CODEWITHSUNDEEP
javascriptecmascript-6evaltemplate-strings
KOLANICH
KOLANICH

Reputation: 3072

Convert a string to a template string

Is it possible to create a template string as a usual string,

let a = "b:${b}";

and then convert it into a template string,

let b = 10;
console.log(a.template()); // b:10

without eval, new Function and other means of dynamic code generation?

Upvotes: 232

Views: 122962

Answers (23)

chethan kamath
chethan kamath

Reputation: 11

Option1:

function replaceTemplate(templateString, objContext) {
        return templateString.replace(/\${[A-Za-z0-9_\$]+}/ig, (data) => { const key = data.replace(/^\${/, '').replace(/}$/, ''); return objContext[key] || data })
     }

or

Option2:

function replaceTemplate(templateString) {
    return templateString.replace(/\${[A-Za-z0-9_\$]+}/ig, (data) => { const key = data.replace(/^\${/, '').replace(/}$/, ''); return this[key] || data })
}

example

const fruits = { apple: 1, banana: 3, stringValue: 'a: ${apple}, b: ${banana}'} 
replaceTemplate(fruits.stringValue, fruits) // Option1: 
// or 
replaceTemplate.bind(fruits)(fruits.stringValue) //Option2: 

output: 'apple: 1, banana: 3, orange: ${orange}'

Upvotes: 1

Giang Thanh Dat
Giang Thanh Dat

Reputation: 11

You can refer to this solution 

const interpolate = (str) =>
  new Function(`return \`${new String(str)}\`;`)();

const foo = 'My';

const obj = {
  text: 'Hanibal Lector',
  firstNum: 1,
  secondNum: 2
}
 
const str = "${foo} name is : ${obj.text}. sum = ${obj.firstNum} + ${obj.secondNum} = ${obj.firstNum + obj.secondNum}";

console.log(interpolate(str));

by returning a new function that has a template literal that copies the full string into that template literal you achieve the correct variable expansion wanted - this is useful if you want to have strings saved in some content management system that you then want to expand.

The code also includes an example of this working.

Upvotes: 1

M3D
M3D

Reputation: 783

TLDR: https://jsfiddle.net/bj89zntu/1/

Everyone seems to be worried about accessing variables. Why not just pass them? I'm sure it won't be too hard to get the variable context in the caller and pass it down. Use ninjagecko's answer to get the props from obj.

function renderString(str,obj){
    return str.replace(/\$\{.+?\}/g,(match)=>{return index(obj,match)})
}

Here is the full code:

function index(obj,is,value) {
    if (typeof is == 'string')
        is=is.split('.');
    if (is.length==1 && value!==undefined)
        return obj[is[0]] = value;
    else if (is.length==0)
        return obj;
    else
        return index(obj[is[0]],is.slice(1), value);
}

function renderString(str,obj){
    return str.replace(/\$\{.+?\}/g,(match)=>{return index(obj,match)})
}

renderString('abc${a}asdas',{a:23,b:44}) //abc23asdas
renderString('abc${a.c}asdas',{a:{c:22,d:55},b:44}) //abc22asdas

Upvotes: 16

Mohamad Abdel Rida
Mohamad Abdel Rida

Reputation: 303

I came up with this implementation and it works like a charm.

function interpolateTemplate(template: string, args: any): string {
  return Object.entries(args).reduce(
    (result, [arg, val]) => result.replace(`$\{${arg}}`, `${val}`),
    template,
  )
}

const template = 'This is an example: ${name}, ${age} ${email}'

console.log(interpolateTemplate(template,{name:'Med', age:'20', email:'[email protected]'}))

You could raise an error if arg is not found in template

Upvotes: 5

Thiago Lagden
Thiago Lagden

Reputation: 301

Faz assim (This way):

let a = 'b:${this.b}'
let b = 10

function template(templateString, templateVars) {
    return new Function('return `' + templateString + '`').call(templateVars)
}

result.textContent = template(a, {b})
<b id=result></b>

Upvotes: 1

joehep
joehep

Reputation: 154

I realize I am late to the game, but you could:

const a =  (b) => `b:${b}`;

let b = 10;
console.log(a(b)); // b:10

Upvotes: -4

Mateusz Moska
Mateusz Moska

Reputation: 2029

In my project I've created something like this with ES6:

