String.prototype.at()

¥Syntax

at(index)

¥Parameters

index

要返回的字符串字符的索引（位置）。当传递负索引时，支持从字符串末尾开始相对索引；即，如果使用负数，则将从字符串末尾倒数找到返回的字符。

¥Return value

由位于指定位置的单个 UTF-16 代码单元组成的 String。如果找不到给定索引，则返回 undefined。

¥A String consisting of the single UTF-16 code unit located at the specified position. Returns undefined if the given index can not be found.

¥Examples

¥Return the last character of a string

以下示例提供了一个函数，该函数返回在指定字符串中找到的最后一个字符。

¥The following example provides a function which returns the last character found in a specified string.

// A function which returns the last character of a given string
function returnLast(arr) {
  return arr.at(-1);
}

let invoiceRef = "myinvoice01";

console.log(returnLast(invoiceRef)); // '1'

invoiceRef = "myinvoice02";

console.log(returnLast(invoiceRef)); // '2'

¥Comparing methods

在这里，我们比较选择 String 的倒数第二个（最后一个）字符的不同方法。虽然以下所有方法都是有效的，但它强调了 at() 方法的简洁性和可读性。

¥Here we compare different ways to select the penultimate (last but one) character of a String. Whilst all below methods are valid, it highlights the succinctness and readability of the at() method.

const myString = "Every green bus drives fast.";

// Using length property and charAt() method
const lengthWay = myString.charAt(myString.length - 2);
console.log(lengthWay); // 't'

// Using slice() method
const sliceWay = myString.slice(-2, -1);
console.log(sliceWay); // 't'

// Using at() method
const atWay = myString.at(-2);
console.log(atWay); // 't'

¥See also

• core-js 中 String.prototype.at 的 Polyfill
• String.prototype.indexOf()
• String.prototype.lastIndexOf()
• String.prototype.charCodeAt()
• String.prototype.codePointAt()
• String.prototype.split()