19 lines
453 B
Markdown
19 lines
453 B
Markdown
---
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title: when
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tags: function,intermediate
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---
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Tests a value, `x`, against a predicate function. If `true`, return `fn(x)`. Else, return `x`.
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- Return a function expecting a single value, `x`, that returns the appropriate value based on `pred`.
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```js
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const when = (pred, whenTrue) => x => (pred(x) ? whenTrue(x) : x);
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```
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```js
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const doubleEvenNumbers = when(x => x % 2 === 0, x => x * 2);
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doubleEvenNumbers(2); // 4
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doubleEvenNumbers(1); // 1
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```
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