How does the "new Function" calculate a string of numeric signs and produce the result? How does this type of function declaration work in general?

Question

let calc = '41+9-(30/2)*2';
let getResult = new Function(`return ${calc}`);
let omg = getResult();

console.log(omg); //20

I really don't understand how it works. Yes, I know that with such a declaration, a function has access only to global variables. As I understand, such a function can turn any string into code. So is it something like "eval"? What is the mechanism of calculating a string and converting it to the result of number type?

If anyone helped me figure this out, I would really appreciate it.

Answer 1

The following code works because we are in the global context already:

const x = 'foo'
const f = new Function(`return x`)
const result = f()
console.log(result) // 'foo'

The following code does not work because we are not in the global context:

(function () {
    const x = 'foo'
    const f = new Function(`return x`)
    const result = f()
    console.log(result) // ReferenceError: x is not defined
}())

However, your code is categorically different. String x is substituted into a template literal... and then the completed string is supplied to the Function constructor. So the function body is not closing over x. The function body is literally return 41+9-(30/2)*2, which is evaluated and the result is returned.

So your code will work, regardless of context:

(function() {
    const x = '41+9-(30/2)*2'
    const f = new Function(`return ${x}`)
    const result = f()
    console.log(result) //20
}())