Imitating @keyframes rule using Javascript only

I'm trying to create a function that uses percentage to get a number. The idea is to have something like this: 0%: 100px 50%: 200px 75%: 210px

So when I run my function at a percentage like 25% it blends 0 and 50 together, same for above 50%.

It sounds kind of complicated, the overall idea is to create something using JS only that will ease the numbers going from 0 to 100%

I've tried using regular percentage calculation, yet it doesn't really work for 3 or more variables.

So if you don't mind I've taken the liberty of turning your percentage into a JSON object:

let input = {
  0: 100,
  50: 200,
  75: 210,
};

We can then do something like this if we assume percentages are always whole numbers:

let output = {};
let previous = null;
// for each whole percent
for(let keyPercent in input)
{
  // cast to number
  let currPercent = parseFloat(keyPercent);
  // get value of percent
  let currValue = input[currPercent];
  if (currValue != null) { // if value is present
    if (previous != null) { // and previous value is not null
      // find the step size (valDiff) / (percentDiff)
      let step = (currValue - previous.value) / (currPercent - previous.percent);
      // count to multiply by step
      let count = 0;
      // this produces the percent value for every step between previous.percent and currPercent
      for (let prevPercent = previous.percent; prevPercent <= currPercent; prevPercent += 1) {
        output[prevPercent] = previous.value + (step * count);
        count += 1;
      }
    }
    // set previous value
    previous = { percent: currPercent, value: currValue };
  }
}
console.log(output);

this outputs something along the lines of:

0: 100
1: 102
2: 104
...
48: 196
49: 198
50: 200
51: 200.4
52: 200.8
...
73: 209.2
74: 209.6
75: 210

Let me know if you need any clarification.

Note: comments are made in code