Functions and scope

Functions turn repeated logic into a named unit. Parameters are inputs, return is the output. A function without a return gives undefined.

function progressPercent(done, total) {
  if (total === 0) return 0;
  return Math.round((done / total) * 100);
}

const label = (percent) => `${percent}% complete`;

console.log(label(progressPercent(7, 10))); // "70% complete"

Parameter

A named input in the function definition.

Argument

The actual value passed when calling the function.

Scope

The region where a variable can be accessed.

Closure

A function remembering variables from where it was created.