Python's %s equivalent in Javascript?

Question

I'm currently learning NodeJS after working with Python for the last few years.

In Python I was able to save a string inside a JSON with dynamic parameters and set them once the string loaded, for example:

MY JSON:

j = {
"dynamicText": "Hello %s, how are you?"
}

and then use my string like that:

print(j['dynamicText'] % ("Dan"))

so Python replaces the %s with "Dan".

I am looking for the JS equivalent but could not find one. Any ideas?

** Forgot to mention: I want to save the JSON as another config file so literals won't work here

[Template_literals](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals) — charlietfl, May 28 '18 at 17:07
the only way (that I know) to do that is with a function that takes a parameter and return an object (in this case with the name that you want pass in as an argument) — manAbl, May 28 '18 at 17:07
@ASDFGerte Thanks, but I want to save this JSON as separated file.. — Dan Monero, May 28 '18 at 17:12
seems like just syntactic sugar for `j['dynamicText'].replace('s%', 'Dan')` — Slai, May 28 '18 at 17:13
Possible duplicate of [NodeJS String format like Python?](https://stackoverflow.com/questions/10788408/nodejs-string-format-like-python) — Frax, May 28 '18 at 17:17

brk · Answer 1 · 2018-05-28T17:14:30.067

4

Use template literal. This is comparatively new and may not support ancient browsers

var test = "Dan"
var j = {
  "dynamicText": `Hello ${test}, how are you?`
}

console.log(j["dynamicText"])

Alternatively you can create a function and inside that function use string.replace method to to replace a word with new word

var test = "Dan"
var j = {
  "dynamicText": "Hello %s, how are you?"
}

function replace(toReplaceText, replaceWith) {
  let objText = j["dynamicText"].replace(toReplaceText, replaceWith);
  return objText;
}


console.log(replace('%s', test))

edited May 28 '18 at 17:14

answered May 28 '18 at 17:07

brk

48,835
10
56
78

Yeah, but my problem is that I want to save this JSON separated, and without defining the variable first – Dan Monero May 28 '18 at 17:10

score 4 · Answer 2 · answered May 28 '18 at 17:16

4

There is no predefined way in JavaScript, but you could still achieve something like below. Which I have done in my existing Application.

function formatString(str, ...params) {
    for (let i = 0; i < params.length; i++) {
        var reg = new RegExp("\\{" + i + "\\}", "gm");
        str = str.replace(reg, params[i]);
    }
    return str;
}

now formatString('You have {0} cars and {1} bikes', 'two', 'three') returns 'You have two cars and three bikes'

In this way if {0} repeats in String it replaces all.

like formatString('You have {0} cars, {1} bikes and {0} jeeps', 'two', 'three') to "You have two cars, three bikes and two jeeps"

Hope this helps.

answered May 28 '18 at 17:16

ngChaitanya

435
2
8

In Node there is [`util.format()`](https://nodejs.org/api/util.html#util_util_format_format_args), so your answer is quite inaccurate, even though it's correct for in-browser JS. – Frax May 28 '18 at 17:20
Yes agree there is .format in nodeJs but could you let me know why would this is inaccurate for node? you mean this would break in NodeJs? – ngChaitanya May 28 '18 at 17:25
My comment was aimed at "There is no predefined way in JavaScript" sentence, as there is a predefined way in Node. Your code would work just the same, but would be kind of reinventing the wheel in this case. – Frax May 28 '18 at 17:47

gtsiam · Answer 3 · 2018-05-28T17:40:10.983

1

You can write a String.format method, using regex, and the String.replace method:

String.prototype.format = function() {
    var args = arguments;
    return this.replace(/{(\d+)}/g, (match, p1) => {
        var i = parseInt(p1);
        return typeof args[i] != 'undefined' ? args[i] : match;
    });
}

After that, running:

console.log("{0}{1}".format("John", "Doe"));

Will output: John Doe

Of course, if you don't like editing the prototype of objects you don't own (it is generally good practice), you can just create a function:

var format = function(str) {
    var args = arguments;
    return str.replace(/{(\d+)}/g, (match, p1) => {
        var i = parseInt(p1);
        return typeof args[i+1] != 'undefined' ? args[i+1] : match;
    });
}

edited May 28 '18 at 17:40

answered May 28 '18 at 17:07

gtsiam

85
10

For some reasons I got a "format is not a function" error – Dan Monero May 28 '18 at 17:09
@DanMonero Ok, I forgot that was a custom function I use. Sorry for that, I fixed it now – gtsiam May 28 '18 at 17:26

score 0 · Answer 4 · answered May 28 '18 at 17:21

For a few nice alternatives, you may want to take a look at JavaScript equivalent to printf/string.format

While it's asking for a C-like printf() equivalent for JS, the answers would also apply to your question, since Python format strings are inspired by C.

score 0 · Accepted Answer · answered Jun 17 '18 at 21:02

0

I've found a great npm module that does exactly what I was needed - string-format.

String-Format in NPM

answered Jun 17 '18 at 21:02

Dan Monero

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Python's %s equivalent in Javascript?

5 Answers5