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JavaScript equivalent of Python's format() function?

Python has this beautiful function to turn this:

bar1 = 'foobar'
bar2 = 'jumped'
bar3 = 'dog'

foo = 'The lazy ' + bar3 + ' ' + bar2 ' over the ' + bar1
# The lazy dog jumped over the foobar


Into this:

bar1 = 'foobar'
bar2 = 'jumped'
bar3 = 'dog'

foo = 'The lazy over the '.format(bar3, bar2, bar1)
# The lazy dog jumped over the foobar


Does JavaScript have such a function? If not, how would I create one which follows the same syntax as Python's implementation?

String.prototype

prototype

10 Answers
10

Another approach, using the String.prototype.replace method, with a "replacer" function as second argument:

String.prototype.replace

String.prototype.format = function ()
var i = 0, args = arguments;
return this.replace(//g, function ()
return typeof args[i] != 'undefined' ? args[i++] : '';
);
;

var bar1 = 'foobar',
bar2 = 'jumped',
bar3 = 'dog';

'The lazy over the '.format(bar3, bar2, bar1);
// "The lazy dog jumped over the foobar"


1

There is a way, but not exactly using format.


var name = "John";
var age = 19;
var message = `My name is $name and I am $age years old`;
console.log(message);

jsfiddle - link

$...

var message = `My name is $ 'default' and I am $ 'default' years old`;

f'My name is name and I am age'

Looking for an answer for the same question, I just found this: https://github.com/davidchambers/string-format, which is "JavaScript string formatting inspired by Python’s str.format()". It seem it's pretty much the same as python's format() function.

str.format()

format()

Taken from YAHOOs library:

YAHOO.Tools.printf = function()
var num = arguments.length;
var oStr = arguments[0];
for (var i = 1; i < num; i++)
var pattern = "\" + (i-1) + "\";
var re = new RegExp(pattern, "g");
oStr = oStr.replace(re, arguments[i]);

return oStr;



Call it like:

bar1 = 'foobar'
bar2 = 'jumped'
bar3 = 'dog'

foo = YAHOO.Tools.printf('The lazy 0 1 over the 2', bar3, bar2, bar1);


Here's my first attempt. Feel free to point out flaws.

Example: http://jsfiddle.net/wFb2p/5/

String.prototype.format = function() ;


EDIT: Made it a bit more efficient by eliminating the .match() call in the while statement.

.match()

while

EDIT: Changed it so that the same error is thrown if you don't pass any arguments.

arguments[i]

This code allows you to specify exactly which brackets to replace with which strings. The brackets don't need to be in the same order as the arguments, and multiple brackets are possible. The format function takes an array of values as its parameter, with each key being one of the bracketed 'variables' which is replaced by its corresponding value.

String.prototype.format = function (arguments)
var this_string = '';
for (var char_pos = 0; char_pos < this.length; char_pos++)
this_string = this_string + this[char_pos];


for (var key in arguments)
var string_key = '' + key + ''
this_string = this_string.replace(new RegExp(string_key, 'g'), arguments[key]);

return this_string;
;

'The time is time and today is day, day, day. Oh, and did I mention that the time is time.'.format(day:'Monday',time:'2:13');
//'The time is 2:13 and today is Monday, Monday, Monday. Oh, and did I mention that the time is 2:13.'


this_string = this

JS:

String.prototype.format = function ()
var str = this;
for (var i = 0; i < arguments.length; i++)
str = str.replace('' + i + '', arguments[i]);

return str;


bar1 = 'foobar';
bar2 = 'jumped';
bar3 = 'dog';

python_format = 'The lazy 2 1 over the 0'.format(bar1,bar2,bar3);

document.getElementById("demo").innerHTML = "JavaScript equivalent of Python's format() function:<br><span id='python_str'>" + python_format + "</span>";


HTML:

<p id="demo"></p>


CSS:

span#python_str
color: red;
font-style: italic;



OUTPUT:

JavaScript equivalent of Python's format() function:

The lazy dog jumped over the foobar

DEMO:

jsFiddle

JavaScript doesn't have such a function AFAIK.

You could create one by modifying the String class's prototype object to add a format() method which takes a variable number of arguments.

In the format method you'd have to get the String's instance value (the actual string) and then parse it for '' and insert the appropriate argument.

Then return the new string to the caller.

JavaScript does not have a string formatting function by default, although you can create your own or use one someone else has made (such as sprintf)

In the file

https://github.com/BruceSherwood/glowscript/blob/master/lib/glow/api_misc.js

is a function String.prototype.format = function(args) that fully implements the Python string.format() function, not limited simply to handling character strings.

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