"Hello" in "Hello World";
// TypeError: cannot use 'in' operator to search for 'Hello' in 'Hello World'

例如，你将需要使用 String.prototype.includes()。

¥Instead you will need to use String.prototype.includes(), for example.

"Hello World".includes("Hello");
// true

¥The operand can't be null or undefined

确保你正在检查的对象实际上不是 null 或 undefined。

¥Make sure the object you are inspecting isn't actually null or undefined.

const foo = null;
"bar" in foo;
// TypeError: cannot use 'in' operator to search for 'bar' in 'foo' (Chrome)
// TypeError: right-hand side of 'in' should be an object, got null (Firefox)

in 运算符始终需要一个对象。

¥The in operator always expects an object.

const foo = { baz: "bar" };
"bar" in foo; // false

"PI" in Math; // true
"pi" in Math; // false

¥Searching in arrays

使用 in 运算符搜索 Array 对象时要小心。in 运算符检查索引号，而不是该索引处的值。

¥Be careful when using the in operator to search in Array objects. The in operator checks the index number, not the value at that index.

const trees = ["redwood", "bay", "cedar", "oak", "maple"];
3 in trees; // true
"oak" in trees; // false

¥See also

• in