How to change individual characters within a string in JavaScript

- QUESTION -

In my Meteor app, I have an array of objects with the following structure

[{ type: "oldValue" }]

When I run

testArray[0].type = 'test'
console.log(testArray[0].type)

'test' is correctly printed in the console. However, when I run

testArray[0].type[1] = 'd'
console.log(testArray[0].type)

'test' is printed in the console instead of 'tdst'. The second letter wasn't changed to a d.

How can I change individual characters within a string?

- ANSWER -

As @Pointy pointed out, strings are immutable. If you want to change certain characters, you can work with an array of characters:

var myString = 'hello there!'
myString = myString.split('')
myString[1] = 'a'
myString = myString.join('')
console.log(myString)     // 'hallo there!'

However, per your comment, you probably want to iterate through the string once, since constantly splitting and joining the string might be slow:

function replaceCharacters (str, chars, conversionFunction) {
    var charBuff = []
    for (var i = 0; i < str.length; i++) {
        if (chars.indexOf(str[i])) {
            charBuff.push(conversionFunction(str[i]))
        } else {
            charBuff.push(str[i])
        }
    }
    return charBuff.join('')
}

examples:

console.log(replaceCharacters('hello', 'al', function (myChar) { return 'Z' }))
// logs 'heZZo'
// you can pass an array or a string as the second parameter
// since `.indexOf` works on both strings and arrays

and if you're working with es6:

function replaceCharacters (str, chars, conversionFunc) {
    return [...str].reduce((p, c) => {
        p.push(chars.indexOf(c) === -1 ? c : conversionFunc(c))
        return p
    }, []).join('')
}