String.prototype.interpolate = function(params) {
  const names = Object.keys(params);
  const vals = Object.values(params);
  return new Function(...names, `return \`${this}\`;`)(...vals);
}

const template = 'Example text: ${text}';
const result = template.interpolate({
  text: 'Foo Boo'
});
console.log(result);

Upvotes: 173

alexpods
alexpods

Reputation: 48505

As your template string must get reference to the b variable dynamically (in runtime), so the answer is: NO, it's impossible to do it without dynamic code generation.

But, with eval it's pretty simple:

let tpl = eval('`'+a+'`');

Upvotes: 129

pekaaw
pekaaw

Reputation: 2597

There are many good solutions posted here, but none yet which utilizes the ES6 String.raw method. Here is my contriubution. It has an important limitation in that it will only accept properties from a passed in object, meaning no code execution in the template will work.

function parseStringTemplate(str, obj) {
    let parts = str.split(/\$\{(?!\d)[\wæøåÆØÅ]*\}/);
    let args = str.match(/[^{\}]+(?=})/g) || [];
    let parameters = args.map(argument => obj[argument] || (obj[argument] === undefined ? "" : obj[argument]));
    return String.raw({ raw: parts }, ...parameters);
}

let template = "Hello, ${name}! Are you ${age} years old?";
let values = { name: "John Doe", age: 18 };

parseStringTemplate(template, values);
// output: Hello, John Doe! Are you 18 years old?
  1. Split string into non-argument textual parts. See regex.
    parts: ["Hello, ", "! Are you ", " years old?"]
  2. Split string into property names. Empty array if match fails.
    args: ["name", "age"]
  3. Map parameters from obj by property name. Solution is limited by shallow one level mapping. Undefined values are substituted with an empty string, but other falsy values are accepted.
    parameters: ["John Doe", 18]
  4. Utilize String.raw(...) and return result.

Upvotes: 18

cesarmarinhorj
cesarmarinhorj

Reputation: 27

I made my own solution doing a type with a description as a function

export class Foo {
...
description?: Object;
...
}

let myFoo:Foo = {
...
  description: (a,b) => `Welcome ${a}, glad to see you like the ${b} section`.
...
}

and so doing:

let myDescription = myFoo.description('Bar', 'bar');

Upvotes: 1

colxi
colxi

Reputation: 8670

You should try this tiny JS module, by Andrea Giammarchi, from github : https://github.com/WebReflection/backtick-template

/*! (C) 2017 Andrea Giammarchi - MIT Style License */
function template(fn, $str, $object) {'use strict';
  var
    stringify = JSON.stringify,
    hasTransformer = typeof fn === 'function',
    str = hasTransformer ? $str : fn,
    object = hasTransformer ? $object : $str,
    i = 0, length = str.length,
    strings = i < length ? [] : ['""'],
    values = hasTransformer ? [] : strings,
    open, close, counter
  ;
  while (i < length) {
    open = str.indexOf('${', i);
    if (-1 < open) {
      strings.push(stringify(str.slice(i, open)));
      open += 2;
      close = open;
      counter = 1;
      while (close < length) {
        switch (str.charAt(close++)) {
          case '}': counter -= 1; break;
          case '{': counter += 1; break;
        }
        if (counter < 1) {
          values.push('(' + str.slice(open, close - 1) + ')');
          break;
        }
      }
      i = close;
    } else {
      strings.push(stringify(str.slice(i)));
      i = length;
    }
  }
  if (hasTransformer) {
    str = 'function' + (Math.random() * 1e5 | 0);
    if (strings.length === values.length) strings.push('""');
    strings = [
      str,
      'with(this)return ' + str + '([' + strings + ']' + (
        values.length ? (',' + values.join(',')) : ''
      ) + ')'
    ];
  } else {
    strings = ['with(this)return ' + strings.join('+')];
  }
  return Function.apply(null, strings).apply(
    object,
    hasTransformer ? [fn] : []
  );
}

template.asMethod = function (fn, object) {'use strict';
  return typeof fn === 'function' ?
    template(fn, this, object) :
    template(this, fn);
};

