Javascript, convert unicode string to Javascript escape?

"み".charCodeAt(0).toString(16);

This will give you the unicode (in Hex). You can run it through a loop:

String.prototype.toUnicode = function(){
    var result = "";
    for(var i = 0; i < this.length; i++){
        // Assumption: all characters are < 0xffff
        result += "\\u" + ("000" + this[i].charCodeAt(0).toString(16)).substr(-4);
    }
    return result;
};

"みどりいろ".toUnicode();       //"\u307f\u3069\u308a\u3044\u308d"
"Mi Do Ri I Ro".toUnicode();  //"\u004d\u0069\u0020\u0044\u006f\u0020\u0052\u0069\u0020\u0049\u0020\u0052\u006f"
"Green".toUniCode();          //"\u0047\u0072\u0065\u0065\u006e"

Demo: http://jsfiddle.net/DerekL/X7MCy/

More on: .charCodeAt

just

escape("みどりいろ")

should meet the needs for most cases, buf if you need it in the form of "\u" instead of "%xx" / "%uxxxx" then you might want to use regular expressions:

escape("みどりいろ").replace(/%/g, '\\').toLowerCase()

escape("みどりいろ").replace(/%u([A-F0-9]{4})|%([A-F0-9]{2})/g, function(_, u, x) { return "\\u" + (u || '00' + x).toLowerCase() });

(toLowerCase is optional to make it look exactly like in the first post)

It doesn't escape characters it doesn't need to in most cases which may be a plus for you; if not - see Derek's answer, or use my version:

'\\u' + "みどりいろ".split('').map(function(t) { return ('000' + t.charCodeAt(0).toString(16)).substr(-4) }).join('\\u');

My version of code, based on previous answers. I use if to convert non UTF8 chars in JSON.stringify().

const toUTF8 = string =>
    string.split('').map(
        ch => !ch.match(/^[^a-z0-9\s\t\r\n_|\\+()!@#$%^&*=?/~`:;'"\[\]\-]+$/i)
            ? ch
            : '\\' + 'u' + '000' + ch.charCodeAt(0).toString(16)
    ).join('');

Usage:

JSON.stringify({key: 'Категория дли импорта'}, (key, value) => {
    if (typeof value === "string") {
        return toUTF8(value);
    }

    return value;
});

Returns JSON:

{"key":"\\u00041a\\u000430\\u000442\\u000435\\u000433\\u00043e\\u000440\\u000438\\u00044f \\u000434\\u00043b\\u000438 \\u000438\\u00043c\\u00043f\\u00043e\\u000440\\u000442\\u000430"}

Above answer is reasonable. A slight space and performance optimization:

function escapeUnicode(str) {
    return str.replace(/[^\0-~]/g, function(ch) {
        return "\\u" + ("000" + ch.charCodeAt().toString(16)).slice(-4);
    });
}