JS Replace variable reference in string with the variable contents

Question

I want create a function which takes a string parameter and replaces variable references within it (designated with flanking %'s) with their corresponding variable values. I have been sufficiently warned about risk of eval() function, but haven't found an alternative. I am not sure how risky this code is. If it is a problem, what approach would be more secure.

Here is what I have:

var a = 1; var b = 2; result = myFunction("a is %a%, b is %b%"); console.log(result); // return "a is 1, b is 2" function myFunction(text) { // escape needed chars in text for regex text = text.replace(/[\-\[\]\/\{\}\(\)\*\+\?\.\\\^\$\|]/g, "\\$&"); var regExp = /%.+?%/g, match; while (match = regExp.exec(text)) { rep = match[0].substr(1).slice(0, -1); // remove flanking %'s text = text.replace(match[0], eval(rep)); } return text }

Based upon MH Souza recommendation, I figured this should work, but output is:

%a% a %b% b a is a, b is b

var a = 1; var b = 2; result = myFunction("a is %a%, b is %b%"); console.log(result); function myFunction(text) { // escape neede chars in text for regex text = text.replace(/[\-\[\]\/\{\}\(\)\*\+\?\.\\\^\$\|]/g, "\\$&"); var regExp = /%.+?%/g, match; while (match = regExp.exec(text)) { var rep = match[0].substr(1).slice(0, -1); // remove flanking %'s var rep = `${rep}`; console.log(match[0], rep); text = text.replace(match[0], rep); } return text }

Do you have any constraints that prevent you from using template literals? — rishat
– rishat, Commented Oct 28, 2017 at 19:36
Other than never using them before and not quite knowing how to implement, I no contraints. — cycle4passion
– cycle4passion, Commented Oct 28, 2017 at 20:05

Matheus Henrique Souza · Accepted Answer · 2017-10-28 21:06:33Z

You can achieve this by using Template Literals.

In your case:

const a = 1; const b = 2; const result = `a is ${a}, b is ${b}`; // a is 1, b is 2

You just need to write your string like this: `My string`

And to concatenate a variable value, you write the variable like this: ${myVariable}

So, the final result would look like this:

const myVariable = 'awesome'; const finalResult = `My string is ${myVariable}` // My string is awesome

code updated above, still not resolving like I would expect.

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