Format a JavaScript string using placeholders and an object of substitutions?

I have a string with say: My Name is %NAME% and my age is %AGE%.

%XXX% are placeholders. We need to substitute values there from an object.

Object looks like: {"%NAME%":"Mike","%AGE%":"26","%EVENT%":"20"}

I need to parse the object and replace the string with corresponding values. So that final output will be:

>My Name is Mike and my age is 26.

The whole thing has to be done either using pure javascript or jquery.

The requirements of the original question clearly couldn't benefit from string interpolation, as it seems like it's a runtime processing of arbitrary replacement keys.

However, if you just had to do string interpolation, you can use:

const str = `My name is ${replacements.name} and my age is ${replacements.age}.`

Note the backticks delimiting the string, they are required.

---

For an answer suiting the particular OP's requirement, you could use String.prototype.replace() for the replacements.

The following code will handle all matches and not touch ones without a replacement (so long as your replacement values are all strings, if not, see below).

var replacements = {"%NAME%":"Mike","%AGE%":"26","%EVENT%":"20"},
    str = 'My Name is %NAME% and my age is %AGE%.';

str = str.replace(/%\w+%/g, function(all) {
   return replacements[all] || all;
});

jsFiddle.

If some of your replacements are not strings, be sure they exists in the object first. If you have a format like the example, i.e. wrapped in percentage signs, you can use the in operator to achieve this.

jsFiddle.

However, if your format doesn't have a special format, i.e. any string, and your replacements object doesn't have a null prototype, use Object.prototype.hasOwnProperty(), unless you can guarantee that none of your potential replaced substrings will clash with property names on the prototype.

jsFiddle.

Otherwise, if your replacement string was 'hasOwnProperty', you would get a resultant messed up string.

jsFiddle.

---

As a side note, you should be called replacements an Object, not an Array.

How about using ES6 template literals?

var a = "cat";
var b = "fat";
console.log(`my ${a} is ${b}`); //notice back-ticked string

More about template literals...

You can use JQuery(jquery.validate.js) to make it work easily.

$.validator.format("My name is {0}, I'm {1} years old",["Bob","23"]);

Or if you want to use just that feature you can define that function and just use it like

function format(source, params) {
    $.each(params,function (i, n) {
        source = source.replace(new RegExp("\\{" + i + "\\}", "g"), n);
    })
    return source;
}
alert(format("{0} is a {1}", ["Michael", "Guy"]));

credit to jquery.validate.js team

Currently there is still no native solution in Javascript for this behavior. Tagged templates are something related, but don't solve it.

Here there is a refactor of alex's solution with an object for replacements.

The solution uses arrow functions and a similar syntax for the placeholders as the native Javascript interpolation in template literals ({} instead of %%). Also there is no need to include delimiters (%) in the names of the replacements.

There are two flavors (three with the update): descriptive, reduced, elegant reduced with groups.

Descriptive solution:

const stringWithPlaceholders = 'My Name is {name} and my age is {age}.';

const replacements = {
  name: 'Mike',
  age: '26',
};

const string = stringWithPlaceholders.replace(
  /{\w+}/g,
  placeholderWithDelimiters => {
    const placeholderWithoutDelimiters = placeholderWithDelimiters.substring(
      1,
      placeholderWithDelimiters.length - 1,
    );
    const stringReplacement = replacements[placeholderWithoutDelimiters] || placeholderWithDelimiters;
    return stringReplacement;
  },
);

console.log(string);

Reduced solution:

const stringWithPlaceholders = 'My Name is {name} and my age is {age}.';

const replacements = {
  name: 'Mike',
  age: '26',
};

const string = stringWithPlaceholders.replace(/{\w+}/g, placeholder =>
  replacements[placeholder.substring(1, placeholder.length - 1)] || placeholder
);

console.log(string);

UPDATE 2020-12-10

Elegant reduced solution with groups, as suggested by @Kade in the comments:

const stringWithPlaceholders = 'My Name is {name} and my age is {age}.';

const replacements = {
  name: 'Mike',
  age: '26',
};

const string = stringWithPlaceholders.replace(
  /{(\w+)}/g, 
  (placeholderWithDelimiters, placeholderWithoutDelimiters) =>
    replacements[placeholderWithoutDelimiters] || placeholderWithDelimiters
);

console.log(string);

UPDATE 2021-01-21

Support empty string as a replacement, as suggested by @Jesper in the comments:

const stringWithPlaceholders = 'My Name is {name} and my age is {age}.';

const replacements = {
  name: 'Mike',
  age: '',
};

const string = stringWithPlaceholders.replace(
  /{(\w+)}/g, 
  (placeholderWithDelimiters, placeholderWithoutDelimiters) =>
  replacements.hasOwnProperty(placeholderWithoutDelimiters) ? 
    replacements[placeholderWithoutDelimiters] : placeholderWithDelimiters
);

console.log(string);

As with modern browser, placeholder is supported by new version of Chrome / Firefox, similar as the C style function printf().

Placeholders:

• %s String.
• %d,%i Integer number.
• %f Floating point number.
• %o Object hyperlink.

e.g.

console.log("generation 0:\t%f, %f, %f", a1a1, a1a2, a2a2);

BTW, to see the output:

• In Chrome, use shortcut Ctrl + Shift + J or F12 to open developer tool.
• In Firefox, use shortcut Ctrl + Shift + K or F12 to open developer tool.

---

@Update - nodejs support

Seems nodejs don't support %f, instead, could use %d in nodejs. With %d number will be printed as floating number, not just integer.

