[ACCEPTED]-MessageFormat in javascript (parameters in localized UI strings)-internationalization

String.prototype.format = function() {
    var args = arguments;

    return this.replace(/\{(\d+)\}/g, function() {
        return args[arguments[1]];
    });
};

// Returns '2 + -1 = 1'.
'{0} + {1} = {2}'.format(2, -1, 1);

Or to fit your requirement:

function replaceParams(string, replacements) {
    return string.replace(/\{(\d+)\}/g, function() {
        return replacements[arguments[1]];
    });

    // Or, if prototype code above...
    String.format.apply(string, replacements);
}

You can add fancy 2 i18n features such as ordinal-i-fying (whatever 1 it's called):

// Not well tested.

i18n.en.filters = {
    ordinal: function(n) {
        // FIXME Doesn't handle all cases.
        switch(('' + n).substr(-1)) {
            case '1':
                return '' + n + 'st';
            case '2':
                return '' + n + 'nd';
            case '3':
                return '' + n + 'rd';
            case '4':
            case '5':
            case '6':
            case '7':
            case '8':
            case '9':
            case '0':
                return '' + n + 'th';
            default:
                return n; // Just in case...
        }

    },
    plural: function(n, singular, plural) {
        if(n == 1) {
            return singular;
        } else {
            return plural;
        }
    }
};

i18n.current = i18n.en;

String.prototype.format = function() {
    var args = arguments;

    return this.replace(/\{((\d+)((\|\w+(:\w+)*)*))\}/g, function() {
        var arg = args[arguments[2]],
            filters = arguments[3].split('|'),
            i, curFilter, curFilterArgs, curFilterFunc;

        for(i = 0; i < filters.length; ++i) {
            curFilterArgs = filters[i].split(':');
            curFilter = curFilterArgs.shift();
            curFilterFunc = i18n.current.filters[curFilter];

            if(typeof curFilterFunc === 'function') {
                arg = curFilterFunc.apply(null, [ arg ].concat(curFilterArgs));
            }
        }

        return arg;
    });
};

'You have {0} {0|plural:cow:cows} but I have {1} {1|plural:cow:cows}.'.format(2,1);
'My horse came in {0|ordinal} place while yours came in {1|ordinal}.'.format(42,1);

Looks like I was only about 3 years late, but 7 in case anyone still needs an actual standalone 6 MessageFormat library for JS:

https://github.com/SlexAxton/messageformat.js

There ya go! Compiles 5 to JS - so it can be really speedy, and 4 supports SelectFormat and PluralFormat.

Note:: This is ICU MessageFormat 3 which is a bit different (read: better) than 2 the stuff that might be built into your 1 language.

@strager answer did not really work for 5 me, but with a little tweak i got it to 4 be just what i was looking for (which is 3 very similar to what @Omesh was aiming to).

String.prototype.format = function() {
    var args = arguments;

    return this.replace(/\{(\d+)\}/g, function(a) {
        return args[0][parseInt(a.match(/(\d+)/g))];
    });
};

Notice 2 the index value of the args array is different.

It 1 should be called just like @strager suggests:

'I like {0} and {1} but not {2}'.format('apples', 'oranges', 'kiwi');