Archives For strings

>For ages, I have relied on Douglas Crockford’s excellent supplant() for my string formatting needs in JS. It works great with whatever objects you have already hanging around in your code:

var myObj = { name: "Conan", surname: "The Barbarian" };

"{name} {surname}".supplant(myObj);

// result:

"Conan The Barbarian"

This is great, reliable and all that…

But it is a bit verbose in the cases where you do not already have an object hanging around with the correct properties. I tried to find an existing implementation, but most of them were still too verbose relying on hacks like: “{$1} {$2}” or “$1 $2” or worse.

I want a “printf” or Python-style string interpolation:

#python

"%s %s" % ("Conan", "The Barbarian",)

// result:

"Conan The Barbarian"

That’s what I want! – only for strings, it doesn’t have to be too feature-rich. Eventually it would be nice if it could handle integers and floats etc.

After some questions and answers on irc in #js, I was able to get something rudimentary going. I started with supplant, and added support for arrays and arguments.

Behold! String.printf():


String.prototype.printf = function (obj) {
var useArguments = false;
var _arguments = arguments;
var i = -1;
if (typeof _arguments[0] == "string") {
useArguments = true;
}
if (obj instanceof Array || useArguments) {
return this.replace(/\%s/g,
function (a, b) {
i++;
if (useArguments) {
if (typeof _arguments[i] == 'string') {
return _arguments[i];
}
else {
throw new Error("Arguments element is an invalid type");
}
}
return obj[i];
});
}
else {
return this.replace(/{([^{}]*)}/g,
function (a, b) {
var r = obj[b];
return typeof r === 'string' || typeof r === 'number' ? r : a;
});
}
};

Examples:

"{f} {b}".printf({f: "foo", b: "bar"});

"%s %s".printf(["foo", "bar"]);

"%s %s".printf("foo", "bar");

// all of which give this result:

"foo bar"

Let me know if you think of more tweaks for this – or problems. It pains me every time I see concatenation in JS code. Question, which is more efficient? ‘+’ or String.replace()?

Cheers!

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