Flow类型中(a: ?string)与(a?: string)的区别是什么？

Great question! Let’s break down the critical differences between these two Flow syntax patterns, using your exact concat function examples to make things crystal clear.

Core Definitions #

First, let’s clarify what each syntax actually means:

• a: ?string (Nullable Type): This tells Flow that the parameter must be provided when calling the function, but its value can be a string, null, or undefined. You can’t skip passing this parameter entirely—you have to send something, even if it’s null or undefined.
• a?: string (Optional Parameter): This means the parameter can be omitted from the function call. If you do pass a value, it has to be a string; if you skip it, the parameter will be undefined inside the function. null isn’t allowed here unless you explicitly add it to the type.

Behavior in Your concat Examples #

Let’s map this to your two function declarations:

Function 1: function concat(a: ?string, b: ?string): string { } #

This function requires both arguments every time you call it—no exceptions. The only flexibility is that each argument can be a string, null, or undefined.

• ✅ Valid calls:

  concat("hello", "world"); // Both strings
  concat(null, undefined); // Nullable values are allowed
  concat("foo", null); // Mix of string and null
  

• ❌ Invalid calls (Flow will throw errors):

  concat("hello"); // Missing the second argument `b`
  concat(); // Missing both arguments entirely
  

Function 2: function concat(a?: string, b?: string): string { } #

This function lets you call it with 0, 1, or 2 arguments. But any value you do pass has to be a proper string—no null allowed by default.

• ✅ Valid calls:

  concat(); // No arguments, both params are undefined
  concat("hello"); // Only first argument provided
  concat("hello", "world"); // Both strings
  

• ❌ Invalid calls (Flow will throw errors):

  concat(null, "world"); // `null` isn't a string type
  concat(undefined, null); // `null` isn't allowed for `b`
  

Want the Best of Both Worlds? #

If you need a parameter that’s optional (can be skipped) and accepts null/undefined when provided, combine the two syntaxes:

function concat(a?: ?string, b?: ?string): string { }

Now you can call concat(), concat(null), or concat("foo", undefined)—all will pass Flow’s checks.

内容的提问来源于stack exchange，提问作者Rohan