str = str.split('');
str[3] = 'h';
str = str.join('');

我用负下标的安全方法

/**
 * @param {string} str 
 * @param {number} index 
 * @param {string} replacement 
 * @returns {string}
 */
static replaceAt (str, index, replacement)
{
    if (index < 0) index = str.length + index
    if (index < 0 || index >= str.length) throw new Error(`Index (${index}) out of bounds "${str}"`)
    return str.substring(0, index) + replacement + str.substring(index + 1)
}

像这样使用它:

replaceAt('my string', -1, 'G') // 'my strinG'
replaceAt('my string', 2, 'yy') // 'myyystring'
replaceAt('my string', 22, 'yy') // Uncaught Error: Index (22) out of bounds "my string"

function dothis() { var x = document.getElementById("x").value; var index = document.getElementById("index").value; var text = document.getElementById("text").value; var length = document.getElementById("length").value; var arr = x.split(""); arr.splice(index, length, text); var result = arr.join(""); document.getElementById('output').innerHTML = result; console.log(result); } dothis(); <input id="x" type="text" value="White Dog" placeholder="Enter Text" /> <input id="index" type="number" min="0"value="6" style="width:50px" placeholder="index" /> <input id="length" type="number" min="0"value="1" style="width:50px" placeholder="length" /> <input id="text" type="text" value="F" placeholder="New character" /> <br> <button id="submit" onclick="dothis()">Run</button> <p id="output"></p>

此方法适用于较小长度的字符串，但对于较大的文本可能很慢。

var x = "White Dog";
var arr = x.split(""); // ["W", "h", "i", "t", "e", " ", "D", "o", "g"]
arr.splice(6, 1, 'F');

/* 
  Here 6 is starting index and 1 is no. of array elements to remove and 
  final argument 'F' is the new character to be inserted. 
*/
var result = arr.join(""); // "White Fog"

使用扩展语法，你可以将字符串转换为数组，在给定位置分配字符，然后转换回字符串:

Const STR = "hello world"; 函数replace (s, i, c) { Const arr =[…s];//将字符串转换为数组 Arr [i] = c;//在pos i处设置字符c 返回arr.join(”);//返回字符串 ｝ //打印"hallo world" console.log(replace (str, 1， 'a'));

你不能。取位置前后的字符并连接成一个新字符串:

var s = "Hello world";
var index = 3;
s = s.substring(0, index) + 'x' + s.substring(index + 1);

你可以试试

var strArr = str.split("");

strArr[0] = 'h';

str = strArr.join("");