如何模拟Deno内置方法，例如Deno.Command()？

在测试中模拟Deno对象上的Command方法，核心思路是临时替换原始的Deno.Command构造函数，让它返回你预设的结果。下面是几种常用的实现方式：

1. 用Deno标准库的mock模块处理 #

Deno标准库的std/testing/mock.ts提供了便捷的mock工具，适合需要快速替换并后续恢复的场景：

import { assertEquals } from "https://deno.land/std@0.208.0/assert/mod.ts";
import { mock } from "https://deno.land/std@0.208.0/testing/mock.ts";

// 待测试的业务函数：调用外部命令
async function runExternalCommand() {
  const cmd = new Deno.Command("echo", { args: ["hello"] });
  const output = await cmd.output();
  return new TextDecoder().decode(output.stdout);
}

Deno.test("模拟Command返回指定输出", async () => {
  // 先保存原始的Command构造函数，测试后要恢复
  const originalCommand = Deno.Command;
  
  // 替换Deno.Command，让它返回的实例拥有预设的output方法
  mock(Deno, "Command", () => ({
    output: async () => ({
      stdout: new TextEncoder().encode("mock output"),
      stderr: new Uint8Array(),
      success: true,
      code: 0,
    })
  }));

  try {
    const result = await runExternalCommand();
    assertEquals(result, "mock output");
  } finally {
    // 必须恢复原始方法，避免影响其他测试用例
    Deno.Command = originalCommand;
  }
});

2. 直接覆盖Deno.Command（简单场景） #

如果不需要复杂的mock逻辑，直接给Deno.Command赋值自定义类即可，代码更简洁：

import { assertEquals } from "https://deno.land/std@0.208.0/assert/mod.ts";

async function runExternalCommand() {
  const cmd = new Deno.Command("echo", { args: ["hello"] });
  const output = await cmd.output();
  return new TextDecoder().decode(output.stdout);
}

Deno.test("直接替换Command实现", async () => {
  const originalCommand = Deno.Command;

  // 自定义模拟类，实现和原始Command一致的接口
  Deno.Command = class MockCommand {
    constructor(_cmd: string, _options: { args: string[] }) {}
    async output() {
      return {
        stdout: new TextEncoder().encode("direct mock result"),
        stderr: new Uint8Array(),
        success: true,
        code: 0,
      };
    }
  } as typeof Deno.Command;

  try {
    const result = await runExternalCommand();
    assertEquals(result, "direct mock result");
  } finally {
    Deno.Command = originalCommand;
  }
});

3. 按命令参数返回不同结果 #

如果需要根据不同的命令或参数返回对应模拟值，可以在mock逻辑里做判断：

import { assertEquals } from "https://deno.land/std@0.208.0/assert/mod.ts";
import { mock } from "https://deno.land/std@0.208.0/testing/mock.ts";

async function runCommand(cmd: string, args: string[]) {
  const command = new Deno.Command(cmd, { args });
  const output = await command.output();
  return new TextDecoder().decode(output.stdout);
}

Deno.test("根据命令参数返回对应结果", async () => {
  const originalCommand = Deno.Command;

  mock(Deno, "Command", (cmd: string, options: { args: string[] }) => ({
    output: async () => {
      // 根据传入的命令和参数返回不同模拟值
      if (cmd === "echo" && options.args[0] === "hello") {
        return { stdout: new TextEncoder().encode("hello from mock"), stderr: new Uint8Array(), success: true, code: 0 };
      } else if (cmd === "ls") {
        return { stdout: new TextEncoder().encode("file1.txt\nfile2.txt"), stderr: new Uint8Array(), success: true, code: 0 };
      }
      // 默认返回失败结果
      return { stdout: new Uint8Array(), stderr: new Uint8Array(), success: false, code: 1 };
    }
  }));

  try {
    assertEquals(await runCommand("echo", ["hello"]), "hello from mock");
    assertEquals(await runCommand("ls", []), "file1.txt\nfile2.txt");
    assertEquals(await runCommand("unknown", []), "");
  } finally {
    Deno.Command = originalCommand;
  }
});

内容的提问来源于stack exchange，提问作者hallvors