How to convert large UTF-8 strings into ASCII?

Any UTF-8 string that is reversibly convertible to ASCII is already ASCII.

UTF-8 can represent any unicode character - ASCII cannot.

As others have said, you can't convert UTF-8 text/plain into ASCII text/plain without dropping data.

You could convert UTF-8 text/plain into ASCII someother/format. For instance, HTML lets any character in UTF-8 be representing in an ASCII data file using character references.

If we continue with that example, in JavaScript, charCodeAt could help with converting a string to a representation of it using HTML character references.

Another approach is taken by URLs, and implemented in JS as encodeURIComponent.

It is impossible to convert an UTF-8 string into ASCII but it is possible to encode Unicode as an ASCII compatible string.

Probably you want to use Punycode - this is already a standard Unicode encoding that encodes all Unicode characters into ASCII. For JavaScript code check this question

Please edit you question title and description in order to prevent others from down-voting it - do not use term conversion, use encoding.

If the string is encoded as UTF-8, it's not a string any more. It's binary data, and if you want to represent the binary data as ASCII, you have to format it into a string that can be represented using the limited ASCII character set.

One way is to use base-64 encoding (example in C#):

string original = "asdf";
// encode the string into UTF-8 data:
byte[] encodedUtf8 = Encoding.UTF8.GetBytes(original);
// format the data into base-64:
string base64 = Convert.ToBase64String(encodedUtf8);

If you want the string encoded as ASCII data:

// encode the base-64 string into ASCII data:
byte[] encodedAscii = Encoding.ASCII.GetBytes(base64);

function utf8ToAscii(str) {
    /**
     * ASCII contains 127 characters.
     * 
     * In JavaScript, strings is encoded by UTF-16, it means that
     * js cannot present strings which charCode greater than 2^16. Eg:
     * `String.fromCharCode(0) === String.fromCharCode(2**16)`
     *
     * @see https://developer.mozilla.org/en-US/docs/Web/API/DOMString/Binary
     */
    const reg = /[\x7f-\uffff]/g; // charCode: [127, 65535]
    const replacer = (s) => {
        const charCode = s.charCodeAt(0);
        const unicode = charCode.toString(16).padStart(4, '0');
        return `\\u${unicode}`;
    };

    return str.replace(reg, replacer);
}

Better way

See Uint8Array to string in Javascript also. You can use TextEncoder and Uint8Array:

function utf8ToAscii(str) {
    const enc = new TextEncoder('utf-8');
    const u8s = enc.encode(str);

    return Array.from(u8s).map(v => String.fromCharCode(v)).join('');
}
// For ascii to string
// new TextDecoder().decode(new Uint8Array(str.split('').map(v=>v.charCodeAt(0))))

Do you want to strip all non ascii chars (slash replace them with '?', etc) or to store Unicode code points in a non unicode system?

First can be done in a loop checking for values > 128 and replacing them.

If you don't want to use "any platform/framework/library" then you will need to write your own encoder. Otherwise I'd just use JQuery's .html();

Your requirement is pretty strange.

Converting UTF-8 into ASCII would loose all information about Unicode codepoints > 127 (i.e. everything that's not in ASCII).

You could, however try to encode your Unicode data (no matter what source encoding) in an ASCII-compatible encoding, such as UTF-7. This would mean that the data that is produced could legally be interpreted as ASCII, but it is really UTF-7.

Here is a function to convert UTF8 accents to ASCII Accents (àéèî etc) If there is an accent in the string it's converted to %239 for exemple Then on the other side, I parse the string and I know when there is an accent and what is the ASCII char.

I used it in a javascript software to send data to a microcontroller that works in ASCII.

convertUtf8ToAscii = function (str) {
    var asciiStr = "";
    var refTable = { // Reference table Unicode vs ASCII
        199: 128, 252: 129, 233: 130, 226: 131, 228: 132, 224: 133, 231: 135, 234: 136, 235: 137, 232: 138,
        239: 139, 238: 140, 236: 141, 196: 142, 201: 144, 244: 147, 246: 148, 242: 149, 251: 150, 249: 151
    };
    for(var i = 0; i < str.length; i++){
        var ascii = refTable[str.charCodeAt(i)];
        if (ascii != undefined)
            asciiStr += "%" +ascii;
        else
            asciiStr += str[i];
    }
    return asciiStr;
}

If you are using node.js you can use the TextDecoder class.

const decoder = new TextDecoder('ascii');
let text = decoder.decode(buffer);

An implementation of the quote() function might do what you want. My version can be found here

You can use eval() to reverse the encoding:

var foo = 'Hägar';
var quotedFoo = quote(foo);
var unquotedFoo = eval(quotedFoo);
alert(foo === unquotedFoo);