如何在JavaScript中使用when（）有条件地更改值

30 Seconds of Code is a brilliant collection of JavaScript snippets, digestible in ≤ 30 seconds. Anyone looking to master JavaScript should go through the entire thing.

30秒的代码是精妙JavaScript片段集合，可在≤30秒内消化。 任何想要精通JavaScript的人都应该仔细研究整个过程。

Inspired by Ramda, I contributed when() to 30secondsofcode’s official GitHub repo. This is one my favorite functions.

受Ramda的启发，我将when()贡献给30secondsofcode的官方GitHub存储库。这是我最喜欢的功能之一。

when() takes 3 parameters:

when()采用3个参数：

pred: A predicate function (must return true or false)

pred ：谓词函数(必须返回true或false )
whenTrue: A function to run if pred returns true.

whenTrue ：如果pred返回true运行的函数。
A value: x.

值： x 。

Here’s the most basic implementation:

这是最基本的实现：

when = (pred, whenTrue, x) => {if (pred(x)) {return whenTrue(x);} else {return x;}
};

Which you can shorten to:

您可以将其缩短为：

when = (pred, whenTrue, x) => (pred(x) ? whenTrue(x) : x);

Let’s say we want to triple even numbers

假设我们想将偶数增加三倍

when((x) => x % 2 === 0, (x) => x * 3, 2);
// 6

We got 6 because 2 is an even number. What if we pass 11?

我们得到6是因为2是偶数。如果我们通过11怎么办？

when((x) => x % 2 === 0, (x) => x * 3, 11);
// 11

更进一步 (A Step Further)

when currently needs all 3 parameters at once–what if we could supply just the first 2, and give x later on?

当前when需要同时使用所有3个参数-如果我们可以仅提供前两个参数，然后再给出x ，该怎么办？

when = (pred, whenTrue) => (x) => (pred(x) ? whenTrue(x) : x);

This version’s what I submitted to 30secondsofcode.org. Now our code’s more flexible.

这个版本是我提交给30secondsofcode.org的版本。现在我们的代码更加灵活。

tripleEvenNums = when((x) => x % 2 === 0, (x) => x * 3);tripleEvenNums(20); // 60
tripleEvenNums(21); // 21
tripleEvenNums(22); // 66

甚至更远 (Even Further Beyond)

We can pass x later because when(pred, whenTrue) returns a function expecting x. What if we curry when()?

我们可以稍后传递x因为when(pred, whenTrue)返回一个期望x的函数。如果我们咖喱when()怎么办？

If you’re new to currying see my article on it.

如果您是新手，请参阅我的文章。

A curried function doesn’t need all its parameters at once. You can supply some and get a function that takes the rest, allowing for powerful patterns.

咖喱函数不需要一次所有的参数。您可以提供一些功能，并获得剩下的功能，以实现强大的模式。

一个愚蠢的例子 (A Silly Example)

Imagine we have two lists of people, both contain a guy named Bobo.

想象一下，我们有两个人名单，每个人都包含一个名叫Bobo 。

Bobo wants a nickname for each list.

Bobo希望为每个列表起一个昵称。

If we find Bobo in list 1, change his name to B Money.

如果我们在列表1中找到Bobo ， Bobo他的名字更改为B Money 。
If we find Bobo in list 2, change his name to Bo-bob.

如果我们在列表2中找到Bobo ， Bobo其名称更改为Bo-bob 。

Currying when allows us to easily write a function for each concern.

Curing when允许我们轻松地为每个关注点编写一个函数。

If you’re following along, here’s a curry function from 30secondsofcode.org.

如果您正在遵循，这里是30secondsofcode.org中的curry函数。

curry = (fn, arity = fn.length, ...args) =>arity <= args.length ? fn(...args) : curry.bind(null, fn, arity, ...args);

We’ll need a predicate to find Bobo.

我们需要一个谓词才能找到Bobo 。

isBobo = (person) => person.name === 'Bobo';

To keep our functions pure, we’ll need a way to immutably change a person’s name.

为了保持功能纯净，我们需要一种不变地更改人名的方法。

changeName = (newName, obj) => ({...obj,name: newName
});

Let’s also curry it so we can supply just newName.

我们还要对其进行咖喱处理，以便仅提供newName 。

changeName = curry((newName, obj) => ({...obj,name: newName
}));

Here’s our lists.

这是我们的清单。

list1 = [{name: 'Bobo',id: 1,iq: 9001},{name: 'Jaime',id: 2,iq: 9000},{name: 'Derek',id: 3,iq: 8999}
];list2 = [{name: 'Sam',id: 1,iq: 600},{name: 'Bobo',id: 2,iq: 9001},{name: 'Peter',id: 3,iq: 8}
];

Let’s map over list1.

让我们映射到list1 。

doIfBobo = when(isBobo);
renameToBMoney = changeName('B Money');list1.map(doIfBobo(renameToBMoney));

Our result:

我们的结果：

[{name: 'B Money',id: 1,iq: 9001},{name: 'Jaime',id: 2,iq: 9000},{name: 'Derek',id: 3,iq: 8999}
];

Because of when, we only changed Bobo and ignored everyone else!

由于when ，我们只换了Bobo而忽略了其他所有人！

Now map over list2.

现在映射到list2 。

renameToBoBob = changeName('Bo-bob');list2.map(doIfBobo(renameToBoBob));

Our result:[{"name": "Sam","id": 1,"iq": 600
},{"name": "Bo-bob","id": 2,"iq": 9001**},{"name": "Peter","id": 3,"iq": 8}
];

Looks good to me! We gave Bobo his nicknames without affecting anyone else.

在我看来很好！我们给了Bobo他的昵称，却没有影响到其他人。

If you’re further interested, consider these links:

如果您进一步感兴趣，请考虑以下链接：

30secondsofcode.org’s collection

30secondsofcode.org的收藏
My article on currying

我关于currying的文章
Ramda

拉姆达

翻译自: https://www.freecodecamp.org/news/30-seconds-of-code-conditionally-change-values-with-when-732b09e46334/