String Formatting in JavaScript. In javascript, there is no built-in string formatting function.
JavaScript Print JavaScript does not have any print object or print methods. You cannot access output devices from JavaScript. The only exception is that you can call the window.print() method in the browser to print the content of the current window.
String. format returns a new String, while System. out. printf just displays the newly formatted String to System.
Building on the previously suggested solutions:
// First, checks if it isn't implemented yet.
if (!String.prototype.format) {
String.prototype.format = function() {
var args = arguments;
return this.replace(/{(\d+)}/g, function(match, number) {
return typeof args[number] != 'undefined'
? args[number]
: match
;
});
};
}
"{0} is dead, but {1} is alive! {0} {2}".format("ASP", "ASP.NET")
outputs
ASP is dead, but ASP.NET is alive! ASP {2}
If you prefer not to modify String
's prototype:
if (!String.format) {
String.format = function(format) {
var args = Array.prototype.slice.call(arguments, 1);
return format.replace(/{(\d+)}/g, function(match, number) {
return typeof args[number] != 'undefined'
? args[number]
: match
;
});
};
}
Gives you the much more familiar:
String.format('{0} is dead, but {1} is alive! {0} {2}', 'ASP', 'ASP.NET');
with the same result:
ASP is dead, but ASP.NET is alive! ASP {2}
From ES6 on you could use template strings:
let soMany = 10;
console.log(`This is ${soMany} times easier!`);
// "This is 10 times easier!
See Kim's answer below for details.
Try sprintf() for JavaScript.
If you really want to do a simple format method on your own, don’t do the replacements successively but do them simultaneously.
Because most of the other proposals that are mentioned fail when a replace string of previous replacement does also contain a format sequence like this:
"{0}{1}".format("{1}", "{0}")
Normally you would expect the output to be {1}{0}
but the actual output is {1}{1}
. So do a simultaneously replacement instead like in fearphage’s suggestion.
It's funny because Stack Overflow actually has their own formatting function for the String
prototype called formatUnicorn
. Try it! Go into the console and type something like:
"Hello, {name}, are you feeling {adjective}?".formatUnicorn({name:"Gabriel", adjective: "OK"});
You get this output:
Hello, Gabriel, are you feeling OK?
You can use objects, arrays, and strings as arguments! I got its code and reworked it to produce a new version of String.prototype.format
:
String.prototype.formatUnicorn = String.prototype.formatUnicorn ||
function () {
"use strict";
var str = this.toString();
if (arguments.length) {
var t = typeof arguments[0];
var key;
var args = ("string" === t || "number" === t) ?
Array.prototype.slice.call(arguments)
: arguments[0];
for (key in args) {
str = str.replace(new RegExp("\\{" + key + "\\}", "gi"), args[key]);
}
}
return str;
};
Note the clever Array.prototype.slice.call(arguments)
call -- that means if you throw in arguments that are strings or numbers, not a single JSON-style object, you get C#'s String.Format
behavior almost exactly.
"a{0}bcd{1}ef".formatUnicorn("FOO", "BAR"); // yields "aFOObcdBARef"
That's because Array
's slice
will force whatever's in arguments
into an Array
, whether it was originally or not, and the key
will be the index (0, 1, 2...) of each array element coerced into a string (eg, "0", so "\\{0\\}"
for your first regexp pattern).
Neat.
I got to this question page hoping to find how to format numbers in JavaScript, without introducing yet another library. Here's what I've found:
The equivalent of sprintf("%.2f", num)
in JavaScript seems to be num.toFixed(2)
, which formats num
to 2 decimal places, with rounding (but see @ars265's comment about Math.round
below).
(12.345).toFixed(2); // returns "12.35" (rounding!)
(12.3).toFixed(2); // returns "12.30" (zero padding)
The equivalent of sprintf("%.2e", num)
is num.toExponential(2)
.
(33333).toExponential(2); // "3.33e+4"
To print numbers in base B, try num.toString(B)
. JavaScript supports automatic conversion to and from bases 2 through 36 (in addition, some browsers have limited support for base64 encoding).
(3735928559).toString(16); // to base 16: "deadbeef"
parseInt("deadbeef", 16); // from base 16: 3735928559
Quick tutorial on JS number formatting
Mozilla reference page for toFixed() (with links to toPrecision(), toExponential(), toLocaleString(), ...)
From ES6 on you could use template strings:
let soMany = 10;
console.log(`This is ${soMany} times easier!`);
// "This is 10 times easier!
Be aware that template strings are surrounded by backticks ` instead of (single) quotes.
For further information:
https://developers.google.com/web/updates/2015/01/ES6-Template-Strings
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/template_strings
Note: Check the mozilla-site to find a list of supported browsers.
jsxt, Zippo
This option fits better.
String.prototype.format = function() {
var formatted = this;
for (var i = 0; i < arguments.length; i++) {
var regexp = new RegExp('\\{'+i+'\\}', 'gi');
formatted = formatted.replace(regexp, arguments[i]);
}
return formatted;
};
With this option I can replace strings like these:
'The {0} is dead. Don\'t code {0}. Code {1} that is open source!'.format('ASP', 'PHP');
With your code the second {0} wouldn't be replaced. ;)
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