Home - News ( United States | United Kingdom | Italy | Germany ) - Football scores

Showing content from https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Errors/Unexpected_type below:

TypeError: "x" is (not) "y" - JavaScript

JavaScript 异常“x is (not) y”在出现与期望不符的类型（通常为意外获得的 undefined 或 null 值）时被抛出。

TypeError: Cannot read properties of undefined (reading 'x') (V8-based)
TypeError: "x" is undefined (Firefox)
TypeError: "undefined" is not an object (Firefox)
TypeError: undefined is not an object (evaluating 'obj.x') (Safari)

TypeError: "x" is not a symbol (V8-based & Firefox)
TypeError: Symbol.keyFor requires that the first argument be a symbol (Safari)

TypeError。

出现了与期望不符的类型。这个错误常常由 undefined 或 null 值引起。

此外，某些方法，例如 Object.create() 或 Symbol.keyFor()，要求必须提供特定类型的参数。

// undefined and null cases on which the substring method won't work
const foo = undefined;
foo.substring(1); // TypeError: foo is undefined

const foo = null;
foo.substring(1); // TypeError: foo is null

// Certain methods might require a specific type
const foo = {};
Symbol.keyFor(foo); // TypeError: foo is not a symbol

const foo = "bar";
Object.create(foo); // TypeError: "foo" is not an object or null

要解决值为 undefined 或 null 的空指针问题，你可以首先测试值是否为 undefined 或 null。

if (foo !== undefined && foo !== null) {
  // Now we know that foo is defined, we are good to go.
}

或者，你如果能确定 foo 的值不会是其他的假值（如："" 或 0），或者排除这些情况不是问题，那你可以简单地测试其是否为真。

if (foo) {
  // Now we know that foo is truthy, it will necessarily not be null/undefined.
}

RetroSearch is an open source project built by @garambo | Open a GitHub Issue

Search and Browse the WWW like it's 1997 | Search results from DuckDuckGo

HTML: 3.2 | Encoding: UTF-8 | Version: 0.7.4