Just use replace()

var values = {"%NAME%":"Mike","%AGE%":"26","%EVENT%":"20"};
var substitutedString = "My Name is %NAME% and my age is %AGE%.".replace("%NAME%", $values["%NAME%"]).replace("%AGE%", $values["%AGE%"]);

You can use a custom replace function like this:

var str = "My Name is %NAME% and my age is %AGE%.";
var replaceData = {"%NAME%":"Mike","%AGE%":"26","%EVENT%":"20"};

function substitute(str, data) {
    var output = str.replace(/%[^%]+%/g, function(match) {
        if (match in data) {
            return(data[match]);
        } else {
            return("");
        }
    });
    return(output);
}

var output = substitute(str, replaceData);

You can see it work here: http://jsfiddle.net/jfriend00/DyCwk/.

If you want to do something closer to console.log like replacing %s placeholders like in

>console.log("Hello %s how are you %s is everything %s?", "Loreto", "today", "allright")
>Hello Loreto how are you today is everything allright?

I wrote this

function log() {
  var args = Array.prototype.slice.call(arguments);
  var rep= args.slice(1, args.length);
  var i=0;
  var output = args[0].replace(/%s/g, function(match,idx) {
    var subst=rep.slice(i, ++i);
    return( subst );
  });
   return(output);
}
res=log("Hello %s how are you %s is everything %s?", "Loreto", "today", "allright");
document.getElementById("console").innerHTML=res;

<span id="console"/>

you will get

>log("Hello %s how are you %s is everything %s?", "Loreto", "today", "allright")
>"Hello Loreto how are you today is everything allright?"

UPDATE

I have added a simple variant as String.prototype useful when dealing with string transformations, here is it:

String.prototype.log = function() {
    var args = Array.prototype.slice.call(arguments);
    var rep= args.slice(0, args.length);
    var i=0;
    var output = this.replace(/%s|%d|%f|%@/g, function(match,idx) {
      var subst=rep.slice(i, ++i);
      return( subst );
    });
    return output;
   }

In that case you will do

"Hello %s how are you %s is everything %s?".log("Loreto", "today", "allright")
"Hello Loreto how are you today is everything allright?"
    

Try this version here

This allows you to do exactly that

NPM: https://www.npmjs.com/package/stringinject

GitHub: https://github.com/tjcafferkey/stringinject

By doing the following:

var str = stringInject("My username is {username} on {platform}", { username: "tjcafferkey", platform: "GitHub" });
 
// My username is tjcafferkey on Git

> I have written a code that lets you format string easily.

Use this function.

function format() {
    if (arguments.length === 0) {
        throw "No arguments";
    }
    const string = arguments[0];
    const lst = string.split("{}");
    if (lst.length !== arguments.length) {
        throw "Placeholder format mismatched";
    }
    let string2 = "";
    let off = 1;
    for (let i = 0; i < lst.length; i++) {
        if (off < arguments.length) {
            string2 += lst[i] + arguments[off++]
        } else {
            string2 += lst[i]
        }
    }
    return string2;
}

Example

format('My Name is {} and my age is {}', 'Mike', 26);

Output

> My Name is Mike and my age is 26

As a quick example:

var name = 'jack';
var age = 40;
console.log('%s is %d yrs old',name,age);

The output is:

> jack is 40 yrs old

Here is another way of doing this by using es6 template literals dynamically at runtime.

const str = 'My name is ${name} and my age is ${age}.'
const obj = {name:'Simon', age:'33'}


const result = new Function('const {' + Object.keys(obj).join(',') + '} = this.obj;return `' + str + '`').call({obj})

document.body.innerHTML = result

const stringInject = (str = '', obj = {}) => {
  let newStr = str;
  Object.keys(obj).forEach((key) => {
    let placeHolder = `#${key}#`;
    if(newStr.includes(placeHolder)) {
      newStr = newStr.replace(placeHolder, obj[key] || " ");
    }
  });
  return newStr;
}

Input: stringInject("Hi #name#, How are you?", {name: "Ram"});
Output: "Hi Ram, How are you?"

ES6:

const strFormat = (str, ...args) => args.reduce((s, v) => s.replace('%s', v), str);

// Use it like:
const result = strFormat('%s is %s yrs old', 'name', 23);

This is a merged solution of Gerson Diniz and Shubham Vyas.

It is possible to pass a set of arguments or an object.

function strSwap(str) {

  if (!str) return null;
  
  let args = [];
  for (let a of arguments) 
    args.push(a);
  args.shift();
  
  if (!args.length) return null;
  
  // replacement by object - {{prop}}
  if (!!(args[0].constructor && args[0].constructor.name.toLowerCase() === 'object')) {
    for (let i in args[0]) {
      let n = `{{${i}}}`;
      str = str.includes(n) ? str.replaceAll(n, args[0][i] + '') : str;
    }
  }
  // replacement by placeholders - %s
  else {
    str = args.reduce((s, v) => s.replace('%s', v), str);
  }
  
  return str;

}

// ---------------------

console.log(strSwap('Hello %s, my name is %s.', 'alice', 'bob'));

console.log(strSwap('Hello {{a}}, my name is {{b}}. Hello {{b}}.', {
  a: 'alice', 
  b: 'bob'
}));

Lots of good/similar answers here. I wanted the ability to easily get a nested key in an object (or perhaps some JSON data structure) for substitution, so I took the following simple approach:

const getKey = (d, path) => {
  // path can be a string like 'key1.key2' or an iterable of keys
  if (typeof(path) === 'string') {
    path = path.split('.')
  }
  return path.reduce((x, y) => x[y], d)
}

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


// Example

> const str = 'there are ${a} ways to ${b.c}'
undefined
> inject(str, {'a':'many', 'b': {'c': 'skin a cat'}})
'there are many ways to skin a cat'

Some inspiration from this and this.