Using template literals for dynamic expression in Javascript

Question

How can i using dynamic values for evaluating expressions? For instance, consider the below example,

var a = 5;
var b = 10;
console.log(`Fifteen is ${a + b} and not ${2 * a + b}.`);

What if a and b are dynamic names? means instead of a and b it can be anything like 'someValue' . how to do something like below? (name can be dynamic)

 var 'someValue' = 1;
 console.log(`*Fifteen is ${'someValue' + b}* and not ${2 * a + b}.`);

In a nutshell i want to evaluate an expression where the field names are dynamic. So i wont be able to define variables statically. How do i overcome this?

Answer 1

You can use bracket notation and this keyword which is a reference to the window object.

 window.name="Vicky"; window.test="test"; const message = (p0, p1) => `Hello, ${p0}. This is a ${p1}`; console.log(message(window['name'],window['test'])); 

Answer 2

Maybe eval will suits your needs. But be careful with this function.

 function test() { var a = 'Hello World', b = 1, c = 2, myVarRefA = 'a', myVarRefB = 'b', myVarRefC = 'c'; console.log(`${eval(myVarRefA)} ${eval(myVarRefB + ' + ' + myVarRefC)}`); } test(); 

Answer 3

You can use a temporary Object with variables as its attributes

let temp = Object.create(null);
temp["someVar"] =3;
temp["anotherVar"]=2;
console.log(temp["someVar"] + temp["anotherVar"]);

But you can also use the more elegant

let temp = Object.create(null);
temp.someVar =3;
temp.anotherVar=2;
console.log(temp.someVar+ temp.anotherVar);

Do be careful about your "this" bindings

Answer 4

If your field names are completely custom, you can get use of Object.keys(..) . This method will let you loop through the properties in an object.

The Object.keys() method returns an array of a given object's own enumerable properties, in the same order as that provided by a for...in loop (the difference being that a for-in loop enumerates properties in the prototype chain as well).

Example (Just to show usage)

  const customObject = { name: 'Foo', age: 23, city: 'Bar City' }; const getFormatedObject = (fieldsObject) => { const fields = {}; Object.keys(fieldsObject).forEach((fieldName, index) => { fields[`f${index}`] = fieldsObject[fieldName]; }); return fields; } const formatedObject = getFormatedObject(customObject); for (let i = 0; i < Object.keys(formatedObject).length; i++) { console.log(formatedObject[`f${i}`]); } 

If you know the number of fields on your object you can go with something like below.

 const customObject = { name: 'Foo', age: 23, city: 'Bar City' }; const keys = Object.keys(customObject); console.log(`His ${keys[0]} is ${customObject[keys[0]]} and ${keys[1]} is ${customObject[keys[1]]} and ${keys[2]} is ${customObject[keys[2]]}`);