Iterator.prototype.take()

¥Syntax

take(limit)

¥Parameters

limit

从迭代开始时要获取的元素数量。

¥Return value

新的 迭代器助手。返回的迭代器辅助程序逐一生成原始迭代器中的元素，然后在生成 limit 个元素后或当原始迭代器耗尽时（以先到者为准）完成（next() 方法生成 { value: undefined, done: true }）。

¥A new iterator helper. The returned iterator helper yields the elements in the original iterator one-by-one, and then completes (the next() method produces { value: undefined, done: true }) once limit elements have been yielded, or when the original iterator is exhausted, whichever comes first.

¥Exceptions

RangeError

如果 limit 变为 NaN 则抛出，或者当 转换为整数 变为负值时抛出。

¥Examples

¥Using take()

以下示例创建一个迭代器，该迭代器生成斐波那契数列中的项，然后记录前三个项：

¥The following example creates an iterator that yields terms in the Fibonacci sequence, and then logs the first three terms:

function* fibonacci() {
  let current = 1;
  let next = 1;
  while (true) {
    yield current;
    [current, next] = [next, current + next];
  }
}

const seq = fibonacci().take(3);
console.log(seq.next().value); // 1
console.log(seq.next().value); // 1
console.log(seq.next().value); // 2
console.log(seq.next().value); // undefined

¥Using take() with a for...of loop

当你不手动滚动迭代器时，take() 最方便。因为迭代器也是可迭代的，所以你可以使用 for...of 循环迭代返回的助手：

¥take() is most convenient when you are not hand-rolling the iterator. Because iterators are also iterable, you can iterate the returned helper with a for...of loop:

for (const n of fibonacci().take(5)) {
  console.log(n);
}

// Logs:
// 1
// 1
// 2
// 3
// 5

因为 fibonacci() 是无限迭代器，所以不能使用 for 循环直接迭代它。

¥Because fibonacci() is an infinite iterator, you can't use a for loop to iterate it directly.

¥Combining drop() with take()

你可以将 take() 与 Iterator.prototype.drop() 组合起来以获得迭代器的一部分：

¥You can combine take() with Iterator.prototype.drop() to get a slice of an iterator:

for (const n of fibonacci().drop(2).take(5)) {
  // Drops the first two elements, then takes the next five
  console.log(n);
}
// Logs:
// 2
// 3
// 5
// 8
// 13

for (const n of fibonacci().take(5).drop(2)) {
  // Takes the first five elements, then drops the first two
  console.log(n);
}
// Logs:
// 2
// 3
// 5

¥Lower and upper bounds of take count

当 limit 为负或 NaN 时，抛出 RangeError：

¥When the limit is negative or NaN, a RangeError is thrown:

fibonacci().take(-1); // RangeError: -1 must be positive
fibonacci().take(undefined); // RangeError: undefined must be positive

当 limit 大于迭代器可以产生的元素总数（例如 Infinity）时，返回的迭代器助手的行为与原始迭代器基本相同：

¥When the limit is larger than the total number of elements the iterator can produce (such as Infinity), the returned iterator helper has essentially the same behavior as the original iterator:

for (const n of new Set([1, 2, 3]).values().take(Infinity)) {
  console.log(n);
}

// Logs:
// 1
// 2
// 3

¥See also

• core-js 中 Iterator.prototype.take 的 Polyfill
• Iterator
• Iterator.prototype.drop()