Returns a value between 0 and 1 representing how often a word appears in a string.

wordrate.js

// Full function
// Returns a value between 0 and 1 representing how often a word appears in a string
function wordRate(word, str) {
  return str.split(word).filter(Boolean).length / str.split(" ").length;
}

// Golfed/minified, 58 bytes
const w = (w,s)=>s.split(w).filter(Boolean).length/s.split` `.length

// Usage
console.log(wordRate("blue", "blue red blue green") * 100 + "%");  // 50%
console.log(w("blue", "blue red blue green") * 100 + "%");         // 50%