An important topic in JavaScript is the concept of closures. When a function is created, it has access to a reference to all the variables declared around it, also known as it's lexical environment. The combination of the function and its enviroment is called a closure. This is a powerful and often used feature of the language.
function createAdder(a) { function f(b) { const sum = a + b; return sum; } return f; } const f = createAdder(3); console.log(f(4)); // 7 another example function createAdder(a){ return function f1(b){ return function f2(c){ const sum=a+b+c; return sum; } } } const f=createAdder(3); const f1=f(4); const f2=f1(5) console.log(f2)//12 
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