Using function context/variables without function arguments

Question

I'm trying to implement my own template literals functionality (in educational purposes), but I can't understand how it works.

My idea is to extend String.prototype with function, that will eval every ${} sequence inside the string. The problem is my new function don't know context (variables inside the string). Here's how I think it should work:

String.prototype.smart_eval = function() { /* find all ${} and eval them */ }

function some_function() {
  let a = 1;
  let b = 2;
  return 'A is ${a}, B is ${b}, the sum is ${a + b}'.smart_eval()
}

This will cause Uncaught ReferenceError: a is not defined since a and b belong to some_function(), not smart_eval(). Is there any elegant way to solve this without using function arguments or .call() / .apply()?

vodkaJedi · Accepted Answer · 2021-03-09 04:05:20Z

0

So... It seems I want something, that is not possible. My first idea to solve this as smooth as possible is Python-like function format. I don't qualify this piece of code as 100% working solution, but maybe somebody will find it usable.

String.prototype.format=function(values) {
  let s = this;
  let r = '';
  for (let i = 0; i < s.length; i++) {
    if (s[i] == '{') {
      for (let t = i+1; t < s.length; t++) {
        if (s[t] == '}') {
          let d = s.substring(i + 1, t);
          try { d = parseInt(d) }
          catch(err) { d = false } 
          finally {
            if (d !== false) {
              if (values.length >= d + 1) { r += values[d]; i = t; break; }
              else { throw new ReferenceError('Literal is out of values range.') }
            }
          }
        }
      }
    } else {
      r += s[i] || ''
    }
  }
  return r
}

Examples:

'Hello {0}!'.format(['there']) // output: 'Hello there!'
'G{0}n{0}ral K{0}n{1}.'.format(['e', 'obi']) // output: 'General Kenobi.'

If you don't need IE11 support and some other browsers you can use rest parameters in function definition:

String.prototype.format=function(...values) {

And call it in more convenient way (without wrapping parameters into array):

'{1} wanna {0}? Let\'s {0}!'.format('play', 'You') // output: 'You wanna play? Let's play!'

edited Mar 9, 2021 at 4:05

answered Mar 9, 2021 at 1:01

vodkaJedi

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1 Comment

epascarello Over a year ago

code is pretty complicated... simple reg exp is how most people would have solved it. stackoverflow.com/questions/36994853/…

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