测试中的断言是什么？

Question

测试中的断言是什么？

Accepted Answer

**断言**是测试中的语句，**检查条件是否为真** — 验证实际结果是否与预期结果相匹配。它们是测试如何确定通过或失败的核心：如果断言失败，测试就失败。

## 断言的作用

```text
An ASSERTION checks that something is as expected:
  → "assert that the actual value EQUALS the expected value"
  → if TRUE → the assertion passes; if FALSE → the assertion FAILS → the TEST FAILS
→ Assertions are how a test VERIFIES behavior (the "check" part of a test).
```

## 常见的断言类型

```js
// equality
expect(result).toBe(5);                  // strict equality
expect(obj).toEqual({ name: 'Ann' });    // deep/value equality (objects)

// truthiness / null
expect(value).toBeTruthy();
expect(value).toBeNull();
expect(value).toBeDefined();

// comparisons
expect(count).toBeGreaterThan(0);
expect(price).toBeLessThanOrEqual(100);

// collections / strings
expect(array).toContain('item');
expect(array).toHaveLength(3);
expect(text).toMatch(/pattern/);          // regex

// errors
expect(() => risky()).toThrow();          // expect a function to throw

// negation
expect(value).not.toBe(null);
```

## 良好的断言

```text
✓ Assert the SPECIFIC expected outcome (clear what's being checked)
✓ Use the RIGHT matcher (toBe vs toEqual; specific > generic)
✓ Clear FAILURE messages (good assertions/matchers explain what failed)
✓ Don't over-assert (test one behavior) or under-assert (verify the actual result)
```

## 为什么这很重要

理解断言是基础性知识，因为它们是**测试验证行为的核心机制** — 决定通过或失败 — 所以这是编写任何测试的必备知识。

断言**检查条件是否为真**（通常是检查实际结果是否与预期结果匹配），它是测试的