Replace string with array elements

You're creating a string templating system. Since you're using ECAMScript 6 syntax already, you could use the new built-in template literals.

let [FIRSTNAME, LASTNAME] = ['Peter', 'Parker'];
let string = `My name is ${FIRSTNAME} ${LASTNAME}`;

console.log(string);

This will also work more cleanly when there are multiple insertions of a given label within the string.

let [FIRSTNAME, LASTNAME] = ['Peter', 'Parker'];
let string = `My name is ${LASTNAME}... ${FIRSTNAME} ${LASTNAME}`;

console.log(string);

patternToBeReplaced.forEach(function (match, index) {
    string = string.replace(match, nameArray[index]);
});

Something like that.

let str = 'My name is [~FIRSTNAME] [~LASTNAME]';
let nameArray = ['Peter', 'Parker'];
let patternToBeReplaced = ['[~FIRSTNAME]', '[~LASTNAME]']


const result = patternToBeReplaced.reduce((str, placeholder, i) => {
return str.replace(placeholder, nameArray[i]);
}, str);
console.log(result);

1. output is a local variable, it's useless to just declaring it.
2. replace(String, String) would only replace the first matched pattern.

Here's my solution:

let replaceWithArrayElements = (patterns, replacements, targetString) => {
    patterns.forEach((pattern, i) => targetString = targetString.replace(new RegExp(pattern, 'g')));
    return targetString;
}

First solution, keeping your base code :

let string = 'My name is [~FIRSTNAME] [~LASTNAME]';
let nameArray = ['Peter', 'Parker'];
let patternToBeReplaced = ['[~FIRSTNAME]', '[~LASTNAME]'];
patternToBeReplaced
.forEach((match, i)=>{
  string = string
           .replace(match, nameArray[i]);
});

Using Map:

let string = 'My name is [~FIRSTNAME] [~LASTNAME]';
let replaceMap = new Map();
replaceMap.set('[~FIRSTNAME]', 'Peter');
replaceMap.set('[~LASTNAME]', 'Parker');
replaceMap.forEach((key, value)=>{
  string = string.replace(key, value);
});

Using ES6 "template" strings:

/*
*Have some ways of getting the first name in "firstName"
*Have some ways of getting the last name in "lastName"
*/
let string = `My name is ${firstName} ${lastName}`;

string.match(/\[~\w+\]/g).forEach((item,index)=>{
  string = string.replace(item, nameArray [index])
})

You can use the above code for you needs