Demo (all the following tests return true):

const info = 'template';
// just string
`some ${info}` === template('some ${info}', {info});

// passing through a transformer
transform `some ${info}` === template(transform, 'some ${info}', {info});

// using it as String method
String.prototype.template = template.asMethod;

`some ${info}` === 'some ${info}'.template({info});

transform `some ${info}` === 'some ${info}'.template(transform, {info});

Upvotes: 1

Matt Browne
Matt Browne

Reputation: 12419

Similar to Daniel's answer (and s.meijer's gist) but more readable:

const regex = /\${[^{]+}/g;

export default function interpolate(template, variables, fallback) {
    return template.replace(regex, (match) => {
        const path = match.slice(2, -1).trim();
        return getObjPath(path, variables, fallback);
    });
}

//get the specified property or nested property of an object
function getObjPath(path, obj, fallback = '') {
    return path.split('.').reduce((res, key) => res[key] || fallback, obj);
}

Note: This slightly improves s.meijer's original, since it won't match things like ${foo{bar} (the regex only allows non-curly brace characters inside ${ and }).

UPDATE: I was asked for an example using this, so here you go:

const replacements = {
    name: 'Bob',
    age: 37
}

interpolate('My name is ${name}, and I am ${age}.', replacements)

Upvotes: 10

Regular Jo
Regular Jo

Reputation: 5510

Since we're reinventing the wheel on something that would be a lovely feature in javascript.

I use eval(), which is not secure, but javascript is not secure. I readily admit that I'm not excellent with javascript, but I had a need, and I needed an answer so I made one.

I chose to stylize my variables with an @ rather than an $, particularly because I want to use the multiline feature of literals without evaluating til it's ready. So variable syntax is @{OptionalObject.OptionalObjectN.VARIABLE_NAME}

I am no javascript expert, so I'd gladly take advice on improvement but...

var prsLiteral, prsRegex = /\@\{(.*?)(?!\@\{)\}/g
for(i = 0; i < myResultSet.length; i++) {
    prsLiteral = rt.replace(prsRegex,function (match,varname) {
        return eval(varname + "[" + i + "]");
        // you could instead use return eval(varname) if you're not looping.
    })
    console.log(prsLiteral);
}

A very simple implementation follows

myResultSet = {totalrecords: 2,
Name: ["Bob", "Stephanie"],
Age: [37,22]};

rt = `My name is @{myResultSet.Name}, and I am @{myResultSet.Age}.`

var prsLiteral, prsRegex = /\@\{(.*?)(?!\@\{)\}/g
for(i = 0; i < myResultSet.totalrecords; i++) {
    prsLiteral = rt.replace(prsRegex,function (match,varname) {
        return eval(varname + "[" + i + "]");
        // you could instead use return eval(varname) if you're not looping.
    })
    console.log(prsLiteral);
}

In my actual implementation, I choose to use @{{variable}}. One more set of braces. Absurdly unlikely to encounter that unexpectedly. The regex for that would look like /\@\{\{(.*?)(?!\@\{\{)\}\}/g

To make that easier to read

\@\{\{    # opening sequence, @{{ literally.
(.*?)     # capturing the variable name
          # ^ captures only until it reaches the closing sequence
(?!       # negative lookahead, making sure the following
          # ^ pattern is not found ahead of the current character
  \@\{\{  # same as opening sequence, if you change that, change this
)
\}\}      # closing sequence.

If you're not experienced with regex, a pretty safe rule is to escape every non-alphanumeric character, and don't ever needlessly escape letters as many escaped letters have special meaning to virtually all flavors of regex.

Upvotes: -1

Bryan Rayner
Bryan Rayner

Reputation: 4330

No, there is not a way to do this without dynamic code generation.

However, I have created a function which will turn a regular string into a function which can be provided with a map of values, using template strings internally.

