Predicate is a fancy word for a function that returns a boolean value.

These functions are very useful because they let you test if a value satisifies certain requirements.

function isNumber(value) {
  return typeof value === "number";
}

isNumber(10); // returns true
isNumber("hello"); // returns false

JavaScript programmers often give predicate functions a name that starts with a verb e.g. isBig, isNegative, isActive, shouldUpdate,

Calling a predicate function is like asking a question: "is this value a number". The return value is the answer to your question.