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mixed, any and asterisk
[ July 25, 2017 ]

asterisk(*) vs any

  • Use *:
const foo: () => Array<*> = () => [777];

// Flow error:
// 13: const foo: () => Array<*> = () => [777];
//                                        ^ number. This type is incompatible with
// 15: (foo(): Array<string>);
//                   ^ string
(foo(): Array<string>);

// No flow errors.
(foo(): Array<number>);
  • Use any:
const bar: () => Array<any> = () => [777];

// No flow errors.
(bar(): Array<string>);

// No flow errors.
(bar(): Array<number>);

用了any基本上跟不檢查沒兩樣，甚至有可能污染到其他值。所以能不用就盡量不用。

關於any

mixed vs asterisk(*)

  • Use mixed:
// Flow error:
// 1: const add5: (mixed) => number = a => a + 5;
//                                         ^ mixed. This type cannot be used in an addition because it is unknown whether it behaves like a string or a number.
const add5: mixed => number = value => value + 5;

此錯誤訊息產生的原因官方文件記載如下：

When you try to use a value of a mixed type you must first figure out what the actual type is.

所以我們在進行+運算前，必須先確認value為number或是string。 官方文件範例是在運算前使用typeof value來確認型別。

  • Use *:
// No flow errors.
const add5: * => number = value => value + 5;

// No flow errors.
(add5(2): number);

// Flow error:
// 1: const add5: (*) => number = a => a + 5;
//                                     ^ string. This type is incompatible with
// 1: const add5: (*) => number = a => a + 5;
//                       ^ number
(add5('3'): number);

暫時還沒想到什麼情況是用*會比較好，之後再補上。

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