I am a relative newcomer to python, and have been blown away by the flexibility of some operations in Python. The string-formatting operator, %, is really wonderful and flexible. This is my attempt at implementing something similar in JavaScript.

Obviously, you can’t create a new operator in JavaScript, and in addition you can’t use % as a JavaScript identifier. So I went for the next-best thing:

String.prototype.format = function string_format(d) { // there are two modes of operation... unnamed indices are read in order; // named indices using %(name)s. The two styles cannot be mixed. // Unnamed indices can be passed as either a single argument to this function, // multiple arguments to this function, or as a single array argument let curindex = 0; if (arguments.length > 1) d = arguments; function r(s, key, type) { let v; if (key == "") { if (curindex == -1) throw Error("Cannot mix named and positional indices in string formatting."); if (curindex == 0 && (!(d instanceof Object) || !(0 in d))) v = d; else if (!(curindex in d)) throw Error("Insufficient number of items in format, requesting item %i".format(curindex)); else v = d[curindex]; ++curindex; } else { key = key.slice(1, -1); if (curindex > 0) throw Error("Cannot mix named and positional indices in string formatting."); curindex = -1; if (!(key in d)) throw Error("Key '%s' not present during string substitution.".format(key)); v = d[key]; } switch (type) { case "s": return v.toString(); case "r": return v.toSource(); case "i": return parseInt(v); case "f": return Number(v); case "%": return "%"; default: throw Error("Unexpected format character '%s'.".format(type)); } } return this.replace(/%(\([^)]+\))?(.)/g, r); }; String.prototype.ø = String.prototype.format;

If you are at all familiar with the python string-formatting operator, this should be very similar:

"%s %s".ø("angry", "monkeys"); == "angry monkeys"; "%(key)s: %(value)s".ø({key: 'bananas', value: 'tasty'}) == "bananas: tasty"; "%i - %i".ø([1, 3]) == "1 - 3"; "%r".ø(['α', 'ω'] == '["α", "ω"]';

I know that there are many sprintf-like libraries out there for JavaScript: I just happen to like mine best. Caution: this code requires JavaScript 1.7… if you replace some “let”s with “var”s it may work in older browsers, but I haven’t tested it.

Being able to pass a function as a replacement in String.replace is a very powerful feature!