Generate Template String Gist

/**
 * Produces a function which uses template strings to do simple interpolation from objects.
 * 
 * Usage:
 *    var makeMeKing = generateTemplateString('${name} is now the king of ${country}!');
 * 
 *    console.log(makeMeKing({ name: 'Bryan', country: 'Scotland'}));
 *    // Logs 'Bryan is now the king of Scotland!'
 */
var generateTemplateString = (function(){
    var cache = {};

    function generateTemplate(template){
        var fn = cache[template];

        if (!fn){
            // Replace ${expressions} (etc) with ${map.expressions}.

            var sanitized = template
                .replace(/\$\{([\s]*[^;\s\{]+[\s]*)\}/g, function(_, match){
                    return `\$\{map.${match.trim()}\}`;
                    })
                // Afterwards, replace anything that's not ${map.expressions}' (etc) with a blank string.
                .replace(/(\$\{(?!map\.)[^}]+\})/g, '');

            fn = Function('map', `return \`${sanitized}\``);
        }

        return fn;
    }

    return generateTemplate;
})();

Usage:

var kingMaker = generateTemplateString('${name} is king!');

console.log(kingMaker({name: 'Bryan'}));
// Logs 'Bryan is king!' to the console.

Hope this helps somebody. If you find a problem with the code, please be so kind as to update the Gist.

Upvotes: 33

Robert Moskal
Robert Moskal

Reputation: 22553

Still dynamic but seems more controlled than just using a naked eval:

const vm = require('vm')
const moment = require('moment')


let template = '### ${context.hours_worked[0].value} \n Hours worked \n #### ${Math.abs(context.hours_worked_avg_diff[0].value)}% ${fns.gt0(context.hours_worked_avg_diff[0].value, "more", "less")} than usual on ${fns.getDOW(new Date())}'
let context = {
  hours_worked:[{value:10}],
  hours_worked_avg_diff:[{value:10}],

}


function getDOW(now) {
  return moment(now).locale('es').format('dddd')
}

function gt0(_in, tVal, fVal) {
  return _in >0 ? tVal: fVal
}



function templateIt(context, template) {
  const script = new vm.Script('`'+template+'`')
  return script.runInNewContext({context, fns:{getDOW, gt0 }})
}

console.log(templateIt(context, template))

https://repl.it/IdVt/3

Upvotes: 2

Mohit Pandey
Mohit Pandey

Reputation: 3813

@Mateusz Moska, solution works great, but when i used it in React Native(build mode), it throws an error: Invalid character '`', though it works when i run it in debug mode.

So i wrote down my own solution using regex.

String.prototype.interpolate = function(params) {
  let template = this
  for (let key in params) {
    template = template.replace(new RegExp('\\$\\{' + key + '\\}', 'g'), params[key])
  }
  return template
}

const template = 'Example text: ${text}',
  result = template.interpolate({
    text: 'Foo Boo'
  })

console.log(result)

Demo: https://es6console.com/j31pqx1p/

NOTE: Since I don't know the root cause of an issue, i raised a ticket in react-native repo, https://github.com/facebook/react-native/issues/14107, so that once they can able to fix/guide me about the same :)

Upvotes: 6

Daniel
Daniel

Reputation: 9464

I liked s.meijer's answer and wrote my own version based on his:

function parseTemplate(template, map, fallback) {
    return template.replace(/\$\{[^}]+\}/g, (match) => 
        match
            .slice(2, -1)
            .trim()
            .split(".")
            .reduce(
                (searchObject, key) => searchObject[key] || fallback || match,
                map
            )
    );
}

Upvotes: 11

s.meijer
s.meijer

Reputation: 3909

I required this method with support for Internet Explorer. It turned out the back ticks aren't supported by even IE11. Also; using eval or it's equivalent Function doesn't feel right.

For the one that notice; I also use backticks, but these ones are removed by compilers like babel. The methods suggested by other ones, depend on them on run-time. As said before; this is an issue in IE11 and lower.

