String.prototype.slice() - JavaScript 编辑

The slice() method extracts a section of a string and returns it as a new string, without modifying the original string.

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Syntax

str.slice(beginIndex[, endIndex])

Parameters

beginIndex

The zero-based index at which to begin extraction. If negative, it is treated as str.length + beginIndex. (For example, if beginIndex is -3, it is treated as str.length - 3.) If beginIndex is not a number after Number(beginIndex), it is treated as 0.

If beginIndex is greater than or equal to str.length, an empty string is returned.

endIndex Optional

The zero-based index before which to end extraction. The character at this index will not be included.

If endIndex is omitted or undefined, or greater than str.length, slice() extracts to the end of the string. If negative, it is treated as str.length + endIndex. (For example, if endIndex is -3, it is treated as str.length - 3.) If it is not undefined and not a number after Number(endIndex), an empty string is returned.

If endIndex is specified and startIndex is negative, endIndex should be negative, otherwise an empty string is returned. (For example, slice(-3, 0) returns "".)

If endIndex is specified, and startIndex and endIndex are both positive or negative, endIndex should be greater than startIndex, otherwise an empty string is returned. (For example, slice(-1, -3) or slice(3, 1) returns "".)

Return value

A new string containing the extracted section of the string.

Description

slice() extracts the text from one string and returns a new string. Changes to the text in one string do not affect the other string.

slice() extracts up to but not including endIndex. str.slice(1, 4) extracts the second character through the fourth character (characters indexed 1, 2, and 3).

As an example, str.slice(2, -1) extracts the third character through the second to last character in the string.

Examples

Using slice() to create a new string

The following example uses slice() to create a new string.

let str1 = 'The morning is upon us.', // the length of str1 is 23.
    str2 = str1.slice(1, 8),
    str3 = str1.slice(4, -2),
    str4 = str1.slice(12),
    str5 = str1.slice(30);
console.log(str2)  // OUTPUT: he morn
console.log(str3)  // OUTPUT: morning is upon u
console.log(str4)  // OUTPUT: is upon us.
console.log(str5)  // OUTPUT: ""

Using slice() with negative indexes

The following example uses slice() with negative indexes.

let str = 'The morning is upon us.'
str.slice(-3)      // returns 'us.'
str.slice(-3, -1)  // returns 'us'
str.slice(0, -1)   // returns 'The morning is upon us'

This example counts backwards from the end of the string by 11 to find the start index and forwards from the start of the string by 16 to find the end index.

console.log(str.slice(-11, 16)) // => "is u"

Here it counts forwards from the start by 11 to find the start index and backwards from the end by 7 to find the end index.

console.log(str.slice(11, -7)) // => " is u"

These arguments count backwards from the end by 5 to find the start index and backwards from the end by 1 to find the end index.

console.log(str.slice(-5, -1)) // => "n us"

Specifications

Browser compatibility

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See also

• String.prototype.substr()
• String.prototype.substring()
• Array.prototype.slice()