使用import * as x作为函数默认参数引发未定义错误的原因

Let's break down exactly what's happening here with your code:

First, here's your original code for reference:

import * as curry from './curry-module.js'
export function example(curry = curry) {
  return {
    foo: (args) => curry.foo(args),
    bar: (args) => curry.bar(args),
  }
}

When you call example() without arguments, the curry variable inside the function ends up as undefined—and this is all down to JavaScript's scoping and variable initialization rules.

The Root Cause #

When you define a function parameter with the same name as an outer variable (in this case, curry), the parameter becomes a local variable that shadows the outer one.

Here's what happens step-by-step when example() runs without arguments:

1. The function creates a local curry parameter variable (this shadows the imported curry from outside).
2. When trying to set the default value curry = curry, the right-hand side curry refers to this uninitialized local parameter—not the imported one.
3. Since local variables are initialized to undefined before any assignment, you're effectively setting curry = undefined.

It's equivalent to writing this explicit version:

function example() {
  let curry; // Local variable, initialized to undefined
  curry = curry; // Assign undefined to itself
  // ... rest of the function
}

The Fix #

The simplest solution is to rename the function parameter so it doesn't clash with the imported variable. This way, the default value will correctly reference the imported curry toolset:

import * as curry from './curry-module.js'
export function example(curryUtils = curry) { // Renamed parameter to avoid shadowing
  return {
    foo: (args) => curryUtils.foo(args),
    bar: (args) => curryUtils.bar(args),
  }
}

Now when you call example() without arguments, curryUtils will default to the real imported curry module, and you can still pass a mock version for testing purposes.

内容的提问来源于stack exchange，提问作者worc