So this is what I came up with:

function get(path, obj, fb = `$\{${path}}`) {
  return path.split('.').reduce((res, key) => res[key] || fb, obj);
}

function parseTpl(template, map, fallback) {
  return template.replace(/\$\{.+?}/g, (match) => {
    const path = match.substr(2, match.length - 3).trim();
    return get(path, map, fallback);
  });
}

Example output:

const data = { person: { name: 'John', age: 18 } };

parseTpl('Hi ${person.name} (${person.age})', data);
// output: Hi John (18)

parseTpl('Hello ${person.name} from ${person.city}', data);
// output: Hello John from ${person.city}

parseTpl('Hello ${person.name} from ${person.city}', data, '-');
// output: Hello John from -

Upvotes: 4

cruzanmo
cruzanmo

Reputation: 777

This solution works without ES6:

function render(template, opts) {
  return new Function(
    'return new Function (' + Object.keys(opts).reduce((args, arg) => args += '\'' + arg + '\',', '') + '\'return `' + template.replace(/(^|[^\\])'/g, '$1\\\'') + '`;\'' +
    ').apply(null, ' + JSON.stringify(Object.keys(opts).reduce((vals, key) => vals.push(opts[key]) && vals, [])) + ');'
  )();
}

render("hello ${ name }", {name:'mo'}); // "hello mo"

Note: the Function constructor is always created in the global scope, which could potentially cause global variables to be overwritten by the template, e.g. render("hello ${ someGlobalVar = 'some new value' }", {name:'mo'});

Upvotes: 1

user2501097
user2501097

Reputation: 1181

I currently can't comment on existing answers so I am unable to directly comment on Bryan Raynor's excellent response. Thus, this response is going to update his answer with a slight correction.

In short, his function fails to actually cache the created function, so it will always recreate, regardless of whether it's seen the template before. Here is the corrected code:

    /**
     * Produces a function which uses template strings to do simple interpolation from objects.
     * 
     * Usage:
     *    var makeMeKing = generateTemplateString('${name} is now the king of ${country}!');
     * 
     *    console.log(makeMeKing({ name: 'Bryan', country: 'Scotland'}));
     *    // Logs 'Bryan is now the king of Scotland!'
     */
    var generateTemplateString = (function(){
        var cache = {};

        function generateTemplate(template){
            var fn = cache[template];

            if (!fn){
                // Replace ${expressions} (etc) with ${map.expressions}.

                var sanitized = template
                    .replace(/\$\{([\s]*[^;\s\{]+[\s]*)\}/g, function(_, match){
                        return `\$\{map.${match.trim()}\}`;
                    })
                    // Afterwards, replace anything that's not ${map.expressions}' (etc) with a blank string.
                    .replace(/(\$\{(?!map\.)[^}]+\})/g, '');

                fn = cache[template] = Function('map', `return \`${sanitized}\``);
            }

            return fn;
        };

        return generateTemplate;
    })();

Upvotes: 3

sarkiroka
sarkiroka

Reputation: 1542

You can use the string prototype, for example

String.prototype.toTemplate=function(){
    return eval('`'+this+'`');
}
//...
var a="b:${b}";
var b=10;
console.log(a.toTemplate());//b:10

But the answer of the original question is no way.

Upvotes: 6

Jason Orendorff
Jason Orendorff

Reputation: 45086

What you're asking for here:

//non working code quoted from the question
let b=10;
console.log(a.template());//b:10

is exactly equivalent (in terms of power and, er, safety) to eval: the ability to take a string containing code and execute that code; and also the ability for the executed code to see local variables in the caller's environment.

There is no way in JS for a function to see local variables in its caller, unless that function is eval(). Even Function() can't do it.

When you hear there's something called "template strings" coming to JavaScript, it's natural to assume it's a built-in template library, like Mustache. It isn't. It's mainly just string interpolation and multiline strings for JS. I think this is going to be a common misconception for a while, though. :(

Upvotes: 32

didinko
didinko

Reputation: 470

The issue here is to have a function that has access to the variables of its caller. This is why we see direct eval being used for template processing. A possible solution would be to generate a function taking formal parameters named by a dictionary's properties, and calling it with the corresponding values in the same order. An alternative way would be to have something simple as this:

var name = "John Smith";
var message = "Hello, my name is ${name}";
console.log(new Function('return `' + message + '`;')());

And for anyone using Babel compiler we need to create closure which remembers the environment in which it was created:

console.log(new Function('name', 'return `' + message + '`;')(name));

Upvotes: 10

Related Questions