Concatenation doesn't seem to work in JavaScript

Question

I am doing something like this in my application. I have sequence of Id values like ID1, ID2 etc. I am trying to fetch the ID value using for loop.

Concatenation doesn't seem to work.

 function Save(count,Id1,Id2,Id3){ var response = []; for(var i=1;i <= count; i++) { value = `${'Id' + i}`; alert(value); } }

 <input type="button" value="Submit" onclick="Save(3,1,2,3)" />

Answer 1

You're creating a String "Id1" , you cannot interpolate a variable name. But with ES6, you can use the rest parameters ( ... ) to convert parts of your arguments to an Array:

 function Save(count, ...ids) { var response = []; for (var i = 0; i < count; i++) { value = ids[i]; alert(value); } }

 <input type="button" value="Submit" onclick="Save(3,1,2,3)" />

Before ES6, you would have to use arguments (that would convert all arguments):

 function Save() { var response = []; for (var i = 1; i <= arguments[0]; i++) { value = arguments[i]; alert(value); } }

 <input type="button" value="Submit" onclick="Save(3,1,2,3)" />

Or use eval , but don't. This is just for the example:

 function Save(count, Id1, Id2, Id3, Id4) { var response = []; for (var i = 1; i <= count; i++) { value = eval(`Id${i}`); alert(value); } }

 <input type="button" value="Submit" onclick="Save(3,1,2,3)" />

Answer 2

That doens't works because will throw a syntax exception. Its not the way you use interpolation. There are lots of ways to do what you need in a simple way:

• Using an array:

 function Save(ids) { var response = []; for(var i=0;i < ids.length; i++) { value = ids[i]; console.log(value); } } Save([1,2,3,4]);

• Using arguments constant:

 function Save(){ var response = []; for(var i=0;i < arguments.length; i++) { value = arguments[i]; console.log(value); } } Save(1,2,3,4);

Answer 3

That's not how interpolation works. It just inserts the value of the expression into a string. For example:

const age = 30
console.log(`I am ${age} years old!`) // Prints "I am 30 years old"

It doesn't interpret a string as an expression, which is what you apparently want to do. You could do that using eval , but this is most likely a step into the wrong direction.

If you want to iterate over the given values, just use an array instead:

 function save (values) { for (const value of values) { alert(value) } }

 <input type="button" value="Submit" onclick="save([9, 8, 7])" />

You can also iterate manually over the indexes, that can be useful if you need the index too, for example:

 function save (values) { for (let i = 0; i < values.length; i++) { alert(`Index ${i} = ${values[i]}`) } }

 <input type="button" value="Submit" onclick="save([9, 8, 7])" />

Or, if you want to keep your method of calling the function with multiple arguments (although the count argument is completely unnecessary), you can destructure the arguments into an array, like this:

 function save (...values) { for (let i = 0; i < values.length; i++) { alert(`Index ${i} = ${values[i]}`) } }

 <input type="button" value="Submit" onclick="save(9, 8, 7)" />

If you really wanted that count argument there, you could use function save (count, ...values) . As a side note, there is also arguments , which is an array-like object that holds all the arguments passed to your current function, so you could iterate over that as well.